Prob. a bit basic for on here but this type of electronics isn't my best? Here is an alarm circuit I have designed to be incorporated in a metal cabinet, can't use a reed switch as the cabinet is steel. Quick talk through Op amp and the 2 pot. dividers compares the light levels inside and outside the circuit, when the one is close to the one on the right it will activate the op amp output, this will need to be set up in situ using variable resistors When this occurs it will activate the thyristor which acts like a latching switch, this allows current to flow to the 555 timer which will let some leds and a buzzer to work in pulses. This can be reset by closing the door to darken the inside of the cabinet and then turning the circuit on again Would this work?