This circuit, designed on request, has proven to be useful to indicate when the voltage in a power supply line is changing from 120V to 240Vac. It can be used in different circumstances and circuits, mainly when an increase in ac or dc supply voltage needs to be detected. D3 illuminates when the line voltage is approaching 120V and will remain in the on state also at 240V supply. On the other hand, D6 will illuminate only when the line voltage is about 240V and will stay on because the latching action of Q1, Q2 and related components. C1, D1 and D2 provide a low dc voltage in the 4.5V - 6V range in order to allow proper operation of latch circuit and LEDs.
Circuit diagram:
Parts:
R1_____________470R 1/2W Resistor
R2_____________220K 1/4W Resistor
R3,R7__________470R 1/4W Resistors
R4_______________1K 1/4W Resistor
R5_______________2K2 1/4W Resistor
R6_____________330R 1/4W Resistor
C1_____________330nF 630V Polyester Capacitor
C2______________10µF 25V Electrolytic Capacitor
D1,D2________1N4007 1000V 1A Diode
D3,D6___________LEDs (Color and shape at will)
D4_________BZX79C10 10V 500mW Zener Diode (See Notes)
D5___________1N4148 75V 150mA Diode
Q1____________BC547 45V 100mA NPN Transistor
Q2____________BC557 45V 100mA PNP Transistor
Notes:
- D4 value could require some adjustment in order to allow precise switching of the circuit at the chosen voltage. If the case, please try values in the 8.2V - 15V range.
- Warning! The circuit is connected to 240Vac mains, then some parts in the circuit board are subjected to lethal potential! Avoid touching the circuit when plugged and enclose it in a plastic box.