Sunday, May 16, 2010

The Guts of the Thermistor Gauge.

I refer you to the link on my earlier post of the circuit by Roy Schmaus. Its based on an LM324 Op Amp. I'm just trying to get my head around these devices now. They seem to be used for everything under the sun. Here is my circuit nearly finished.

I've included a 7815 voltage regulator on the board too. You can see that on the right. The three components behind the black croc clip. There is a 7 watt resistor on there which is waaaay too big for the thermistor I used but it was 470 ohm, just what I needed, so I went for it!

At this stage I've yet to put two trimmers on. One to be part of the LM324 divider circuit. And the other to drop the output voltage/current to be used in a moving coil metre.

I got this cheapo multimeter from a Chinese Shop in Barcelona. I don't always go so far to buy components! I took the board out, cut the battery holder and screw posts. This gave me a flush back which fit neatly on to a project box.

So I put in a DC socket and an old mobile phone charger cable out for the thermistor connection.
I managed to squash the circuit in.
Here it is, with thermistor soldered on for testing.

The circuit is amazingly sensitive to air movements around the thermistor. A couple of tips if your building this are:
-There will be a delay after you power up before things get to the levels/temperatures/voltages they are supposed to be at. So give your circuit a few seconds before you take a hammer to it.
-Try putting the thermistor in a little cardboard tube to protect from drafts and your breath which may cause weird readings.

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