Photocell Circuit

[nprichards]

I’m trying to build a circuit that uses a photocell to turn on and off a small light. The power source is 12v. I was looking at using RadioShack photocells that produce a dark (i.e. less light) resistance of about 10,000 ohms and a bright resistance (i.e. more light) of about 700 ohm resistance.

How can I use this photocell to turn on a light when the photocell reads dark and turns off the light when the photocell reads bright.

Please keep in mind I don’t have any background in electronics other than what I have been able to research on the Web.

Thanks for any help.

 

[user_88]

The attached file shows an elementary transistor switch using the photocell.
You did not specify how much current you required to flow through your light ... or what kind of light it is.

The circuit here produces about 30 ma through the light .... using the specified transistor.

 

[Boncuk]

Hi,the simplest way to make a light switch using an LDR takes two resistors and one transistor.Connect the fixed value transistor and the variable (LDR) in series and from the junction feed the base of a transistor.Calculate the value of the fixed value transistor for a voltage divider giving you ~0.7V at the base of the transistor when light conditions change. Connecting the LDR to ground the transistor will switch on the load if it gets dark. Reversing LDR and resistor the function will be reversed.Here is a sample circuit using a load (lamp) of 300Ω

 

[Boncuk]

Hi,

the simplest way to make a light switch using an LDR takes two resistors and one transistor.

Connect the fixed value transistor and the variable (LDR) in series and from the junction feed the base of a transistor.

Calculate the value of the fixed value transistor for a voltage divider giving you ~0.7V at the base of the transistor when light conditions change.

Connecting the LDR to ground the transistor will switch on the load if it gets dark. Reversing LDR and resistor the function will be reversed.

Here is a sample circuit using a load (lamp) of 300Ω. The transistor starts conducting if the LDR has reached 1KΩ of resistance. At full darkness the lamp glows brightly.

If the lamp turns on too early increase the value of R1.

Boncuk

 

[Mikebits]

What keeps the lamp from turning on the LDR?

 

[Boncuk]

What keeps the lamp from turning on the LDR?

Nothing! A good and cheap flashing circuit though.

Of course I assumed the lamp won't be near the LDR.