Issue 258: EQ Answers

These are the answers to the EQ questions that appeared in Circuit Cellar 258(January 2012).

Answer 1—Q1 and Q2 are wired as a differential amplifier, but in this digital application, it’s probably better to think of them as a current switch. Whenever VIN < VTH, the current through R1 is shunted to ground, and whenever VIN > VTH, the current passes instead through Q2 to the base of Q3, turning it on. As long as Q1 and Q2 are reasonably well matched, VTH is the input voltage at which the switchover occurs, and this is relatively stable with respect to temperature. For example, if VIN is being driven by standard TTL logic, you might set VTH to 1.5 V.

Answer 2—R1, combined with VTH, sets the amount of current flowing through Q2 to drive the base of Q3. This current should be sufficient to drive Q3 into saturation, given the expected load on it (R3 plus the external circuit). R2 serves to make sure that Q3 isn’t turned on by any leakage current through Q2 when it’s supposed to be off. For example, a value of 100 kΩ would bypass currents of up to 6 µA or so.

Answer 3—When VIN is high, there is virtually no current flowing through the input terminal—just the leakage current through Q1’s base-collector junction. When VIN is low, the driving circuit must sink the current set by R1 divided by the beta (current gain) of Q1. For example, if R1 is 4,300 Ω, giving a current through Q1 of about 1 mA, and the beta of Q1 is 50, then the driving circuit must sink about 20 µA.

Answer 4—Better: The component count is reduced by one transistor. Better: VTH is stabilized by the forward drop of a diode, making it less dependent on the exact value of the positive supply. Worse: The switching voltage is now determined by the combination of D1 and the base-emitter drop of Q2, both of which vary with temperature. Worse: Now the driving circuit must supply all of the base current for Q3 when it is in the high state.

Contributed by David Tweed (eq at

Solid-State Lighting Solutions Project

Electronics system control, “green design,” and energy efficiency are important topics in industry and academia. Here we look at a project from San Jose-based Echelon Corp.’s 2007 “Control Without Limits” design competition. Designers were challenged to implement Pyxos technology in innovative systems that reduced energy consumption. Daryl Soderman and Dale Stepps (of INTELTECH Corp.) took First Prize for their Solid State Lighting Solutions project.

The Pyxos chip is on the board (Source: Echelon & Inteltech)

So, how does it work? Using the Pyxos FT network protocol, this alternative lighting project is a cost-effective, energy-efficient solution that’s well-suited for use in residential, commercial, or public buildings. You can easily embed the LED lighting and control system—which features SSL lighting, a user interface, motion detectors, and light sensors—in an existing network. In addition, you can control up to five zones in a building by using the system’s fully programmable ESB-proof touchpad.

Another view of the Pyxos chip is on the board (Source: Echelon & Inteltech)


For more information about Pyxos technology, visit

This winning project, as well as others, was promoted by Circuit Cellar based on a 2007 agreement with Echelon.




RFI Bypasssing

With GPS technology and audio radio interfaces on his personal fleet of bikes, Circuit Cellar columnist Ed Nisley’s family can communicate to each other while sending GPS location data via an automatic packet reporting system (APRS) network. In his February 2012 article, Ed describes a project for which he used a KG-UV3D radio interface rigged with SMD capacitors to suppress RF energy. He covers topics such as test-fixture measurements on isolated capacitors and bypassing beyond VHF.

Photo 2 from the Febuary article, "RFI Bypassing (Part 1)." A pair of axial-lead resistors isolate the tracking generator and spectrum analyzer from the components under test. The 47-Ω SMD resistor, standing upright just to the right of the resistor lead junction, forms an almost perfect terminator. (Source: Ed Nisley CC259)

Ed writes:

Repeatable and dependable measurements require a solid test fixture. Although the collection of parts in Photo 2 may look like a kludge, it’s an exemplar of the “ugly construction” technique that’s actually a good way to build RF circuits. “Some Thoughts on Breadboarding,” by Wes Hayword, W7ZOI, gives details and suggestions for constructing RF projects above a solid printed circuit board (PCB) ground plane.

You can read this article now in Circuit Cellar 259. If you aren’t a subscriber, you can purchase a copy of the issue here.


GPS-Based Vehicle Timing & Tracking Project

The KartTracker’s Renesas kit (Source: Steve Lubbers CC259)

You can design and construct your own vehicle timing system at your workbench. Steve Lubbers did just that, and he describes his project in Circuit Cellar 259 (February 2012). He calls his design the “Kart Tracker,” which he built around a Renesas Electronics Corp. RX62N RDK. In fact, Steve writes that the kit has most of what’s need to bring such a design to fruition:

Most of the pieces of my KartTracker are already built into the Renesas Electronics RX62N development board (see Figure 1). The liquid crystal display (LCD) on the development board operates as the user interface and shows the driver what is happening as he races. The integrated accelerometer can be used to record the G forces experienced while racing. A serial port provides connections to a GPS receiver and a wireless transmitter. Removable flash memory stores all the race data so you can brag to your friends. You now have all of the pieces of my KartTracker.

The following block diagram depicts the relationship between the CPU, base station, flash drive, and other key components:

KartTracker Diagram (Source: Steve Lubbers CC259)

The software for the system is fairly straightforward. Steve writes:

The KartTracker software was built around the UART software sample provided with the RX62N development kit. To provide file system support, the Renesas microSD/Tiny FAT software was added. Finally, my custom GPS KartTracker software was added to the Renesas samples. My software consists of GPS, navigation, waypoints, and display modules. Support software was added to interface to the UART serial port, the file system, and the user display and control on the RX62N circuit board.

Pseudocode for the main processing loop (Source: Steve Lubbers CC259)

Read Steve’s article in the February issue for more details.

If you want to build a similar system, you should get familiar with the Renesas RX62N RDK. In the following video, Dave Jones of EEVBlog provides a quick and useful introduction to the RX62N RDK and its specs (Source: Renesas).

Good luck with this project. Be sure to keep Circuit Cellar posted on your progress!

Click here to purchase Circuit Cellar 259.