When I constructed the circuit on a breadboard it worked fine. But when I build it on the PCB it doesn't clearly recognize darkness.
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Hi! I made an alarm circuit that is control by a light sensor. This light sensor is made out of a LDR, Transistor(2N3904) and a couple of resistors as shown below. When I constructed the circuit on a breadboard it worked fine. But when I build it on the PCB it doesn't clearly recognize darkness. i.e. when in darkness still the alarm circuit is on when it should be off. What could be the reason for this? I tried changing the resistor to 330R but still the same. Could this be because of a faulty LDR?
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This sentence is really the key. It means that there is a mistake you made somewhere, or that you damaged a component during the soldering process.
Do you have a DMM? If so probe around for resistance values (non powered up) and voltages (powered up).
Try first to wash your board with alcohol. The sensitivity can be adjusted by the resistor connected from transistor base to ground- the smaller its value, the less sensitivity. If nothing helps, the light sensor might be damaged by soldering. Replace it with a new one.
Also,,,, add a resistor (470 to 1k) in series with the LDR.
I've measured LDRs that have a resistance lower than 100 ohms in bright sunlight. Too much current will flow in that instance, and you can damage either the LDR or the transistor.
You have an LDR to detect light and an LED as an alarm that makes light. Does the LDR "see" light from the LED? If it does then it will never turn off.
Can you measure the the resistance of the LDR in light and in dark? Also check the PCB traces are clean...
I measured the resistance, when there is light its about 0.3k and when there its dark about 0.3M.
Please check the PCB, clean once again and try once more. The LDR is working fine as expected. Please check the transistor connections. Please also see that there is nothing that can short in between the PCB tracks.
I cannot think of anything that can go wrong in the circuit...
It should be any wrong connections in the PCB. As you said it is worked fine in the bread board the same is expected in the PCB too. Trace the connections node to node using DMM.
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