Simple Photoresistor Circuit

Thread Starter

gwpmike

Joined Oct 18, 2015
1
I want to create a simple circuit that is essentially the opposite of a dawn/dusk circuit.

A dawn/dusk circuit would allow a light to turn on (or close the circuit) when it is dark outside, and would turn the light off (open the circuit) when it is light outside. I want to do the opposite.

I want a LED to be able to turn on ONLY when it is light outside. I want the LED to not turn on when it is dark outside.

In other words, close the circuit when the photoresistor detects light, and open the circuit when the photoresistor does not detect light. Please see the attached diagram.

 

ISB123

Joined May 21, 2014
1,236
You are going to need to play with VR1 value since we have no idea of whats the resistance of LDR under certain Lux.

Here:
1.png
 
Last edited:

GS3

Joined Sep 21, 2007
408
I like to have hysteresis to prevent instability. If it is just an LED for play it does not matter but if it is a relay controlling something it shortens the life of the device and is annoying.
 

AnalogKid

Joined Aug 1, 2013
8,101
In the post #4 schematic, add a large value resistor from the comparator + input to its output.

ak
 

ian field

Joined Oct 27, 2012
6,543
http://www.sentex.ca/~mec1995/circ/relayldr.html
http://www.eleccircuit.com/light-sensitive-switch-with-ldr-2n2926/

They do the opposite of what you want but you can easily adapt them by switching the LDR with the resistor.

They should both have good hysteresis which other designs don't have.

Another one:

View attachment 93151
Here's a hand trace of an outdoor light fitting that shuts off in daylight:-

The photodiode looks very like the IR PD in the front panel of a 90s CTV, but apparently it worked just fine for sun light.

The basic circuit is a dual op amp with a voltage follower stage to handle the high resistance of the PD, the second stage is a Schmitt trigger for clean switching.

The mains aspect of the circuit can be disregarded, it ends up requiring 9V DC for its supply.
 

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