In Fig.1 A precision DC under?voltage relay switch. The opamp is wind as a voltage comparator, with a reference voltage applied to pin 2 and the test voltage applied to pin 3: the relay turns on when the pin 3 voltage exceeds that of pin 2. The circuit can be made to trip at any voltage in excess of 5 volts by suitable choice of R1 value. In Fig. 2 An over-voltage switch that can be used to trip at any pre-set voltage in excess of about 10 mV. The input voltage can be connected directly to pin 2 if trip values in the range 10mV to 3Volts are required. For voltages in excess of 3 Volts, a suitable range resistor must be connected in the position shown, to keep the pin 2 Voltage drive to suitable level. The circuit can be converted to an under?voltage switch by transposing the pin 2 and pin 3 connections of the op-amp.
The circuit can be used as an AC voltage switch by first rectifying the AC input signal.
R1=2Kohm/V minus 10Kohm R6-11=2.2Kohm D3-5=1N4001
R2-9-10=1Kohm R7-12=4.7Kohm D4=3V3 0.5W Zener
R3-8=10Kohm RV1-2=10Kohm pot. IC1-2=LM741
R4=1.2Kohm D1=6V8 0.5W Zener Q1-2=BC214
R5=47Kohm D2=5V6 0.5W Zener RL1-2=12V >120ohm Relay