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555 timer temperature monitor circuit

With this electronic circuit can monitor temperature in up to four points. Using this temperature monitor circuit can choose if the alarm should be triggered when temperature is increased or decreased. This depends on the resistance R10. If you connect R10 between positive supply conductor and output 6 of IC2 then circuit sound the alarm when the temperature rise over the reference temperature. The reference temperature is set by potentiometer P1. Measured temperature is monitored by NTC resistance R9. Its value decreases when temperature increases, as a result, the voltage at the inverting input is greater than the noninverting input, IC2's output so provide a void potential. Temperature relay now absorbs a current of about 20 mA. It is so great that the resistance R1 appear a voltage of 0.65 V enough to pass transistor T1 in conduction state and trigger the alarm sound.
If you want to lower temperature alarm trigger, then resistance R10 should be connected between the output 6 (IC2) and ground. If the monitored temperature falls below the adjusted to the reference value, then the NTC resistance R9 increases. Inverting input of IC2 becomes more negative, so positive potential output increases. Now take the relay of the temperature again 20 mA current T1 alarm triggers. Timer circuit 555 is connected as astabil multivibrator which produces a frequency of about 4 kHz.

Circuit Diagram: 
555 timer temperature monitor circuit

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