AC Mains Bistable Switch

This AC mains-operated bistable  switch turns on or turns off a  device using a miniature neon  lamp and a few discrete components.  This switch can be used for control pan-els, appliances and lighting controls.  A push-to-on switch is used to  light up the neon lamp. The light emit-ted by the neon lamp, in turn, enables  the switching action of the circuit. Use  of a 555 timer wired for bistable operation makes the circuit act as a bistable  switch.

Circuit diagram :

AC Mains Bistable Switch Circuit Diagram

 

The neon lamp (NL1) and the  push-to-on switch (S1) are directly connected to 230V AC mains. The 12V DC  supply for timer 555 (IC1) is derived  from 230V AC mains through capacitive dropper C1, resistor R1 and a 12V  zener diode. IC1 works as a flip-flop  circuit, with the signal at its output  pin 3 toggling every time it receives a  pulse at its pins 2 and 6. 

The operation of the circuit is simple. When you press switch S1 momentarily, the neon lamp glows, making  phototransistor T1 conduct to provide  a pulse at pins 2 and 6 of IC1.  When switch S1 is pressed, the output of IC1  goes high and LED1 glows. Pressing S1  again makes the output of IC1 low and  LED1 stops glowing.

In place of LED1, you can use an  optodiac or suitable relay (not shown  in the circuit) along with a suitable  driver circuit to drive AC loads. Assemble the circuit on a general-purpose PCB with the neon lamp and  the phototransistor housed in a small  black tube isolated from the external  light source, and enclose in a suitable  cabinet. Fix switch S1 on the  front panel of the cabinet,  and mains power cord at  the rear. At the rear, also fix  a 3-pin socket to connect the  AC load. 

Caution.  Take care  when operating this circuit  as it is directly connected to  230V AC mains. Better still,  don’t attempt this circuit  if you have no experience  in handling high-voltage  circuits.

 

 

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