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Newbie needs help with day/night circuit

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MrPike

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I know almost nothing about this stuff but have a task I would like to accomplish, but I need help!

I would like to build as simple a circuit as possible that allows a relay coil to be energized after a trigger, but only at night (reasonably close to dusk/daylight is plenty good enough). Here's what I have at the moment: after a trigger, a 3.6 volt signal will be available from another electronics board. On the other end, I will have a normally open relay wired to some other device and would like the voltage to be passed to the relay only at night. I was thinking that a photocell would be one way to do this, but can't seem to find what I need to do. Basically, I just want the circuit to be completed at night (allow the 3.6V to close the relay), but not in the day.

Can anyone help? Sorry if this is a really dumb question.
 
Here is a design where I used the 555 timer in an unusual way. The 555 timer (not CMOS version) can drive small 12V relays directly as shown. Also the way I used it will give a nice margin (hysteresis) between light and dark (eliminating flicker or relay chatter), as long as you prevent any light from a lamp, if you are switching one, to interfere with the light sensor. The LDR (light dependant resistor) I measured give < 100k under light conditions and over 2M with dark conditions. The pot in the circuit (here simulating the LDR) should be replaced with a pot and your LDR in series so that you can adjust (calibrate) the proper trip points according to the LDR you will use, as my values are applicable to my LDR measurements.

Your 3.6V control signal connects to pin 5. If this point is low (pulled to 0V) then the relay will not switch on regardless of the light conditions.

The 2 diagrams show the switching conditions between LIGHT and DARK
 

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