A precision voltage comparator using zener diode as a reference can be implemented with a general purpose operational amplifier such as the 741. The circuit of figure 1 shows how this can be done.
Of course, other equivalent operational amplifiers can be used. In this circuit the output remains at the high level until the moment the input voltage reaches the reference value given by the zener diode.
The zener diode can be any type of 400 mW and the power supply does not need to be symmetrical. This circuit can be used in security systems, sensor power detection and in many other consumer applications and in industry.
The printed circuit board for the case where a common operational amplifier 741 is used is shown in figure 2.
Material list
CI-1 - 741 - integrated circuit, operational amplifier
Vz - Zener from 1.2 to 12 V x 400 mW
R1, R3 - 10 k ohm x 1/8 W - resistors
R2 - 4.7 k ohm x 1/8 W - resistor
Several:
Printed circuit board, wires, solder, etc.
