Using this handy circuit, we are able to transform an ordinary transistor and a multimeter (in the VDC scale) perfectly into a digital temperature meter having a range of -30 ° C to 150 ° C. The sensor is structured on a transistor hooked up in a way as to supply the base-emitter junction voltage, a voltage which is inversely proportionate to the temperature where the transistor is subjected. This voltage is processed by an operational amplifier which amplifies the difference between it and a predetermined reference voltage.
The reference is defined via the variable resistor R2 and its status is tweaked during the time of calibration. The signal supplied by the amplifier is fed directly to the multimeter at its negative input. The positive input is really a voltage that needs to be fine-tuned through R8 during the time of calibration. To adjust, two recognized temperature values are necessary and with R2 and R8 we have to adjust so that the values in our multimeter match.
The potentiometers R2 and R8 must essentially be of the Trimmer model in order to get a decent stability of their value in ohms.
The multimeter needs to be on the 20VDC range. The operational amplifier can be LM1458, LM358 or equivalent, possibly even the LM324 may be applied however only 1 out of the 4 will used, the remaining 3 operational ones will have to beleft unused.