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Alternate on/off circuit

An alternate on/off circuit is basically a bi-stable multivibrator in the sense that it has two stable state (high and low). A slight difference between this circuit and the common bi-stable multivibrator that you might have come across in books is that those one have two triggering point, set and reset. In the case of this circuit, there only one triggering point which is provided with a momentary switch which changes the state (high/low OR on/off) of the output point of the circuit each time the switch is pressed. This circuit allows you to use a momentary switch to turn on and off your electrical or electronic system by momentarily pressing the switch.

alternate on/off circuitclick to view the enlarged circuit


At the instance of power-up, the base of T1 is at ground potential and it is blocked. Base voltage of T2 equals to supply voltage, hence bias it so it conducts. C1 charges up via R3. This circuit will remain in this state if not altered.

If switch SW is momentarily pressed, it causes a change of state in the circuit. Capacitor C1 (which has been charged up to supply voltage) will force T1 to conduct as it releases stored charges to bias it. at this time, T2 with base connected to collector of T1 is at ground potential (note: base and collector of T1 and T2 are cross-connected to each other). The circuit remains in this state till SW is pressed again.


This depends on how you want your system to be (running or stand-by) at the instance of power-up. if desired that the system should start working immediately the power is switched on, the relay driver should be connected to O/P1. If desired on the other hand that the system should be on stand-by when power is switched on until the SW is pressed, connect the relay drive to O/P2.

alternate on/off circuit- stand byclick to view the enlarged circuit

alternate on/off circuit- runclick to view the enlarged circuit

Area of application

Alternate on/off circuit can be used in wide range of application depends on you, how you can apply it. Click DC motor reverse controller circuit to see how it is being used to control dc motor to rotate in both directions. It can also be used in application like remote controlling of electrical or electronic systems if the controlling operation is limited to only switching on and off.

Other alternate on/off circuits

Alternate on/off circuit using 2 logic INVERTERs
Alternate on/off circuit using logic AND & INVERTER
Alternate on/off circuit using 555


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