How to build a circuit that will turn a light off when it gets dark.

 

How to build a circuit that will turn a light off when it gets dark.

I thought that usually you want to do the opposite: turn a light ON when it gets dark.

 

I have found circuits that create a night light where a light turns on when it gets dark. I need the opposite.

I need a circuit that will allow a light to run continuously on 9 volts until it gets dark. Then I would like it to turn the light off.

Can anyone help?

 

One thread is enough. Please stick to one thread per topic and one topic per thread.

 

I have found circuits that create a night light where a light turns on when it gets dark. I need the opposite.

I need a circuit that will allow a light to run continuously on 9 volts until it gets dark. Then I would like it to turn the light off.

Can anyone help?

Most light sensitive circuits consist of a light dependent resistor (LDR) and a bias or load resistor.

One way to reverse the effect is to interchange the LDR and bias resistor.

Another way of reversing the effect is to add an inverting stage in the circuit as shown below:

And there is still a third method and even simpler method. Typically, a relay is being controlled by the light sensitive circuit. Simply change the connections to the relay from normally open (NO) to normally closed (NC) contacts.

 

Thank you for the help. I was able to build the circuit and it works as described. The light turns off when it gets dark.

Next question I have is that the light is very dim. I assume the resistors are reducing the current flow? Is there a way for me to have full current running to the light?

I did change the transistor BC547 for a 2N3904 would that make a difference?

 

Thank you for the help. I was able to build the circuit and it works as described. The light turns off when it gets dark.

Next question I have is that the light is very dim. I assume the resistors are reducing the current flow? Is there a way for me to have full current running to the light?

I did change the transistor BC547 for a 2N3904 would that make a difference?

What circuit?
Please post the circuit you have built.

 

This is the circuit I built. My battery is a 9 volt....but the output voltage to my light is less than 3 volts. As I mentioned I substituted a 2N3904 Transistor for the BC547 recommended in the schematic.

 

the output voltage to my light is less than 3 volts

If you mean the voltage across the LED, that's to be expected. What is the maximum current rating/specification of your LED?

 

This is the circuit I built. My battery is a 9 volt....but the output voltage to my light is less than 3 volts. As I mentioned I substituted a 2N3904 Transistor for the BC547 recommended in the schematic.

It depends on the LDR chosen. Try replacing R2 with a higher value resistor such as 10kΩ or replace it with a 10k-100kΩ variable resistor to allow you to adjust the switching threshold.