Voltage Controlled Switch by BC547

This switch circuit operates when a voltage level which is previously adjusted is applied to the input. Circuit is designed for 24V supply but can be used in a range of 18V-36V. Voltage level which triggers the circuit can be adjusted by P1 potentiometer. A stabilized voltage is applied to this potentiometer through zener diode and Tr5. When the input voltage level exceeds this value TR1 starts conducting and afterwards Tr2, Tr3, Tr4 transistors conduct and relay switches on.

Switch S1 that is connected to relay, carries the signal from Tr3 to Tr1 through R1 and keeps Tr1 conducting.

When the input is short circuited, D1 diode keeps the circuit operating and you can omit it if you want.

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voltage-controlled-switch-by-bc547.html
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