You can make things simpler if you forget the SCR and use a relay. The power supply will drive the relay when the power is on. When the power goes off the relay will release and connect the battery to the circuit you want. Is it possible or you have to do it with an SCR?I know that a SCR is triggered by a positive pulse and stays ON but for this assignment, it has to be triggered when the power supply is OFF. Thats where I am stuck.
Ok see the attachment then. But note that when the power comes back again on the power supply the SCR wont shut off. if you want you can replace the SCR by an NPN transistor as when the power comes back to release the relay. Or if you still want the SCR use a transistor in parallel with the SCR to reset it when power comes back.I have thought about the relay but a scr is what is needed to be used.
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