Secure Power Switch Circuit

Most of the diagnostic, treatment, and life support devices used in healthcare require regular power supply in order to function properly. Accidentally turning off a medical device can have serious consequences. How can you prevent an accidental or unexpected equipment shutdown?

Here is a little circuit to prevent such costly disasters. It is nothing but a digital toggle switch with an additional switching protection logic. Here two momentary push button switches (S1 &S2) are used to control the switching of the output load, ie one switch is for normal power swtching & other switch is for enabling the first switch!

Schematic of the Power Switch Circuit

secure power switch circuit

Working of this 12VDC operated circuit is very simple, and self-explanatory. Initially, when 12VDC power supply is applied to the circuit, the electro-magnetic relay (RL1) is in ‘off’ state. When on/off switch S1 is pushed, the first Flip-Flop (arranged here as R-S latch) wired around IC1A (½ of CD4013B) is set and its output (at pin 13) goes high. This clocks IC1B (½ of CD4013B) on pin 3 and the bistable toggles to switch on the relay through driver transistor T1 (BC547). Next, the load connected through the relay contacts is switched to ‘on’ state. The Red LED (LED1) indicates this condition.

However, switching ‘off’ the load is not as easy as the ‘on’ process. In order to switch off the load, you must first push the ‘Enable’ switch (S2) and thereafter the ‘on/off’ switch (S1). This prevents accidental shutdown of the connected equipment. When S2 is pushed, IC1A is resetted and nothing happens in the relay driver section. It is only when the S1 is pushed again, IC1A changes state (pin 13 again goes to high level) which in turn toggle IC1B again, to de-activate the relay.

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