Back in 2008 I wrote about a little circuit to turn the OLPC XO-1 Measure application into a digital thermometer. That circuit required two 9 volt batteries, 11 components and a PCB (plus connectors)
A few weeks ago I got asked about making a commercial version of the probe which naturally led to thinking about how to simplify the circuit. I've now got the entire circuit down to a single component that costs 50p in bulk. I've eliminated all the rest (except the connectors) and the circuit is entirely powered by the OLPC itself.
I actually tried a total of four designs for this circuit.
Design 1: The Original The original circuit looked like this:
It works, but it's a bit awkward since it requires those external batteries.
Design 2: Dump the op-amp One simple thing to do is just make a parallel adder with a few resistors and a reference voltage (the original 0.45v from Design 1) from a voltage divider and not worry about all the stability that the op-amp brings.
Here's that circuit under test. It works, but it can be made even simpler.
Design 3: Diode bias After mentioning this project on Hacker News jacquesm suggested trying an even simpler circuit: the original LM35 temperature sensor with a diode inserted between its ground pin and ground to bias ground to the forward voltage drop of the diode (typically 0.6v) which would bring the output voltage for household temperatures into the range of the OLPC.
That worked nicely (I had a 1N4007 lying around which has a forward voltage of up to 1.1v) and here's the circuit.
Design 4: Switch the sensor And then I discovered that there's a pin compatible replacement for the LM35 called the TMP36 which output 0.75v at 25C with 0.01v per C. That means that at 0C it outputs 0.5v and at 100C it output 1.75v. That puts its output inside the range the OLPC can sense. And it can run on the 5V from the USB port. And it has low output impedance.
So the final circuit has a single component. Here it is under test:
And here it is soldered to a connector ready for connection to the OLPC via my original USB/Mic In connector cable.
So if you want to make a really simple temperature probe for the OLPC XO-1 then get a TMP36.
Now all I need to do is find a supplier of stainless steel probes to put the TMP36 in and I can start making them.
PS Ever since I had eye surgery and stopped wearing glasses I've been worried about splashing solder in my eyes. So I wear a pair of Panther Vision LED Lighted Safety Glasses which protect the eyes and let you see what you are doing at the same time.