Light On, Light Off

I'm trying to create a circuit that will arm when light hits it, and then light an LED when the light goes back off. I've posted a picture of the circuit I was trying, but the relay would click open, then back closed immediately. Any help would be great, thanks.

What is the resistance of the LDR when illuminated? When dark? What is the battery voltage? What type of battery? How much current does it take to light the LED?

wildebeest87 said:

Any help would be great, thanks.

Click to expand...

Just one idea to kick around.

Is this what you had in mind?

YouTube - VID00005

The switch is an NC. to be used as a reset.
When light hits the LDR, the circuit is activated, however, Q2, keeps the LED driver transistor Q3 in a off condition, as long as light is still hitting the LDR, when the LDR has the light removed from it, then Q2 shuts off, and allows Q3 to turn full on thereby lighting the LED output.
To reset the circuit, you push the reset button, this deactivates the SCR from it's latched state.

12345678901234567890 said:

Is this what you had in mind?

YouTube - VID00005


View attachment 49378

Click to expand...

This is exactly what I had in mind, that should be perfect. Can I use a 9v battery? And are there specific transistors I should look for?

Hi,
The circuit tested well from 6 - 10 volts, which covers the range of a 9 v. battery.
The transistors used were, 2N3904, or equivalent general purpose NPN.
The SCR is a EC103B type.
The diodes are the general purpose 1N914 or equivalent type.
The LDR, has a very high resistance in the megohm region, then drops to around 13K ohms with the source of lighting as demonstrated.