# LDR Circuit Diagram

#### Stelio.rs

Joined Apr 29, 2023
2
Hello!
I need your help. I'm not sure if this is the right place to ask my question, but I don't understand anything and I would be very grateful if you could help me. Could someone please make a simple diagram for controlling an LED strip? The goal is for it to shine at 100% during the brightest part of the day, and for the brightness to decrease to around 15-20% (in complete darkness) as night falls. I'm thinking of using an LDR sensor, and the LED strip is 12V 1.5A. Please make the diagram as simple as possible and with readily available parts.
Thank you!

#### Ian0

Joined Aug 7, 2020
8,400
It would need the LDR spec to be able to calculate the value of R2, but when R2=R1 the output will be at 33% brightness.
(R3 represents the resistor inside 12V LED strip)
It could be done with an transistor and two resistors, but the transistor will get hot. 555s are cheaper than heatsinks.

#### Stelio.rs

Joined Apr 29, 2023
2
Thank you for the quick response, lan0.
The LDR sensor has a bright resistance of 8-20k and a dark resistance of 1M. What value should C1 have? And if possible, could you recommend a PMOS?

#### Ian0

Joined Aug 7, 2020
8,400
MOSFET: anything that can handle the current.
C1: the 555 will oscillate slower the darker it gets, but you don't want it below 50Hz at the darkest otherwise it will flicker, so (0.7*C1*LDR resistance) < 20ms.
If it gets too dim when it's really really dark, then put a resistor in parallel with the LDR.

#### DickCappels

Joined Aug 21, 2008
10,067
You can put another resistor in series with the LDR to keep the frequency from getting too low. Depending upon how the light is used, in some applications, such as in a machine shop with powered machines, you might need to keep the frequency at a few kilohertz.

If this causes a problem in your application, let us know, there are variations to improve the situation. For most circumstances this is a very good circuit.