LED to come on with battery power when it's too dark for the solar panel

[John1397]

I am trying to get relay to come on when it gets dark outside this works with one LED and 600 ohm resistor but it does not seem to turn on relay I get 20 ma draw when applying 12 volts to coil it seems you need transistor or SCR to to turn on relay. Relay I think was from a TV it's only 1/2" X 1/2" X 1/2"

 

[jedishrfu]

Something like this:

http://www.learningaboutelectronics.com/Articles/Dark-activated-light-circuit.php

This is a great circuit to study to understand how a transistor can amplify a small input to control a device. In the circuit, the photoresistor varies its resistance depending on the light striking it. This in turn varies the current to the base of the transistor. The transistor in turn controls the current flowing through the LED.

Basically during daylight, current flows through the photoresistor line and not through the LED+transistor line and during the night the current is blocked by the high resistance of the photoresistor and instead powers the transistor base allowing more current to flow the LED+transistor line.

 

[osilmag]

If you are still there, I would have the relay coil only controlled by the solar panel. Therefore, a normally closed relay would turn on the LED at night.

 

[Tom.G]

Just change the wiring a bit.
Wire the left end of the relay coil to the Battery "-", not "+"
Connect the left end of the LED to the relay "Off" contact, not "On"

When the Solar Panel is generating voltage it will pull in (turn On) the relay.
When the battery is supplying power, the Diode will block the battery voltage from the relay, turning the relay Off.

Cheers,
Tom