Microcontroller-Based Heating System Monitor

Checking a heating system’s consumption is simple enough.

Heating system monitor

Determining a heating system’s output can be much more difficult, unless you have this nifty design. This Atmel ATmega microcontroller-based project enables you to measure heat output as well as control a circulation pump.

Heating bills often present unpleasant surprises. Despite your best efforts to economise on heating, they list tidy sums for electricity or gas consumption. In this article we describe a relatively easy way to check these values and monitor your consumption almost continuously. All you need in order to determine how much heat your system delivers is four temperature sensors, a bit of wiring, and a microcontroller. There’s no need to delve into the electrical or hydraulic components of your system or modify any of them.

A bit of theory
As many readers probably remember from their physics lessons, it’s easy to calculate the amount of heat transferred to a medium such as water. It is given by the product of the temperature change ΔT, the volume V of the medium, and the specific heat capacity CV of the medium. The power P, which is amount of energy transferred per unit time, is:

P= ΔT × CV × V // Δt

With a fluid medium, the term V // Δt can be interpreted as a volumetric flow Vt. This value can be calculated directly from the flow velocity v of the medium and the inner diameter r of the pipe. In a central heating system, the temperature difference ΔT is simply the difference between the supply (S) and return (R) temperatures. This yields the formula:

P = (TS – TR) × CV × v × pr2

The temperatures can easily be measured with suitable sensors. Flow transducers are available for measuring the flow velocity, but installing a flow transducer always requires drilling a hole in a pipe or opening up the piping to insert a fitting.

Measuring principle
Here we used a different method to determine the flow velocity. We make use of the fact that the supply and return temperatures always vary by at least one to two degrees due to the operation of the control system. If pairs of temperature sensors separated by a few metres are mounted on the supply and return lines, the flow velocity can be determined from the time offset of the variations measured by the two sensors…

As the water flows through the pipe with a speed of only a few metres per second, the temperature at sensor position S2 rises somewhat later than the temperature at sensor position S, which is closer to the boiler.

An ATmega microcontroller constantly acquires temperature data from the two sensors. The time delay between the signals from a pair of sensors is determined by a correlation algorithm in the signal processing software, which shifts the signal waveforms from the two supply line sensors relative to each other until they virtually overlap.The temperature signals from the sensors on the return line are correlated in the same manner, and ideally the time offsets obtained for the supply and return lines should be the same.

To increase the sensitivity of the system, the return line sensor signals are applied to the inputs of a differential amplifier, and the resulting difference signal is amplified. This difference signal is also logged as a function of time. The area under the curve of the difference signal is a measure of the time offset of the temperature variations…

Hot water please
If the heating system is also used to supply hot water for domestic use, additional pipes are used for this purpose. For this reason, the PCB designed by the author includes inputs for additional temperature sensors. It also has a switched output for driving a relay that can control a circulation pump.

Under certain conditions, controlling the circulation pump can save you a lot of money and significantly reduce CO2 emissions. This is because some systems have constant hot water circulation so users can draw hot water from the tap immediately. This costs electricity to power the pump, and energy is also lost through the pipe walls. This can be remedied by the author’s circuit, which switches on the circulation pump for only a short time after the hot water tap is opened. This is detected by the temperature difference between the hot water and cold water supply lines…

Circuit description
The easiest way to understand the schematic diagram is to follow the signal path. It starts at the temperature sensors connected to the circuit board, which are NTC silicon devices.

Heating system monitor schematic

Their resistance varies by around 0.7–0.8% per degree K change in temperature. For example, the resistance of a KT110 sensor is approximately 1.7 kΩ at 5 °C and approximately 2.8 kΩ at 70 °C.

The sensor for supply temperature S forms a voltage divider with resistor R37. This is followed by a simple low-pass filter formed by R36 and C20, which filters out induced AC hum. U4a amplifies the sensor signal by a factor of approximately 8. The TL2264 used here is a rail-to-rail opamp, so the output voltage can assume almost any value within the supply voltage range. This increases the absolute measurement accuracy, since the full output signal amplitude is used. U4a naturally needs a reference voltage on its inverting input. This is provided by the combination of R20, R26 and R27. U5b acts as an impedance converter to minimise the load on the voltage divider…

Thermal power

PC connection
The circuit does not have its own display unit, but instead delivers its readings to a PC via an RS485 bus. Its functions can also be controlled from the PC. IC U8 looks after signal level conversion between the TTL transmit and receive lines of the ATmega microcontroller’s integrated UART and the differential RS485 bus. As the bus protocol allows several connected (peer) devices to transmit data on the bus, transmit mode must be selected actively via pin 3. Jumper JP3 must be fitted if the circuit is connected to the end of the RS485 bus. This causes the bus to be terminated in 120 Ω, which matches the characteristic impedance of a twisted-pair line…

[Via Elektor-Projects.com]

Infrared Communications for Atmel Microcontrollers

Are you planning an IR communications project? Do you need to choose a microcontroller? Check out the information Cornell University Senior Lecturer Bruce Land sent us about inexpensive IR communication with Atmel ATmega microcontrollers. It’s another example of the sort of indispensable information covered in Cornell’s excellent ECE4760 course.

Land informed us:

I designed a basic packet communication scheme using cheap remote control IR receivers and LED transmitters. The scheme supports 4800 baud transmission,
with transmitter ID and checksum. Throughput is about twenty 20-character packets/sec. The range is at least 3 meters with 99.9% packet receive and moderate (<30 mA) IR LED drive current.

On the ECE4760 project page, Land writes:

I improved Remin’s protocol by setting up the link software so that timing constraints on the IR receiver AGC were guaranteed to be met. It turns out that there are several types of IR reciever, some of which are better at short data bursts, while others are better for sustained data. I chose a Vishay TSOP34156 for its good sustained data characteristics, minimal burst timing requirements, and reasonable data rate. The system I build works solidly at 4800 baud over IR with 5 characters of overhead/packet (start token, transmitter number, 2 char checksum , end token). It works with increasing packet loss up to 9000 baud.

Here is the receiver circuit.

The receiver circuit (Source: B. Land, Cornell University ECE4760 Infrared Communications
for Atmel Mega644/1284 Microcontrollers)

Land explains:

The RC circuit acts a low-pass filter on the power to surpress spike noise and improve receiver performance. The RC circuit should be close to the receiver. The range with a 100 ohm resistor is at least 3 meters with the transmitter roughly pointing at the receiver, and a packet loss of less then 0.1 percent. To manage burst length limitations there is a short pause between characters, and only 7-bit characters are sent, with two stop bits. The 7-bit limit means that you can send all of the printing characters on the US keyboard, but no extended ASCII. All data is therefore sent as printable strings, NOT as raw hexidecimal.

Land’s writeup also includes a list of programs and packet format information.

Electrostatic Cleaning Robot Project

How do you clean a clean-energy generating system? With a microcontroller (and a few other parts, of course). An excellent example is US designer Scott Potter’s award-winning, Renesas RL78 microcontroller-based Electrostatic Cleaning Robot system that cleans heliostats (i.e., solar-tracking mirrors) used in solar energy-harvesting systems. Renesas and Circuit Cellar magazine announced this week at DevCon 2012 in Garden Grove, CA, that Potter’s design won First Prize in the RL78 Green Energy Challenge.

This image depicts two Electrostatic Cleaning Robots set up on two heliostats. (Source: S. Potter)

The nearby image depicts two Electrostatic Cleaning Robots set up vertically in order to clean the two heliostats in a horizontal left-to-right (and vice versa) fashion.

The Electrostatic Cleaning Robot in place to clean

Potter’s design can quickly clean heliostats in Concentrating Solar Power (CSP) plants. The heliostats must be clean in order to maximize steam production, which generates power.

The robot cleaner prototype

Built around an RL78 microcontroller, the Electrostatic Cleaning Robot provides a reliable cleaning solution that’s powered entirely by photovoltaic cells. The robot traverses the surface of the mirror and uses a high-voltage AC electric field to sweep away dust and debris.

Parts and circuitry inside the robot cleaner

Object oriented C++ software, developed with the IAR Embedded Workbench and the RL78 Demonstration Kit, controls the device.

IAR Embedded Workbench IDE

The RL78 microcontroller uses the following for system control:

• 20 Digital I/Os used as system control lines

• 1 ADC monitors solar cell voltage

• 1 Interval timer provides controller time tick

• Timer array unit: 4 timers capture the width of sensor pulses

• Watchdog timer for system reliability

• Low voltage detection for reliable operation in intermittent solar conditions

• RTC used in diagnostic logs

• 1 UART used for diagnostics

• Flash memory for storing diagnostic logs

The complete project (description, schematics, diagrams, and code) is now available on the Challenge website.

 

The Future of 8-Bit Chips (CC 25th Anniversary Preview)

Ever since the time when a Sony Walkman retailed for around $200, engineers of all backgrounds and skill levels have been prognosticating the imminent death of 8-bit chips. No matter your age, you’ve likely heard the “8-bit is dead” argument more than once. And you’ll likely hear it a few more times over the next several years.

Long-time Circuit Cellar contributor Tom Cantrell has been following the 8-bit saga for the last 25 years. In Circuit Cellar‘s 25th Anniversary issue, he offers his thoughts on 8-bit chips and their future. Here’s a sneak peek. Cantrell writes:

“8-bit is dead.”  Or so I was told by a colleague. In 1979. Ever since then, reports of the demise of 8-bit chips have been greatly, and repeatedly, exaggerated. And ever since then, I’ve been pointing out the folly of premature eulogizing.

I’ll concede the prediction is truer today than in 1979—mainly, because it wasn’t true at all then. Now, some 30-plus years later, let’s reconsider the prospects for our “wee” friends…

Let’s start the analysis by putting on our Biz101 hats. If you Google “Product Life Cycle” and click on “Images,” you’ll see a variety of somewhat similar graphs showing how products pass through stages of growth, maturity, and decline. Though all the graphs tell a rise-and-fall story, it’s interesting to note the variations. Some show a symmetrical life cycle that looks rather like a normal distribution. But the majority of the graphs show a “long-tail” variation in which the maturity phase lasts somewhat longer and the decline is relatively gradual.

Another noteworthy difference is how some graphs define life and death in terms of “sales” and others “profits.” It stands to reason that no business will continue to sell at a loss indefinitely, but the market knows how to fix that. Even if some suppliers wave the white flag, those that remain can raise prices and maintain profitability as long as there is still demand.

One of the more interesting life cycle variations shows that innovation, like a fountain of youth, can stave off death indefinitely. An example that comes to mind is the recent introduction of ferroelectric RAM (FRAM) MCUs. FRAM has real potential to reduce power consumption and also streamlines the supply chain because a single block of FRAM can be arbitrarily partitioned to emulate any mix of read-mostly or random access memory (see Photo 1). They may be “mature” products, but today the Texas Instruments MSP430 and Ramtron 8051 are leading the way with FRAM.

Photo 1: Ongoing innovation, such as the FRAM-based “Wolverine” MCU from Texas Instruments, continues to expand the market for mini-me MCUs. (Source: Cantrell CC25)

And “innovation” isn’t limited to just the chips themselves. For instance, consider the growing popularity of the Arduino SBC. There’s certainly nothing new about the middle-of-the-road, 8-bit Atmel AVR chip it uses. Rather, the innovations are with the “tools” (simplified IDE), “open-source community,” and “sales channel” (e.g., RadioShack). You can teach an old chip new tricks!

Check out the upcoming anniversary issue for the rest of Cantrell’s essay. Be sure to let us know what you think about the future of the 8-bit chip.

Do Small-RAM Devices Have a Future? (CC 25th Anniversary Preview)

What does the future hold for small-RAM microcontrollers? Will there be any reason to put up with the constraints of parts that have little RAM, no floating point, and 8-bit registers? The answer matters to engineers who have spent years programming small-RAM MCUs. It also matters to designers who are hoping to keep their skills relevant as their careers progress in the 21st century.

In the upcoming Circuit Cellar 25th Anniversary Issue—which is slated for publication in early 2013—University of Utah professor John Regehr shares his thoughts on the future of small-RAM devices. He writes:

For the last several decades, the role of small-RAM microcontrollers has been clear: they are used to perform fixed (though sometimes very sophisticated) functionality in environments where cost, power consumption, and size need to be minimized. They exploit the low marginal cost of additional transistors to integrate volatile RAM, nonvolatile RAM, and numerous peripherals into the same package as the processor core, providing a huge amount of functionality in a small, cheap package. Something that is less clear is the future of small-RAM microcontrollers. The same fabrication economics that make it possible to put numerous peripherals on a single die also permit RAM to be added at little cost. This was brought home to me recently when I started using Raspberry Pi boards in my embedded software class at the University of Utah. These cost $25 to $35 and run a full-sized Linux distribution including GCC, X Windows, Python, and everything else—all on a system-on-chip with 256 MB of RAM that probably costs a few dollars in quantity.

We might ask: Given that it is already the case that a Raspberry Pi costs about the same as an Arduino board, in the future will there be any reason to put up with the constraints of an architecture like Atmel’s AVR, where we have little RAM, no floating point, and 8-bit registers? The answer matters to those of us who enjoy programming small-RAM MCUs and who have spent years fine-tuning our skills to do so. It also matters to those of us who hope to keep our skills relevant through the middle of the 21st century. Can we keep writing C code, or do we need to start learning Java, Python, and Haskell? Can we keep writing stand-alone “while (true)” loops, or will every little MCU support a pile of virtual machines, each with its own OS?

Long & Short Term

In the short term, it is clear that inertia will keep the small-RAM parts around, though increasingly they will be of the more compiler-friendly varieties, such as AVR and MSP430, as opposed to earlier instruction sets like Z80, HC11, and their descendants. But will small-RAM microcontrollers exist in the longer term (e.g., 25 or 50 years)? I’ll attempt to tackle this question by separately discussing the two things that make small-RAM parts attractive today: their low cost and their simplicity.

If we assume a cost model where packaging and soldering costs are fixed but the marginal cost of a transistor (not only in terms of fabrication, but also in terms of power consumption) continues to drop, then small-RAM parts will eventually disappear. In this case, several decades from now even the lowliest eight-pin package, costing a few pennies, will contain a massive amount of RAM and will be capable of running a code base containing billions of lines…

Circuit Cellar’s Circuit Cellar 25th Anniversary Issue will be available in early 2013. Stay tuned for more updates on the issue’s content.