This circuit was designed on request and can be useful to those whishing to have, say, a red LED illuminated when an appliance is on and a green LED illuminated when the same appliance is off.
Any mains operated appliance can be monitored by this circuit, provided a suitable mains switch, capable of withstanding the full load current, is used for SW1. When SW1 is closed, the load and D4 are energized, Q1 is saturated and shorts D3, thus preventing its illumination.
Change R1 and R2 to 15K 1W for 115Vac mains operation.
SW1 must be capable of withstanding the appliance’s full load current and voltage.
R1,R2__________27K 1W Resistors
R3______________6K8 1/4W Resistor
D1,D2_______1N4007 1000V 1A Diodes
Q1_____________BC337 45V 800mA NPN Transistor
SW1____________SPST Mains switch
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