DC power supply undervoltage and overvoltage alarm circuit

The electronic alarm's setting and return adjustments are very convenient and accurate, and its operation is stable and
reliable. In the figure, Ai and Az are two sets of amplifiers in the LM324 quad operational amplifier, which are the core components of the comparison and discrimination circuit.
Ai is used for over-voltage, and A2 is used for under-voltage. By adjusting the potentiometers RPi and RP2, you can easily change the lower limit value (undervoltage
) and upper limit value (overvoltage). The transistor VT plays the role of signal amplification (here used as a switching circuit) to drive the relay to work
. Capacitors Cl, Cz, C3, and C4 are used for delay (1~3s) (to prevent malfunction due to voltage
fluctuation) and filtering out high-frequency spurious interference.