Switch a 555 osc on when it gets dark? Circuit needed..

[Paul Perry (Frostwave)]

Well a DARK sensitive really. I'd like a 555 driven flasher that only goes on when dark (using a LDR as sensor).
And goes off again when the room is light again.
It is an application that needs to have very little drain in off mode. Was thinking of a CMOS 555 for that reason.. I confess, it isn't exactly a stompbox app, but I'm sure there must be SOME wacky e-music app

[gez]

I'm going by memory here Paul, but I think the reset pin keeps the output low if it remains grounded, in which case it could be used as an 'enable' switch: large value resistor in series with the LDR across the rails, then hook the centre of this 'divider' to the 'reset' pin.  A Schmidt buffer would ensure the pin is kept at logic low.  In which case, you could use a 4093 chip for the whole circuit (they can be wired up as Schmidt oscillators, but can be enabled/disabled as they're two input logic gates)

Don't shoot me if I've got that wrong! 

[R.G.]

Good one, gez.

The CD4093 is about the simplest gated oscillator you can make. The oscillator part is just a resistor feedback to one of the inputs with a cap to ground. The gate does the schmitt function and the resistor/cap is the integrator. The second input gates the thing on and off.

You can drive the other input with a resistor and LDR in series. The LDR should be arranged to turn the input on or off with light, as you prefer, by pulling the input up or down in response to light. The other three gates in parallel might be able to drive an LED directly. For large currents you'd need a driver transistor.

[gez]

The other three gates in parallel might be able to drive an LED directly. For large currents you'd need a driver transistor.

Yes, the limited drive current of the 4093 is the only drawback to the idea I outlined, though as you say, paralleling the remaining gates can drive a low current (2mA) LED well.  With only a few millamps available, they won't be as bright as standard LEDs with higher current through them, but this shouldn't be a problem seeing as the circuit only needs to be visible in the dark.