How to use optical sensor to increase voltage to LEDs when ambient light drops

[nelly]

Hi

I would like to use an opto sensor to brighten yard lamps when I turn off the room lights ( or when it’s dark outside).

This is the complete opposite to what my opto sensor does.

Are devices available which do what I wish it to or can someone suggest a module or simple circuit that will let me achieve this.

Thanks.

[Mark Forrest]

Not really an answer, but an observation.   I'd have thought that in the real world yard lamps would be either on or off rather than dim/bright.  Are you over complicating things by looking for a sensor that automatically controls dimming? Would a situation where dark = on, light = off be easier to set up and more prototypical?

[RobertW]

Hi,

 

I think that the reason that the lights get dimmer as the ambient light level falls is because Light Dependent Resistors tend to have a response where their highest resistance is when it is dark.

 

You could get over this problem with a LDR in a potential divider. LDRs typically have a light resistance of about 8k, and a dark resistance of about 1M. From experiments, the best way to do it is with NPN Darlington transistor arrangement. This gives a sharp response between on and off, as Mark says, it's rare for the lamps to come from low brightness to high brightness, its more typical to be on or off. The LDR forms the bottom of the Potential Divider between the base of the transistor and ground, with a 1M resistor between the base of the transistor and supply. I've attached a sample circuit diagram. The supply voltage is uncritical, so it could be fed from a rectified 16VAC supply from the controller. If you wanted a manual override, you could connect a switch across the collector and emitter of Q2, which are pins 2 and 3.

 

Let me know if you've got any comments or questions.

 

Thanks,

 

Rob

 

[Stevebr]

I would replace the 10k resistor with a 5k fixed and 5 k preset to give some ability to adjust.

[nelly]

Hi guys

thanks for the response.

I just want the lights to come on automatically at night when ambient light drops.

Its on-off  that I’m looking to achieve.

 

[Crosland]

1 hour ago, nelly said:

I just want the lights to come on automatically at night when ambient light drops.

Its on-off  that I’m looking to achieve

 

It might have helped had you said that in the first post

 

You can still use a similar circuit to turn on a relay when the light drops. Connect the lights to their power source  through the relay.

[nelly]

Sorry about that...