ZULU2 Radio Module

RF Solutions recently released its ZULU2 radio module range. Featuring a telemetry module, modem module, and a firmware-free module, the new range’s functionality is on par with its predecessor, with the advantage that no external components are required to provide a complete RF solution. RF Solutions ZULU2The new range includes:

  • ZULU2-M: A highly integrated RF modem and intelligent controller with a simple interface to the host controller. It handles all RF data communications automatically and without any requirement from the user.
  • ZULU2-T: Telemetry module providing a reliable transceiver based remote switch with up to 2km range. Each unit is supplied ready to operate, once paired with another, a remote control system is created.
  • ZULU2: A hardware platform module containing a SiLabs RF Transceiver and Processor allowing the user to programme the device to suit their own requirements. With no firmware supplied by RF Solutions, this would appeal to somebody with a confident programming ability.

The 25 mm × 11 mm smart modules can achieve a range of 2 km. License-free and operating on the 868- or 915-MHz frequency bands, the ZULU-2 range is available in surface-mount and dual-inline options, making it a good option for applications such as remote control, security, and data logging.

Source: RF Solutions

Tiny M10578 Modules Add GPS and GNSS to Small Devices and Wearables

Antenova recently released two new modules for GPS and GNSS. The M10578-A2 and M10578-A3 provide an easy drop-in receiver solution, which is a useful way to add location capability to very small consumer devices.Antenova M10578-A2

The modules—based on the MediaTek processor—both measure 9 × 9 × 1.8 mm with low current consumption, which makes them suitable for smaller portable devices, such as smart watches, navigation devices, OBD II modules, asset tracking, personal safety, and sports cameras.

The M10578-A2 module operates with GPS, with a 1-to-5-Hz update rate. The M10578-A3 operates with GPS, GLONASS, BEIDOU, and Gallileo with an update rate of 1 to 10 Hz. Included is internal self-generated orbit prediction that uses two GNSS systems simultaneously to give a faster time to fix and a second high-quality low noise amplifier (LNA) to boost low powered satellite signals. Both modules are pin compatible. As for the antenna function, Antenova offers the small Sinica SR4G008 GNSS antenna. Built on high-grade FR4 substrates with a high density, the modules’ multilayered design places the critical RF functions in the best position for location finding and performance.

Source: Antenova

The GAMMA Smart Module

RF Solutions’s GAMMA smart plug-in RF module offers incredible range and simple setup. You can it as a remote control, data modem, or a bidirectional switch.RF Solutions GAMMA

The latest addition to RF Solutions’s SMART Radio family, the module uses spread-spectrum technology and extensive algorithm enhancements to achieve 16-km range. With eight switch inputs or eight digital outputs, you can use two GAMMA’s modules as a receiver/transmitter remote control. Alternatively, you can set up a transceiver between two GAMMAs, where outputs on the receiving GAMMA will follow the inputs on the transmitting GAMMA.

Ready to use right out of the box, the GAMMA works either on its own or as a part of an existing system. It’s available in 868 and 915 MHz (with an option of SMT or SIL), you can use the GAMMA for a variety of applications.

Source: RF Solutions

GaN Devices for Mobile Base Station Transmitters

Infineon Technologies recently introduced its first devices in a family of Gallium Nitride (GaN) on Silicon Carbide (SiC) RF power transistors. The devices enable mobile base station manufacturers to build smaller, more powerful and more flexible transmitters. With higher efficiency, improved power density, and more bandwidth than currently used RF power transistors, the new devices improve the economics of building infrastructure to support today’s cellular networks. Additionally, they will enable the transition to 5G technology with higher data volumes and enhanced user-experience.Infineon-gan-group

The new RF power transistors leverage the performance of GaN technology to achieve ten percent higher efficiency and five times the power density of the LDMOS transistors commonly used today. This translates to smaller footprints and power requirements for the power amplifiers (PA) of base station transmitters in use today, which operate in either the 1.8-2.2 GHz or 2.3-2.7 GHz frequency range. Future GaN on SiC devices will also support 5G cellular bandsup to the 6 GHz frequency range. This roadmap allows Infineon to build on its long-standing expertise and state-of-the-art production technologies for RF transistor technology.

Design flexibility and support for the next-generation of 4G technology are additional benefits of GaN devices for RF power applications. The new devices have twice the RF bandwidth of LDMOS, so that one PA can support multiple operating frequencies. They also have increased instantaneous bandwidth available for transmitters, which lets a carrier offer higher dates using the data aggregation technique specified for 4.5G cellular networks.

Source: Infineon Technologies

RF Direct Conversion Zero-IF, Near Zero-IF and Low-IF Receiver ICs

CML Microcircuits recently released the CMX994A and CMX994E Direct Conversion Receiver (DCRx) ICs. The CMX994A and CMX994E are RF receiver ICs—which feature I/Q demodulators with low-power consumption and high-performance features—are targeted at narrowband and wideband Software Defined Radios (SDR) for wireless data and two-way radio applications. Their design provides the optimum route for high integration, allowing a small RF receiver to be realised with a minimum of external components in zero-IF, near zero-IF and low-IF systems.

The CMX994A and the CMX994E ICs build on the success of the popular CMX994 and are the first devices to use CML’s PowerTrade technology. PowerTrade enables the devices to dynamically balance power consumption and performance characteristics to suit varying operating requirements. Very low power consumption can also be achieved in standby mode whilst looking for an RF signal, using intelligent control of power cycling, phase control and I/Q channel selection.

The CMX994A delivers a very low power DCRx device while the CMX994E also includes the low power mode of the CMX994A but in addition, offers a high performance mode with improved IP3 performance.

The CMX994, CMX994A and CMX994E DCRx ICs offer excellent RF performance, exceptional IP2 from I/Q mixers and are suitable for modulation schemes including: QAM, 4FSK, GMSK and pi/4-DQPSK. Key features of the device include on-chip VCO for VHF applications, on-chip LNA, precision baseband filtering with selectable bandwidths and the smallest PCB area, typically less than 50% of a dual superhet.

The CMX994A and CMX994E are available, operating at 3 to 3.6 V, and come in a Q4 40-pin VQFN package.

Source: CML Microcircuits

Wireless Data Link

In 2001, while working on self-contained robot system called “Scout,” Tom Dahlin and Donald Krantz developed an interesting wireless data link. A tubular, wheeled robot, Scout’s wireless data link is divided into separate boards, one for radio control and another containing RF hardware.

Dahlin and Krantz write:

This article will describe the hardware and software design and implementation of a low-power, wireless RF data link. We will discuss a robotic application in which the RF link facilitates the command and control functions of a tele-operated miniature robot. The RF Monolithics (RFM) TR-3000 chip is the core of the transceiver design. We use a straightforward interface to a PIC controller, so you should be able to use or adapt much of this application for your needs…

Photo 1: The robot measures a little over 4″. Designed for tele-operated remote surveillance, it contains a video camera and transmitter. Scout can hop over obstacles by hoisting its tail spring (shown extended) and quickly releasing it to slap the ground and propel the robot into the air.

Photo 1: The robot measures a little over 4″. Designed for teleoperated remote surveillance, it contains a video camera and transmitter. Scout can hop over obstacles by hoisting its tail spring (shown extended) and quickly releasing it to slap the ground and propel the robot into the air.

The robot, called Scout, is packed in a 38-mm diameter tube with coaxial-mounted wheels at each end, approximately 110-mm long. The robot is shown in Photo 1. (For additional information, see the “Key Specifications for Scout Robot” sidebar.) Scout carries a miniature video camera and video transmitter, allowing you to tele-operate the robot by sending it steering commands while watching video images sent back from Scout. The video transmitter and data transceiver contained on the robot are separate devices, operating at 915 and 433MHz, respectively. Also contained on Scout are dual-axis magnetometers (for compass functions) and dual-axis accelerometers (for tilt/inclination measurement).

Figure 1: For the radio processor board, a PIC16F877 provides the horsepower to perform transceiver control, Manchester encoding, and packet formatting.

Figure 1: For the radio processor board, a PIC16F877 provides the horsepower to perform transceiver control, Manchester encoding, and packet formatting.

Scout’s hardware and software were designed to be modular. The wireless data link is physically partitioned onto two separate boards, one containing a PIC processor for radio control, message formatting, and data encoding (see Figure 1). The other board contains the RF hardware, consisting of the RFM TR3000 chip and supporting discrete components. By separating the two boards, we were able to keep the digital noise and trash away from the radio.

Read the full article.

Utilize Simple Radios with Simple Computers

I ordered some little UHF transmitters and receivers from suppliers on AliExpress, the Chinese equivalent of Amazon.com, in order to extend my door chimes into areas of my home where I could not hear them. These ridiculously inexpensive units are currently about $1 per transmitter-receiver pair in quantities of five, including shipping, and are available at 315 and 433.92 MHz. Photo 1 shows a transmitter and receiver pair.  Connections are power and ground and data in or out.

Photo 1: 315 MHz Transmitter-Receiver Pair (Receiver on Left)

Photo 1: The 315-MHz transmitter-receiver pair (receiver on left)

The original attempt at a door chime extender modulated the transmit RF with an audio tone and searched for the presence of that tone at the receiver with a narrow audio filter, envelope detector, and threshold detector. This sort of worked, but I started incorporating the same transmitters into another project that interfered, despite the audio filter.

The other project used Arduino Uno R3 computers and Virtual Wire to convey data reliably between transmitters and receivers. Do not expect a simple connection to a serial port to work well. As the other project evolved, I learned enough about the Atmel ATtiny85 processor, a smaller alternative to the Atmel ATmega328 processor in the Arduino Uno R3, to make new and better and very much simpler circuits. That project evolved to come full circle and now serves as a better doorbell extender. The transmitters self identify, so a second transmit unit now also notifies me when the postman opens the mailbox.

Note the requirement for Virtual Wire.  Do not expect a simple connection to a serial port to work very well.

Transmitter

Figure 1 shows the basic transmitter circuit, and Photo 2 shows the prototype transmitter. There is only the ATtiny85 CPU and a transmitter board. The ATtiny85 only has eight pins with two dedicated to power and one to the Reset input.

Figure 1: Simple Transmitter Schematic

Figure 1: Simple transmitter schematic

One digital output powers the transmitter and a second digital output provides data to the transmitter.  The remaining three pins are available to serve as inputs.  One serves to configure and control the unit as a mailbox alarm, and the other two set the identification message the transmitter sends to enable the receiver to discriminate among a group of such transmitters.

Photo 2: 315 MHz Transmitter and ATtiny85 CPU

Photo 2: The 315-MHz transmitter and ATtiny85 CPU

When input pin 3 is high at power-up, the unit enters mailbox alarm mode. In mailbox alarm mode, the input pins 2 and 7 serve as binary identification bits to define the value of the single numeric character that the transmitter sends, and the input pin 3 serves as the interrupt input. Whenever input pin 3 transitions from high-to-low or low-to-high, the ATtiny85 CPU wakes from SLEEP_MODE_PWR_DOWN, makes a single transmission, and goes back to sleep. The current mailbox sensor is a tilt switch mounted to the door of the mailbox. The next one will likely be a reed relay, so only a magnet will need to move.

When in SLEEP_MODE_PWR_DOWN, the whole circuit draws under 0.5 µA. I expect long life from the three AAA batteries if they can withstand heat, cold, and moisture. I can program the ATtiny to pull the identification inputs high, but each binary identification pin then draws about 100 µA when pulled low. In contrast, the 20- or 22-MΩ pull-up resistors I use as pull-ups each draw only a small fraction of a microampere when pulled low.

When input pin 3 is low at power-up, the unit enters doorbell extender alarm mode. In doorbell extender alarm mode, the input pins 2 and 7 again serve as binary identification bits to define the value of the single numeric character that the transmitter sends; but in doorbell extender mode, the unit repetitively transmits the identification character whenever power from the door chimes remains applied.

Receiver

Figure 2 shows the basic receiver circuit, and Photo 3 shows the prototype receiver. There is only the ATtiny85 CPU with a 78L05 voltage regulator and a receiver board.

Figure 2: Simple Receiver Schematic

Figure 2: Simple receiver schematic

The receiver output feeds the input at pin 5. The Virtual Wire software decodes and presents the received character. Software in the CPU sends tone pulses to a loudspeaker that convey the value of the identification code received, so I can tell the difference between the door chime and the mailbox signals. Current software changes both the number of beep tones and their audible frequency to indicate the identity of the transmit source.

Photo 3: The 315-MHz receiver with ATtiny85 CPU and 78L05 voltage regulator

Photo 3: The 315-MHz receiver with ATtiny85 CPU and 78L05 voltage regulator

Note that these receivers are annoyingly sensitive to power supply ripple, so receiver power must either come from a filtered and regulated supply or from batteries.

Photo 4 shows the complete receiver with the loudspeaker.

Photo 4: Receiver with antenna connections and loudspeaker

Photo 4: Receiver with antenna connections and a loudspeaker

Link Margin

A few inches of wire for an antenna will reach anywhere in my small basement. To improve transmission distance from the mailbox at the street to the receiver in my basement, I added a simple half-wave dipole antenna to both transmitter and receiver. Construction is with insulated magnet wire so I can twist the balanced transmission line portion as in Photo 5. I bring the transmission line out through an existing hole in my metal mailbox and staple the vertical dipole to the wooden mail post. My next mailbox will not be metal.

Photo 5: Simple half-wave dipole for both Tx and Rx increases link distance

Photo 5: Simple half-wave dipole for both Tx and Rx increases link distance

I don’t have long term bad weather data to show this will continue to work through heavy ice and snow, but my mailman sees me respond promptly so far.

Operating Mode Differences

The mailbox unit must operate at minimum battery drain, and it does this very well. The doorbell extender operates continuously when the AC door chime applies power. In order to complete a full message no matter how short a time someone presses the doorbell push button, I rectify the AC and store charge in a relatively large electrolytic capacitor to enable sufficient transmission time.

Photo 6: New PCBs for receive and transmit

Photo 6: New PCBs for receive and transmit

Availability

This unit is fairly simple to fabricate and program your self, but if there is demand, my friend Lee Johnson will make and sell boards with pre-programmed ATtiny85 CPUs. (Lee Johnson, NØVI, will have information on his website if we develop this project into a product: www.citrus-electronics.com.) We will socket the CPU so you can replace it to change the program. The new transmitter and receiver printed circuit boards appear in Photo 6.


Dr. Sam Green (WØPCE) is a retired aerospace engineer living in Saint Louis, MO. He holds degrees in Electronic Engineering from Northwestern University and the University of Illinois at Urbana. Sam specialized in free space and fiber optical data communications and photonics. He became KN9KEQ and K9KEQ in 1957, while a high school freshman in Skokie, IL, where he was a Skokie Six Meter Indian. Sam held a Technician class license for 36 years before finally upgrading to Amateur Extra Class in 1993. He is a member of ARRL, a member of the Boeing Employees Amateur Radio Society (BEARS), a member of the Saint Louis QRP Society (SLQS), and breakfasts with the Saint Louis Area Microwave Society (SLAMS). Sam is a Registered Professional Engineer in Missouri and a life senior member of IEEE. Sam is listed as inventor on 18 patents.

Impedance Matching Matters (EE Tip #145)

RF designers, as well as more and more digital-oriented designers, are used to thinking about impedance matching. But it is very easy to forget it when you are designing a non-RF project. A non-matched circuit will generate power losses as well as nasty reflection phenomena. (Refer to my article, “TDR Experiments,” Circuit Cellar 225, 2009.)

Impedance matching must be managed at the schematic stage, for example, by adding provisional matching pads for all integrated antennas, which will enable you to correct a slightly mis-adapted antenna (see Figure 1).

Figure 1: Impedance matching requirements must be anticipated. In particular, any embedded antenna will surely need manual matching for optimal performance. If you forget to include some area for a matching network like this one on your PCB, you won’t achieve the best performance.

Figure 1: Impedance matching requirements must be anticipated. In particular, any embedded antenna will surely need manual matching for optimal performance. If you forget to include some area for a matching network like this one on your PCB, you won’t achieve the best performance.

Impedance matching is also a PCB design issue. As rule of thumb, you can’t avoid impedance-matched tracks when you are working with frequencies higher than the speed of light divided by 10 times the board size. A typical 10-cm board would translate to a cutoff frequency of 300 MHz. A digital designer would then say: “Cool, my clock is only 100 MHz. No problem!” But a 100-MHz square-ware clock or digital signal means harmonic frequencies in the gigahertz range, so it would be wise to show some concern.

The problem could also happen with very slow clocks when you’re using fast devices. Do you want an example? Last year, one of my colleagues developed a complex system with plenty of large and fast FPGAs. These chips were programmed through a common JTAG link and we ended up with nasty problems on the JTAG bus. We still had issues even when we slowed down the JTAG speed to 100 kHz. So, it couldn’t have been an impedance matching problem, right? Wrong. It was. Simply because the JTAG is managed by the FPGA with the same ultra-fast logic cells that manage your fast logic so with stratospheric skew rates which translated into very fast transitions on the JTAG lines. This generated ringing due to improper impedance matching, so there were false transitions on the bus. Such a problem was easy to solve once we pinpointed it, but we lost some days in between.—Robert Lacoste, CC25, 2013

 

HumDT Wireless UART Data Transceiver

Linx Technologies recently announced the launch of its 11.5 mm × 14.0 mm HumDT wireless UART data transceiver with built-in networking with encryption. Each module can act as one of three components in a wireless network: an access point that controls a network, a range extender (to repeat messages and expand the network’s range, or an end device.

Linx Technologies HumDT

Linx Technologies HumDT

Each access point can connect to up to 50 range extenders and end devices. The access point also supports routing so end devices can communicate with each. The transceiver automatically manages all routing and network maintenance functions.

The 900-MHz HumDT version outputs up to 10 dBm, which results in a line-of-sight range of up to 1,600 m (1 mile), depending on the antenna implementation. The 2.4-GHz version outputs up to 1 dBm, resulting in a line-of-sight range of 100 m (300′).

To aid rapid development, the HumDT Series transceiver is available as part of a newly conceived type of Master Development System. This development kit is designed to assist in the rapid evaluation and integration of the HumDT Series data transceiver modules. The all-inclusive system features several preassembled evaluation boards, which include everything needed to quickly test the operation of the transceiver modules.

At below $9 in volume, the Hummingbird platform is a low-cost complete wideband transceiver with microcontroller module.

Source: Linx Technologies

 

EMC Measurement Technology

LangerSX near-field probes enable electromagnetic compatibility (EMC) analyses of interferences emitted by electronic boards, components, and IC pins with high internal frequencies. The SX-R3-1 magnetic H-field probe is designed to detect high-frequency magnetic fields with a high geometrical resolution. The field orientation and distribution can be detected by moving the probe around conductor runs, bypass capacitors, EMC components, and within IC pin and supply system areas. The SX-E03 E-field probe detects bus structures and larger components.

The probes have a 1-to-10-GHz frequency range. Their high resolution (the SX R3-1 achieves 1 mm and the SX E03 covers up to 4 mm × 4 mm) enables them to pinpoint RF sources on densely packed boards or on IC pins. The magnetic-field probe heads are electrically shielded. The probes are connected to a spectrum analyzer input via a shielded cable and SMA connectors during measurement. High clock rates of 2 GHz, for example, may result in fifth-order harmonics of up to 10 GHz. These harmonics are coupled out by RF sources on the board (e.g., conductor-run segments, ICs, and other components). They may stimulate other structural parts of the board to oscillate and emit interferences.

Contact Langer for pricing.

Langer EMV-Technik
www.langer-emv.com

Build an Automated Vehicle Locator

Several things inspired Electrical and Computer Engineering Professor Chris Coulston and his team at Penn State Erie, The Behrend College, to create an online vehicle-tracking system. Mainly, the team wanted to increase ridership on a shuttle bus the local transit authority provided to serve the expanding campus. Not enough students were “on board,” in part because it was difficult to know when the bus would be arriving at each stop.

So Coulston’s team created a system in which a mobile GPS tracker on the bus communicates its location over a radio link to a base station. Students, professors, or anyone else carrying a smartphone can call up the bus tracker web page, find out the bus’ current location, and receive reliable estimates of the bus’ arrival time at each of its stops. Coulston, computer engineering student Daniel Hankewycz, and computer science student Austin Kelleher wrote an article about the system, which appears in our June issue.

Circuit Cellar recently asked Coulston if the system, implemented in the fall 2013 semester, had accomplished its goals and might be expanded.

“The bus tracker team is tracking usage of the web site using Google Analytics,” Coulston said. “The data reveals that we get on average 100 hits a day during cold weather and fewer on warmer days. Ridership has increased during this past year, helping assure the long-term presence of the shuttle on our campus.”

“Over winter break, shuttle service was increased to a distant location on campus,” he added. “In order to better track the location of the shuttle, a second base station was added. The additional base station required a significant rework of the software architecture. The result is that the software is more modular and can accept an arbitrary number of base stations. There are no plans at present to add a second bus—a good thing, because this change would require another significant rework of the software architecture.”

Initially, Coulston looked to other real-time vehicle trackers for inspiration: “There are a variety of live bus trackers that motivated my early ideas, including the University of Utah’s Live Tracker  and the Chicago Transit Authority’s CTA Bus Tracker. Given our single bus route on campus, I was motivated to keep the interface simple and clean to minimize the amount of time needed to figure out where the bus is and how long it’s going to take to get to my stop.”

The system, as it was originally implemented in August 2013, is fully described in the June issue, now available for single-issue purchase or membership download. The following article excerpt provides a broad overview and a description of the team’s hardware choices.

THE BIG PICTURE
Figure 1 shows the bus tracker’s hardware, which consists of three components: the user’s smartphone, the base station placed at a fixed location on campus, and the mobile tracker that rides around on the bus.

The bus tracking system includes a Digi International XTend radio, a Microchip Technology PIC18F26K22 microcontroller, and a Raspberry Pi single-board computer.

Figure 1: The bus tracking system includes a Digi International XTend radio, a Microchip Technology PIC18F26K22 microcontroller, and a Raspberry Pi single-board computer.

Early on, we decided against a cellular-based solution (think cell phone) as the mobile tracker. While this concept would have benefited from wide-ranging cellular coverage, it would have incurred monthly cellar network access fees. Figure 1 shows the final concept, which utilizes a 900-MHz radio link between the mobile tracker and the base station.

Figure 2 shows the software architecture running on the hardware from Figure 1. When the user’s smartphone loads the bus tracker webpage, the JavaScript on the page instructs the user’s web browser to use the Google Maps JavaScript API to load the campus map. The smartphone also makes an XMLHttpRequests request for a file on the server (stamp.txt) containing the bus’ current location and breadcrumb index.

Figure 2: The bus tracker’s software architecture includes a GPS, the mobile tracker, a smartphone, and the base station.

Figure 2: The bus tracker’s software architecture includes a GPS, the mobile tracker, a smartphone, and the base station.

This information along with data about the bus stops is used to position the bus icon on the map, determine the bus’ next stop, and predict the bus’ arrival time at each of the seven bus stops. The bus’ location contained in stamp.txt is generated by a GPS receiver (EM-408) in the form of an NMEA string. This string is sent to a microcontroller and then parsed. When the microcontroller receives a request for the bus’ location, it formats a message and sends it over the 900-MHz radio link. The base station compares the bus position against a canonical tour of campus (breadcrumb) and writes the best match to stamp.txt.

Early in the project development, we decided to collect the bus’ position and heading information at 2-s intervals during the bus’ campus tour. This collection of strings is called “breadcrumbs” because, like the breadcrumbs dropped by Hansel and Gretel in the eponymously named story, we hope they will help us find our way around campus. Figure 3 shows a set of breadcrumbs (b1 through b10), which were collected as the bus traveled out and back along the same road.

Figure 3: Breadcrumbs (b1 through b10) containing the bus’ position and orientation information were taken every 2 s during a test-run campus tour.

Figure 3: Breadcrumbs (b1 through b10) containing the bus’ position and orientation information were taken every 2 s during a test-run campus tour.

The decision to collect breadcrumbs proved fortuitous as they serve an important role in each of the three hardware components shown in Figure 1.

MOBILE TRACKER
The bus houses the mobile tracker (see Photo 1). Figure 4 shows the schematic, which is deceptively simple. What you see is the third iteration of the mobile tracker hardware.

Figure 4: The mobile tracker includes a Microchip Technology PIC18F26K22 microcontroller, a Micrel MIC5205 regulator, a Digi International XTend RF module, and a Texas Instruments TXS0102 bidirectional translator

Figure 4: The mobile tracker includes a Microchip Technology PIC18F26K22 microcontroller, a Micrel MIC5205 regulator, a Digi International XTend RF module, and a Texas Instruments TXS0102 bidirectional translator

An important starting point in the design was how to step down the bus’ 12-V supply to the 5-V required by our circuit. In terms of hardware, the best decision we made was to abandon the idea of trying to integrate a 12-to-5-V converter onto the mobile tracker PCB. Instead we purchased a $40 CUI VYB15W-T DC-DC converter and fed the mobile tracker 5-V inputs…

We used Micrel’s MIC5205 regulator to step down the 5 V for the 3.3-V GPS receiver, which easily supplied its peak 80 mA. Since we ran a Digi International XTend radio at 5 V for the best range, we ended up with mixed voltage signals. We used a Texas Instruments TXS0102 bidirectional voltage-level translator, which handles voltage-interfacing duties between the 5-V radio and the 3.3-V microcontroller.

The mobile tracker unit

Photo 1: The mobile tracker unit

We selected Microchip Technology’s PIC18F26K22 because it has two hardware serial ports, enabling it to simultaneously communicate with the GPS module and the radio modem when the bus is traveling around campus. We placed two switches in front of the serial ports. One switch toggles between the GPS module and the Microchip Technology PICkit 3 programming pins, which are necessary to program the microcontroller. The second switch toggles between the radio and a header connected to a PC serial port (via a Future Technology Devices FT232 USB-to-serial bridge). This is useful when debugging at your desk. An RGB LED in a compact PLCC4 package provides state information about the mobile tracker.

The XTend RF modules are the big brothers to Digi International’s popular XBee series. These radios come with an impressive 1 W of transmitting power over a 900-MHz frequency, enabling ranges up to a mile in our heavily wooded campus environment. The radios use a standard serial interface requiring three connections: TX, RX, and ground. They are simple to set up. You just drop them into the Command mode, set the module’s source and destination addresses, store this configuration in flash memory, and exit. You never have to deal with them again. Any character sent to the radio appears on the destination modem’s RX line.

The GPS receiver utilizes the CSR SiRFstarIII chipset, which is configured to output a recommended minimum specific (RMC) string every 2 s…

The mobile tracker’s firmware listens for commands over the serial port and generates appropriate replies. Commands are issued by the developer or by the base station…

Burning breadcrumbs into the mobile tracker’s flash memory proved to be a good design decision. With this capability, the mobile tracker can generate a simulated tour of campus while sitting on the lab bench.

BASE STATION
The base station consists of an XTend RF module connected to a Raspberry Pi’s serial port (see Photo 2). The software running on the Raspberry Pi does everything from running an Nginx open-source web server to making requests for data from the mobile tracker.

From Figure 1, the only additional hardware associated with the base station is the 900-MHz XTend radio connected to the Raspberry Pi over a dedicated serial port on pins 8 (TX) and 10 (RX) of the Raspberry Pi’s GPIO header.

The only code that runs on the base station is the Python program, which periodically queries the mobile tracker to get the bus’ position and heading. The program starts by configuring the serial port in the common 9600,8,N,1 mode. Next, the program is put into an infinite loop to query the mobile tracker’s position every 2 s.

Photo 2: The base station includes an interface board, a Raspberry Pi, and a radio modem.

Photo 2: The base station includes an interface board, a Raspberry Pi, and a radio modem.

Remote-Control Powered Trapdoor Lift

William Wachsmann, a retired electronic technologist from Canada, has more than 35 years of experience working with minicomputers, microcomputers, embedded systems, and programming in industries ranging from nuclear and aerospace to voicemail and transportation systems.

But despite the complexity of the work he has done over the years, when it came to building a remote-controlled, powered trapdoor lift system for his home, he had two priorities: simplicity and price.

“Although it can be fulfilling to design your own hardware, why reinvent the wheel if you don’t have to? Many reasonably priced modules can be wired together,” Wachsmann says in his article about the project, which appears in Circuit Cellar’s May issue. “Add some software to express the functionality required and you can quickly and inexpensively put together a project. Not only is this method useful for a homebuilt one-of-a-kind application, but it can also be used for proof-of-concept prototyping. It leaves you free to concentrate on solving the problems pertinent to your application.”

Wachsmann’s project relies on off-the-shelf modules for the electrical functions of a trapdoor lift system that provides access to his basement.

“Lifting the trapdoor was hard on my wife’s back,” he says. “If her arms were full when she came upstairs and the door was open, she had to twist her body to release the mechanical latching mechanism while simultaneously stopping the door from falling on her head.”

The multidisciplinary project includes mechanical, electronic, and software components. For the full details—including programming of the project’s Freescale Semiconductor FRDM-KL25Z microprocessor using the mbed online IDE—check out the May issue now available for membership download and single-issue purchase.

(And if you’re interested in other articles about remote-control projects, check out Raul Alvarez’s Home Energy Gateway system, which enables users to remotely monitor home energy consumption and monitor household devices. Alvarez’s project won second place in the 2012 DesignSpark chipKIT challenge administered by Circuit Cellar.)

Excerpts from Wachsmann’s article below describe his system’s mechanical and hardware elements.

INS AND OUTS
I used a screw lift from an old treadmill. It has a 48-VDC motor that draws about 1 A under a 100-lb load. The screw mechanism has a 6” travel distance. Built-in limit switches shut off the motor when the screw reaches the end of its travel in each direction. The screw’s length is nominally 30” when closed and 36” when open. The length can be adjusted slightly by rotating the screw by hand, but the overall travel distance is still 6”.

A simple switch would have sufficed to control the screw lift’s DC motor, but I wanted it to be remotely controlled so the trapdoor could be raised and lowered from anywhere upstairs and from the basement. When someone is in the basement with the trapdoor closed there is no visible way for them to know if the door is obstructed. Initially, I was going to install a warning beeper that the door was opening, but that wouldn’t help if an inanimate object (e.g., a bag of groceries) was on top of the door. I needed to incorporate some form of sensing mechanism into the design.

Figure 1: This diagram represents the trapdoor mechanics. The arm’s down position is shown in red; the up position is shown in blue. Vertical red arrows are labeled with the downward force in pounds

Figure 1: This diagram represents the trapdoor mechanics. The arm’s down position is shown in red; the up position is shown in blue. Vertical red arrows are labeled with the downward force in pounds

THE MECHANICS
I needed a levered system design that used a pivoted bar and the motorized screw lift. The lift also had to enable the door to be manually opened in case of a power failure.
I used IMSI/Design’s TurboCAD program for the mechanical design. By using CAD software, I could experiment with the pivot position, moment arms, and torque requirements to meet the mechanical constraints imposed by the screw lift and the trapdoor’s size and weight.

Photo 1: The screw lift and pivot arm mechanism with a spring assist are shown.

Photo 1: The screw lift and pivot arm mechanism with a spring assist are shown.

Figure 1 shows a diagram of the trapdoor, which is hinged on the left. The opposite side of the door exerts a 15.2-lb downward force. This means the torque (force × distance) required to open the door is 509.2 in-lbs. The pivot arm in red is the position when the door is closed. The blue pivot arm shows the position when the door is open to an 80° angle.

To keep within the 6” lift constraint, I used a 4.25” moment arm to pull down on the pivot arm. This left me with the force required to initially lift the door at 119.5 lb. Also this did not include the added torque due to the pivot arm’s weight.

After mulling this over for a couple of days (and nights) I had an idea. I realized that 119.5 lb is only needed when the door is completely closed. As the door opens, the torque requirement lessens. I incorporated a heavy spring (see Photo 1). When the door is closed the spring extension provides an additional downward force of about 35 lb. This is enough to lessen the load on the screw lift and to compensate for the pivot arm’s additional 2.2 lb. Using a screw lift meant the arm would not spring up if the door was manually opened.

I used an angle iron for the pivot arm. It is 28” long because it had to push up on the door to the right of the door’s center of gravity at 16.75” without adding too much additional torque. The roller is the type used as feet on beds. I used an arbor and 0.75”-diameter bolt through the floor joist for the pivot (see Photo 2).

An arbor is used as a bearing with a 0.75” bolt through the floor joist. The lift mechanism pivots at this point.

Photo 2: An arbor is used as a bearing with a 0.75” bolt through the floor joist. The lift mechanism pivots at this point.

THE HARDWARE
I had set an arbitrary $100 limit for the rest of the system and I quickly realized I would easily come in under budget. I used a $24.25 two-channel RF wireless garage door remote-control receiver, which I purchased from eBay (see Photo 3). This controller can be used in a latched or an unlatched mode. The latched mode requires a momentary push of one of the buttons to cause one of the relays to switch and stay in the On position. When the controller is in unlatched mode, you must hold the button down to keep the relay switched.

Photo 3: The two-channel wireless remote control is shown with the cover removed from the receiver. It came with two keychain-style remotes, which I marked with Up and Down arrows.

Photo 3: The two-channel wireless remote control is shown with the cover removed from the receiver. It came with two keychain-style remotes, which I marked with Up and Down arrows.

Unfortunately, this remote control and any similar ones only come with single-pole double-throw (SPDT) relays. What I really wanted were double-pole double-throw (DPDT) relays to switch both sides of the motor to enable current reversal through the motor.
A remote control system with two remotes seemed ideal and was possible to design with SPDT, so I purchased the relays. Figure 2 shows the circuit using two bridge rectifier DC power supplies. It turns out there were problems with this approach.

SW1 and SW2 represent the Up and Down relays. In latched mode, the door would open when SW1 was energized using the A button on a remote. Pressing the A button again would stop the motor while the door was opening. So would pressing the B button, but then to continue opening the door you needed to press the B button again. Pressing the A  button in this state would cause the door to close because SW2 was still energized. Added to this confusion was the necessity of pressing the A button again when the door was fully opened and stopped due to the internal limit switches. If you didn’t do this, then pressing the B button to close the door wouldn’t work because SW1 was still energized.

Figure 2: It would be theoretically possible to use dual-power supplies and single-pole double-throw (SPDT) switches to control a motor in two directions. When SW1 (b,c) is connected, current flows through D2. When SW2 (b,c) is connected, current flows through D1 in the opposite direction.

Figure 2: It would be theoretically possible to use dual-power supplies and single-pole double-throw (SPDT) switches to control a motor in two directions. When SW1 (b,c) is connected, current flows through D2. When SW2 (b,c) is connected, current flows through D1 in the opposite direction.

I decided to just use the door in unlatched mode and continuously hold down the A button until the door was fully open. What was the problem with this? Noise! Interference from the motor was getting back into the control and causing the relay to frequently switch on and off. This is not good for mechanical relays that have a finite contact life.

After playing around for a while with both operation modes, I noticed that even in the latched mode the motor would sometimes stop and it would occasionally even reverse itself. This was really bad and it became worse.

If both SW1 and SW2 happened to switch at the same time and if the current was at a maximum and there was arcing at the terminal, there could conceivably be a momentary short through a diode in each of the bridge rectifiers that would burn them out. Arc suppression devices wouldn’t help because when active at high voltages, they would almost look like a short between the switch’s terminals A,C. I needed to step back and rethink this.

I found an $8.84 two-channel DPDT relay switch board module on eBay. The module enabled me to use a single-power supply and isolated the motor current from the remote-control board. These relays boards have TTL inputs, so it is tricky to use relays on the remote control board to control the relays on the second relay board. You have to contend with contact bounce. Even if I incorporated debounce circuits, I still didn’t have a way stop the door from opening if it was obstructed.

It was time to get with the 21st century. I needed to use a microcontroller and handle all the debounce and logic functions in firmware.

I bought a $12.95 Freescale Semiconductor FRDM-KL25Z development board, which uses the Kinetis L series of microcontrollers. The FRDM-KL25Z is built on the ARM Cortex-M0+ core. This board comes with multiple I/O pins, most of which can be programmed as required. It also has two micro-USB ports, one of which is used for downloading your program onto the microcontroller and for debugging (see Figure 3).

Figure 3: This is the system’s complete wiring diagram. On the left is a 48-V AC supply and an unregulated 12-V DC motor. A 2.7-Ω, 5-W resistor, which is used for current sensing, is in series with the motor.

Figure 3: This is the system’s complete wiring diagram. On the left is a 48-V AC supply and an unregulated 12-V DC motor. A 2.7-Ω, 5-W resistor, which is used for current sensing, is in series with the motor.

Low-Power Remote-Control Transceivers

LinxThe TT Series remote-control transceiver is designed for bidirectional, long-range, remote-control applications. The module includes an optimized frequency-hopping spread spectrum (FHSS) RF transceiver and an integrated remote-control transcoder.

The FHSS is capable of reaching more than 2 miles in typical line-of-sight environments with 0-dB gain antennas. An amplified version increases the output power from 12.5 to 23.5 dBm, boosting the range to more than 8 miles in line-of-sight environments with 0-dB antennas.

The TT Series transceiver features best-in-class receive sensitivity (up to −111 dBm) and low power consumption (only 19.2 mA in receive mode and 36 mA in transmit mode at 12.5 dBm). The initial version operates in the 902-to-928-Hz frequency band for North and South America.

The transceiver is housed in a compact reflow-compatible surface-mount technology (SMT) package. It doesn’t require any external RF components except an antenna, which simplifies integration and reduces assembly costs.

Programming is not required for basic operation. The transceiver’s primary settings are hardware-selectable, which eliminates the need for an external microcontroller or other digital interface. Eight status lines can be set up in any combination of inputs and outputs to transfer button or contact states. A selectable acknowledgement indicates that the transmission was successfully received. For advanced features, a UART interface provides optional software configuration.

A simple pairing operation configures two modules to operate together. A single button press on each side causes the modules to automatically swap their 32-bit addresses and store them in nonvolatile memory. It can be configured to automatically send an acknowledgement to the transmitting unit either after receiving a command or with external circuitry when an action has taken place. An optional external processor can send two data bytes with the acknowledgement.

The TT Series transceiver module is available as part of Linx Technologies’s master development system that comes with two development boards for benchmarking and prototyping. Each board is populated with a transceiver, two remote-control development boards, and programming boards. The system also includes antennas, a daughterboard with a USB interface, demonstration software, extra modules, and connectors.

Contact Linx Technologies for pricing.

Linx Technologies
www.linxtechnologies.com

A Wire Is an Inductor (EE Tip #126)

I’m confident you know that you should keep wires and PCB tracks as short as possible. But I’m also sure that you will underestimate this problem fairly frequently.

Remember that 1 cm of a 0.25-mm-wide PCB track is roughly equivalent to an inductance of 10 nH. If this 10 nH is paired with, say, a 10-pF capacitor, that gives a resonant frequency as low as 500 MHz, which is easily below the third or fifth harmonics of the clock frequencies commonly seen on modern high-speed digital boards. Similarly, a 1-cm-long track will jeopardize the performances of any RF system such as a 2.4-GHz transceiver. There is only one solution: keep tracks and wires as short as possible. If you can’t, then use impedance-matched tracks.

Remember this rule especially for the ground connections: any grounded pad of any part working in high frequencies should be directly connected by avia to the underlying ground plane. And this via must be as close as possible to the pad, not some millimeters away.

Just yesterday I did a design review of a customer’s RF PCB. A small 0402 inductance was grounded through a via that was 3 mm away. It was a bad idea because the inductance was as low as 1 nH. Those 3 mm changed its value completely.—Robert Lacoste, “Mixed-Signal Designs,” CC25:25th Anniversary Issue, 2013. 

Wireless Data Links (Part 2): Transmitters and Antennas

If you built your own ham radio “back in the day,” you’ll recall the frustration of putting it together with components that were basic at best.

But as columnist George Novacek points out in the second installment of his series examining wireless data links: “Today you can purchase excellent, reasonably priced low-power gear for data communications off the shelf.”

Transmitter and receiver

Photo 1: SparkFun Electronics’s WRL-10524 transmitter and WRL-10532 receiver are low cost, basic, and work well.

Part 2 of Novacek’s series, appearing in the March issue, looks at transmitters and antennas.

In one section, Novacek expands upon the five basic data-transmitter modules—a data encoder, a modulator, a carrier frequency generator, an RF output amplifier, and an antenna:

Low-power data transmitters often integrate the modulator, the carrier frequency generator, and the amplifier into one circuit. A single transistor can do the job. I’ll discuss antennas later. When a transmitter and a receiver are combined into one unit, it’s called a transceiver.

Modulation may not be needed in some simple applications where the mere presence of a carrier is detected to initiate an action. A simple push button will suffice, but this is rarely used as it is subject to false triggering by other transmitters working in the area in the same frequency band.

Digital encoder and decoder ICs are available for simple devices (e.g., garage door openers) or keyless entry where just an on or off output is required from the receiver. These ICs generate a data packet for transmission. If the received packet matches the data stored in the decoder, an action is initiated. Typical examples include Holtek Semiconductor HT12E encoders and HT12D decoders and Freescale Semiconductor MC145026, MC145027, and MC145028 encoder and decoder pairs. For data communications a similar but more advanced scheme is used. I’ll address this when I discuss receivers (coming up in Part 3 of this series).

Novacek’s column goes on to explain modulation types, including OOK and ASK modulation:

OOK modulation is achieved by feeding the Data In line with a 0-to+V-level  datastream. ASK modulation can be achieved by the data varying the transistor biasing to swing the RF output between 100% and typically 30% to 50% amplitude. I prefer to add a separate modulator.

The advantage of ASK as opposed to OOK modulation is that the carrier is always present, thus the receiver is not required to repeatedly synchronize to it. Different manufacturers’ specifications claim substantially higher achievable data rates with ASK rather than OOK.

For instance, Photo 1 shows a SparkFun Electronics WRL-10534 transmitter and a WRL-10532 receiver set for 433.9 MHz (a 315-MHz set is also available), which costs less than $10. It is a bare-bones design, but it works well. When you build supporting circuits around it you can get excellent results. The set is a good starting point for experimentation.

The article also includes tips on a transceiver you can purchase to save time in developing ancillary circuits (XBee), while noting a variety of transceiver, receiver, and transmitter modules are available from manufacturers such as Maxim Integrated, Micrel, and RF Monolithics (RFM).  In addition, the article discusses design and optimization of the three forms of antennas: a straight conductor (monopole), a coil (helical), and a loop.

“These can be external, internal, or even etched onto the PCB (e.g., keyless entry fobs) to minimize the size,” Novacek says.

Do you need advice on what to consider when choosing an antenna for your design?  Find these tips and more in Novacek’s March issue article.