A few simple components are all it takes to create an over-temperature alarm. The circuit in Figure 1 requires only an ordinary negative-temperature-coefficient (NTC) thermistor, two common ICs, and a handful of discrete components. The circuit uses a unipolar 12-V supply and consumes only a few milliamps in the idle state. This circuit is particularly suitable for monitoring the temperature of heatsink plates.
The UA741 op amp (IC1) acts as a comparator. At normal temperatures, the NTC thermistor’s resistance is comparatively very high—in the kilohm range—and the voltage at IC1’s non-inverting input (pin 3) is higher than that at its inverting input (pin 2). In this case, the op amp’s output (pin 1) goes high and switches transistor T1 on. As a result, the low-frequency astable multivibrator, wired using the NE555 timer (IC2), is disabled (Fig. 2).
If the temperature increases above a preset level (determined by potentiometer VR1), IC1’s output goes low and T1 is switched off. Consequently, IC2’s reset terminal (pin 4) receives a positive level through R5. This enables the audio oscillator, and its output drives the loudspeaker. The oscillator’s output frequency can be varied by changing the value of RC timing components R7 or C2.
As far as the temperature sensor is concerned, the choice isn’t critical. So, you can select from a wide range of sensors. With some modifications, you can even use a diode. The p-n junction of a diode has a temperature coefficient of -2 mV/°C, and that characteristic can be employed for temperature measurement. You can even try an old-fashioned germanium transistor like an AC128.