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.
* 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.
Source: [url=http://www.redcircuits.com/]RED Free Circuit Design[/url]
Copyright Flavio DellepianeParts:1x 470R 1/2W Resistor (R1)
1x 220K 1/4W Resistor (R2)
2x 470R 1/4W Resistors (R3,R7)
1x 1K 1/4W Resistor (R4)
1x 2K2 1/4W Resistor (R5)
1x 330R 1/4W Resistor (R6)
1x 330nF 630V Polyester Capacitor (C1)
1x 10µF 25V Electrolytic Capacitor (C2)
2x 1N4007 1000V 1A Diode (D1, D2)
2x LEDs (Color and shape at will) (D3, D4)
1x BZX79C10 10V 500mW Zener Diode (See Notes) (D4)
1x 1N4148 75V 150mA Diode (D5)
1x BC547 45V 100mA NPN Transistor (Q1)
1x BC557 45V 100mA PNP Transistor (Q2)