Dark Activated Circuit

[denled]

Hi
I am looking for a simple and cheap circuit to operate a relay around dusk Ideally it would need to allow for increase of light during the night as I only want it to work the once in a 24 hour period This being at dusk. Also looking for a timer operated by the relay this would run from say 15 seconds to 2 minutes max
Preferable all 12VDC as the timer will be driving a car windscreen wiper motor
Any help would be appreciated
Thanks
Den

 

[green]

There is a light detector circuitry from http://www.geocities.com/bagtool/ldr_opamp.html

green :idea:

 

[Roff]

I think that if you take the circuit here, and swap the photocell with R4 and R5, it will do what you want. Connect the output (to rooster reset) to your relay coil (other end of coil to GND). Connect a 1N4148 diode in parallel with the relay, anode to GND. You might have to use a bipolar LM555 instead of CMOS to get adequate drive for the relay.
The wiper motor will be switched by the relay contacts.

 

[denled]

Thanks for the circuits but I need something that will allow for changes in light during the night say for example the moon comes up and then clouds over this would activate the circuit again which I dont want to happen is there perhaps a time delay cct that could be incorporated into it to allow for this. Are things getting to complicated ??
Dont want it to be to expensive as it may be stolen !
THX
Den

 

[Misterbenn]

i admit i havent looked at the circuits above. but i sugest using a shmit inverter. this will add histaresis (prob not spelt correctly sorry) to your circuit, solving the changes in light level problem.

 

[Misterbenn]

how about this, the output can go to your monostable, then use a trasducer driver to power whatever u want powering.
this will give you two diferent switching levels for on and off, so you'll get less of a problem will small changes in light.

you can also build you own shmit trigger using an op-amp.

sorry about the bad drawing, i only have paint on this computer... :roll:

 

[Misterbenn]

stuff my diagram look circuit Ron did, thats basicly what i'm trying to say.....

 

[Nigel Goodwin]

i admit i havent looked at the circuits above. but i sugest using a shmit inverter. this will add histaresis (prob not spelt correctly sorry) to your circuit, solving the changes in light level problem.

It's got a 'y' in it somewhere, but I'm not trying to spell it either :lol:

Anyway, no need for a schmitt trigger, simply add a positive feedback resistor across the opamp (output back to non-inverting input), this makes it work as a schmitt trigger. The resistor should be a high value (usually megaohms), and the lower it gets the larger the hysteresis (I cheated and looked the spelling up!)