How to increase the light sensitivity of a DARK SENSOR CIRCUIT?

I recently managed to replicate a dark sensor circuit using a LDR, a 220K & 330 ohms resistor, a BC547 transistor and a LED. But the problem is that it is very little sensitive to light. During daytime there is no problem but at night, under the CFL light it doesn't sense the light and turns on. To turn off the light, I have to illuminate the LDR with a external light source and that too very close. What I want is that the LED should light up only when there is complete darkness. How should I modify the circuit to increase the sensitivity? 

The circuit diagram is as given below.

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Can you build or construct something that would help reflect / funnel more light to the sensor?

G4224T3 years ago

Swap R1 for a 100K pot / variable resistor then you can adjust the sensitivity.

Won't work. Read the question.

Thanks. I re-read it and have hopefully provided a helpful, positive and constructive answer this time.

G4224T3 years ago

Oops Maybe I should have read the question a bit better before answering. What I actually meant was to put a POT in to allow you to adjust the sensitivity as opposed to just trying different resistors.

A Higher Value Resistor could work although it could potentially affect the LED current too much too. Photoresistors can have anything between around 600K and a few Meg in darkness so I still think a POT would be the way to go.

Try adding a second transistor to help increase the LED current with higher resistances which should make the photoresistor more responsive to darkness. The attached circuit will allow you to alter the threshold. You might also consider moving the photoresistor away from the light source a bit too which should help.

Hope it helps anyway.


Try increasing R1 to 1Meg


But if you don't have a 1Meg, I bet you have another 220K to put in series.