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Color-Changing Hot & Cold LED Glowies

Last fall, I was on a walk in a ravine and felt a strong breeze blow through. I watched it move along the branches and grass and thought, "wouldn't it be interesting if I could see it blow through the air, too?"

If I could spread some simple temperature sensors around the ravine and have them show the relative temperature with colors, I sould be able to "see" the chilly wind blow through the air by watching the colors change.

These glowies are the result of me finding a way to make simple temperature sensors. Whether they are reactive enough for the purpose I originally imagined will have to wait until next fall, but they are fun to have around.

As a bonus, these are solar powered!

What They Are and How They Work

The Glowies use a small microcontroller, but they are really quite simple in both parts and function. The core of the unit is a silicon diode used as a temperature sensor (actually, two of them). This is very simple, and is very well-understood application - and you can't get much cheaper!

The electrical characteristics of a silicon diode changes depending on temperature. Specifically, the forward voltage will change depending on the temperature. It's not much, but it's enough to measure.

The microcontroller used (PIC12F675) has built-in Analog-to-Digital convertors. These are used to measure the forward voltage of two identical diodes. One of the diodes sticks out a little from the Glowie and serves as the main temperature sensor. The other diode is lightly insulated from the ambient temperature. This makes it slower to "catch up" to temperature changes. Temperature changes are detected by comparing the main sensor to the insulated one.

As a result, the Glowie doesn't actually know what temperature it is - at least, not in degrees. But it is quite good at detecting temperature changes. A temperature change upwards lights the red LED. A change downwards lights the blue LED. Since the Glowie measures and reacts many times per second, borderline changes in temperature result in a flickering of the LEDs. Strong changes have the LED lit solid.

In the animation, you can see the Glowie responding to me blowing compressed air on the sensor to cool it, followed by pressing my finger to the sensor to warm it.

The Glowie goes to sleep if there are no changes in the temperature detected. This means that the power consumption is quite low unless an LED is lit, in which case it consumes about 25mA.

As a result, it's feasible to have the small rechargeable cells of the Glowie recharged by an attached solar cell. This is enough to keep the Glowie running, and feed the occasional LED lighting frenzy. If the Glowie spends a lot of time lighting its LEDs, however, it will run out of juice and have to recharge before it can function again.

Technical Details

Schematic (click for larger version)

Source Code for 12F675 Microcontroller:

Optional Improvements

  • I have found it useful to put a small switch between the Glowie and the battery. This can come in useful either to reset the unit, or to prevent it from running (if you want the battery to be allowed to charge without the Glowie doing anything, for example). This can come in handy especially if the Glowie starts acting strange - which is likely to happen if it ever runs the power source dry. Microcontrollers tend to get their brains scrambled when the power goes flakey.

  • A little personality added to your glowie can go a long way. A ping-pong ball over the LEDs and some "mad scientist hair" seems to work for some of them.

Here's the gang of prototypes:

And here's the gang getting blown with a heat gun and reacting to the heat (lighting up red, then going out as the gun moves away. They'd do the same with blue if it were a cold-gun):

Speaking of cute little electronic things, don't forget to take a look at these Spider-Robot Sculptures!

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