plz help identifying a problem in circuit

Thread Starter


Joined Nov 2, 2008
I have posted many mails previously regarding my ldr based sensor circuit.The purpose of circuit is to turn on or off the relay depending on the intensity of sunlight. Other side of relay is connected to 220v and bulb as a load.

I have built this circuit using four methods.
1. simple transistors.
2. latching transistors.
3. 741 comparator
4. finaly with window comparator and sr latch.

But all circuits have one common problem. At no load all circuits work well but at load on other side of relay flickering of load starts. May be due to AC some noise disturbs relay contacts.I am using electromagnetic relay. What about using solid state relay. I have never used one.


Joined Apr 5, 2008

Can you post a drawing of the circuits used?
Also can the LDR see the lamp?
If the LDR can see the lamp the circuit will go oscillating.
You can place the LDR in a tube and point it away from the lamp.



Joined Apr 2, 2009
There could be two reasons for the relay flickering.
1. The 12 Vcc cannot handle the relay current, the loading may cause the voltage drop thus switching off the transistor.

2. Most likely is that your circuit is creating hysteresis. The switch which you use to turn on the relay is in the active region.
The slow change in light causes the switching voltage to change slowly, if this happens the relay will flicker before turning on. What you need is a Schmidt trigger, or the window comparator window can be made wider.


Thread Starter


Joined Nov 2, 2008
i have added hysteresis in my circuit as well as i have increased window size, but problem does not seem to solve. Can anyone tell what are the possible reasons and solutions. and how to control LDR exactly


Joined Jan 18, 2008
I think Beenthere's explanation is best. The light flickers at the mains frequency, which makes the LDR output flicker.

What happens if you take the center schematic in post#4 and add a capacitor from the transistor base to ground? A rough guess is 10 to 100 uF, without knowing the base current or RV1. You could also add a resistor to the base (e.g., 1 K), which would effectively give you a low-pass filter.