Temperature-control project for freshman design seminar

Although I was planning to work on my book for the circuits class today, while in the shower I had a couple of thoughts about the freshman design seminar that I’d like to record before I forget them.

I need to develop some specific projects that students can work either as preliminaries to doing the main project or as the main project.  I want to incorporate some Arduino programming and electronics into the projects, but not too much.  Previously, I’d been thinking mainly about using LEDs and phototransistors to do a colorimeter, as it is possible to do the mechanical design fairly simply (using foamcore, for example, to build prototypes).  The biggest problem is that amplification is needed, and I don’t think I want to cover amplifiers in a 2-unit freshman course.

Today’s idea for a project was to do temperature regulation.  It is pretty easy to measure temperature with a thermistor, and I’ve already got a lab project for the circuits class that calibrates a thermistor and records temperature vs. time.  Turning a heater on and off (the simplest form of control) is also pretty easy to program, so the students should be able design a closed-loop system that keeps the temperature of something constant.  The biggest problem is what they should control the temperature of.  I was initially thinking of a small water bath (say a coffee cup with about 200ml of water) and was trying to work out how powerful a heater would be needed.

Since water has a heat capacity of about 4.18 J/(g ºC), heating 200ml by 1ºC requires about 836 J, and raising it 0.2ºC/sec requires 167W.  That’s a lot more power than a microcontroller can handle, and it is more than the bench power supplies in the lab can supply (5A@6V, 1A@25V, and 1A@–25V). The little immersion heaters for heating coffee in a cup run around 500W and could be controlled by a relay board, but I think that they are too dangerous to use in the lab (if they are powered without being immersed, they can fail spectacularly).  A 1-liter electric tea kettle runs about 1kW (about as much as the 10A relays on the cheap relay control boards can handle), but already includes some thermostatic control.  Do I want freshman playing with mains electricity, though?

Air is easier to heat, about 1J/(g ºC), and much lighter than water, about 1–1.3 kg/m³ or 1–1.3 g/l, so heating a liter of air only takes about 1–1.3 J/ºC and a 5W heater should be able to raise air temperature in a closed box by 4 ºC/s. A small cardboard box suitable for building a little temperature-controlled space has an interior volume around 6l and the biggest size cardboard box they’d likely want to work with would be about 44l. So a 10W heater in the little box would raise the temperature about 1 ºC/s and in the big box about 0.2 ºC/s. Those are reasonable rates to be working with.

I can get 10W resistors for under 50¢ each and 30W resistors for about $2.50, so the parts cost is low enough also. We could use relays or nFETs to control the resistors. The AOI518 nFETS that I used in the circuits class this year have only about an 8mΩ on-resistance under the conditions we’d be using them (2A–5A source current and 5V V_GS).

Students could build up the project gradually, starting with a thermistor temperature measurement, adding a heater and on-off control, adding a 12v power supply, adding a circulation fan, maybe adding a servo-controlled vent, adding insulation, maybe adding proportional control instead of on-off control, … .

Construction, using bread boards, an Arduino, and cardboard boxes, is pretty simple and does not require any tools more sophisticated than an exacto knife.

The controlled-temperature box looks cheap enough and easy enough (as long as it only has to heat and not cool) to be a useful project for the freshman design course.