Dark sensitive switch - Mod 12V supply to 5V

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Your link didn't work for me but I found the attached circuit muddling through the site. If this is the circuit it could be modified to work on 5 Volts but the first thing that is a problem is a 12 volt relay coil that likely will not work at all on 5 volts. You would need to sub a 5 volt coil relay and make sure that the transistor could drive it. Then change some resistor values maybe. The values of R3 & R4 would be reduced so the LEDs work.

Ron
 

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Now the link works. I know the resistors for these 2 leds must be changed also the relay with a 5v coil but i don't know if the transistor can drive the relay.

The caps can be left to the values as it is?
 
The caps would be fine. You may need to play with R1 a little. R1 plus the pot plus the value (light & dark resistance) of the LDR determine when the transistor is turned on (saturated) to drive the relay. As long as you go with a relay coil that draws the same or less current than what is there it should work just fine.

Ron
 
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