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 panels,
appliances and lighting controls.
A push-to-on switch is used to
light up the neon lamp. The light emitted
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.
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
opto-diac or suitable relay (not shown
in the circuit) along with a suitable
driver circuit to drive AC loads.
Assemble the circuit on a generalpurpose
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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