Temperature Sensor

Schematic:

Construction:
  1. Insert the thermister's leads through the 2 holes in the sides.
  2. Solder leads to two short 3/16 rods either directly or place a wire between the leads and rods.
  3. Insert the rods into the large holes to replace the two pins. Glue into place with super glue or epoxy.

 

Implementing:

Plug this sensor into the RCX and set it on temperature. The temperature will read 10o F too high, but other than that it works ok. Here is an explanation for this problem found on Mindstorm Sensor Input web page at http://www.plazaearth.com/usr/gasperi/lego.htm:

The RCX really wants a thermistor with 12.2kohms at 25C with a -3.7%/C Temp Coef. This isn't equal to anything I can find. I think it is a 10kohm at 25C thermistor with either a trim pot or a fixed 2.2kohm resistor in series. A thermistor with this resistor in series would need a -4.4%/C Temp Coef to fit the RCX and this is an available part. (Mouser 527-0503-10k) The Radio Shack thermistor with a 2.2kohm resistor in series isn't too bad either. The RCX will read 25C at 25C and be off a little everywhere else since the Radio Shack has a lower Temp Coef of -3.7%/C to start with and the 2.2kohm only makes it worse. I've never tried the Mouser part but it looks like a good fit to me.

How it works:

A thermister simply changes resistance with changes in temperature. The RCX is programmed to read this resistance and change the reading into temperature.