How to Make an Electronic Toggle Switch Circuit

A toggle switch refers to a device which is used for switching an electrical circuit ON and OFF alternately whenever required. Normally mechanical switches are used for such operations and are extensively employed wherever electrical switching is required. However mechanical switches have one big drawback, they are prone to wear and tear and have the tendency of producing sparking and RF noise.

A simple circuit explained here provides an electronic alternative to the above operations. Using a single op amp and a few other cheap passive parts, a very interesting electronic toggle switch can be built and used for the said purpose.






Though the circuit also employs a mechanical input device but this mechanical switch is a tiny micro switch which just requires alternate pushing for implementing the proposed toggling actions. 

A micro switch is a versatile device and very much resistant to mechanical stress and therefore does not affect the efficiency of the circuit.

The figure shows a straightforward electronic toggle switch circuit design, incorporating a 741 opamp as the main part.

The IC is configured as a high gain amplifier and therefore its output has the tendency of easily getting triggered to either logic 1 or logic 0, alternately.

A tiny portion of the output potential is applied back to the non inverting input of op amp
When the push button is operated, C1 connects with the inverting input of the opamp. 

Assuming the output was at logic 0, the opamp immediately changes state.

C1 now begins charging through R1.

However keeping the switch pressed for a longer period of time will only charge C1 fractionally and only when it’s released does C1 begins charging up and continues to charge up to the supply voltage level.

Because the switch is open, now C1 gets disconnected and this helps it to “retain” the output information.

Now if the switch is pressed once again, the high output across the fully charged C1 becomes available at the inverting input of the opamp, the opamp yet again changes state and creates a logic 0 at the output so that C1 starts discharging bringing the position of the circuit to the original condition.

The circuit is restored and is ready for the next repetition of the above cycle.

The output is a standard triac trigger set up used for responding to the outputs of the opamp for the relevant switching actions of the connected load.


Parts List

R1, R8 = 1M,
R2, R3, R5, R6 = 10K,
R4 = 220K,
R7 = 1K
C1 = 0.1uF,
C2,C3 = 474/400V,
S1 = micro-switch Push Button,
IC1 = 741
Triac  BT136

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10 comments

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January 28, 2012 at 11:11 PM delete

what is the value of zener Z1 used in the circuit.

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January 29, 2012 at 9:19 AM delete

12V............ the circuit is directly linked with AC mains, so be extremely careful....

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August 6, 2013 at 9:07 PM delete

Hi Swagatam

Can this circuit be used with some modification for switching on say 12V devices? Or a latch circuit will be a better option. If yes, what mods?

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August 7, 2013 at 9:17 AM delete

Hi Abu-Hafss,

Yes this circuit can be effectivey used as a flip flop for operating any load may it be AC or DC at its output.

For DC loads simply connect a mosfet in place of the triac and do the connections appropriately as necessary for a mosfet.

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November 4, 2013 at 9:14 PM delete

Hi,
how to connect led indicator to this circuit .so when switch is ON led is ON & when off led is off. what will be the limiting resistor for led?

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November 5, 2013 at 12:47 PM delete

positive---------^^^------->I-------pin6-----------^^^--------->I-----------ground

limiting resistor 10k

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July 15, 2015 at 11:18 PM delete

Hi Swagatam,
I need to know what is the power of the resistors (watts), the power of the zener diode (watts) and the capacitors in the circuit: Which type? Polyester? All?

Best Regards.
NC

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July 16, 2015 at 11:55 AM delete

Hi Nelio, the resistors are all 1/4 watt rated, the zener diode is 1 watt

C1 is ceramic disc, C3 can be an electrolytic type 220uF/25V....and C2 must be a PPC or metalized polyester

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July 17, 2015 at 8:37 PM delete

Hi Swagatam,
I noticed that there is no rectification (AC to DC conversion) in this circuit. Shouldn't there be a diode after C2, to convert AC to DC for the 741 IC?

Best Regards,
NC.

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July 18, 2015 at 9:54 AM delete

Hi Nelio,

A diode may not be required because here the zener diode is doing both the job, it's restricting the voltage to 12V and also in the course rectifying the AC by allowing only the positive half cycles to pass on to the circuit....C3 is filtering the ripples, you can increase the value if C3 to any higher level for improving the DC quality.

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