Sensing element Ql is a 2N3904 general-purpose npn transistor, although any general-purpose npn unit in a T0-92 style case will do. IC1, an LM334, supplies Ql with a constant current that is independent of temperature. An LM324 quad op amp, IC2, forms a high input-impedance differential amplifier (IC2a, IC2b, and IC2c) with a gain of about 99. IC2d is used as a voltage comparator. When Ql senses a rise or fall in temperature, the base-to-emitter voltage decreases.
Electronic-heat-sniffer - schematic

That decrease in voltage causes the input to IC2a at pin 3 to deviate from the reference voltage that"s fed to IC2b at pin 5, which is set by potentiometers R5. The difference between the input and the reference is amplified by IC2c. That amplified voltage is fed to IC2d where it is compared to a control voltage set by potentiometer R13. The setting of Rl3 determines the threshold and is set at a point that"s equal to the ambient temperature. The output of IC2d at pin 14 is fed to the base of transistor Q2. When the output of IC2d is high, LEDllights and Q2 turns on. With Q2 turned on, a ground path through the transistor is provided for buzzer PBl. The circuit can be built on perforated construction board using point-to-point wiring. All components, except Ql, are mounted on the board. Transistor Ql is mounted at the tip of the heat-sensing probe.

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