i already solved it using a 555 timer powered by a battery and a relay to the output.but the project coordinator has now specified that a battery cannot be used for powering the delay circuit.
How about to use the CMOS 555 Timer (LMC555) instate and use a big capacitor instate a battery ?
basically a 555 monostable circuit respond to a falling edge as trigger , the rising edge has no effect on trigging
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if you could manage to use the mains going off to generate a falling edge , this may work
555 inverting buffer circuit
(a NOT gate)
NOT gate symbol
The buffer circuit's input has a very high impedance (about 1M) so it requires only a few µA, but the output can sink or source up to 200mA. This enables a high impedance signal source (such as an LDR) to switch a low impedance output transducer (such as a lamp).
It is an inverting buffer or NOT gate because the output logic state (low/high) is the inverse of the input state:
Input low (< 1/3 Vs) makes output high, +Vs
Input high (> 2/3 Vs) makes output low, 0V
When the input voltage is between 1/3 and 2/3 Vs the output remains in its present state. This intermediate input region is a deadspace where there is no response, a property called hysteresis, it is like backlash in a mechanical linkage. This type of circuit is called a Schmitt trigger.
If high sensitivity is required the hysteresis is a problem, but in many circuits it is a helpful property. It gives the input a high immunity to noise because once the circuit output has switched high or low the input must change back by at least 1/3 Vs to make the output switch back.
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