Wiring And Schematic

The circuit is comprised of three parts (the differential temperature sensor circuit (ii) a differential amplifier to provide gain (iii) a switching circuit to monitor the output from the differential amplifier.
Two diodes D1 and D2 are used as probes for the sensor. A small preset, RV1 provides fine adjustment of the current through each branch so as to give zero differential output between D1 and D2 when they are at the same temperature. A gain of 500 must be provided at the differential output to provide a useful voltage to switch the LEDs ( lV corresponding to 1¤C.)·RV2 provides fine adjustment of the gain and RV3 adjusts the CMRR. A potential divider network is set up by RV4, R9, R10, RV5 to provide the necessary switching voltages for the voltage comparators, thus enabling LED1 or LED2 or LED3 for voltages set up by RV4 and RV5ie.. -3V and +5v.



SETTING UP .

1. Adjust offset·null on all Op. Amps for zero output by connecting input terminals together and taking to ground and adjusting either RV6, RV7 and RV8. ° 2. Adjust CMRR for differential amplifier by shorting input terminals and connecting to +15V line, then adjusting RV3. 3. Apply probes D1 and D2 to a liquid, say at room temperature, and adjust RV1 until there is zero output across collectors of T1 and T2.  4. Apply probe D1 to a liquid at a temperature 10*70 different from above, then adjust gain control `RV2 until there is IQV se the diff. amplifier output. The CMRR  should again be set; 5. Adjust RV4 and RV5 so that the comparators switch at 3V and +5V corresponding to -30C and +50C.