Dark Sensor LDR Circuit

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Rip

Joined Apr 28, 2016
12
How would I go about designing a dark sensor circuit using an LDR and transistors? The LDR value for light is around 7k ohms and when its dark it is around 1M ohm. The transistors are 2N3904. The circuit has a 5V buzzer and a 2V (10mA) LED. The circuit should work as follows: when there is sufficient light falling on the LDR, the transistor should be in saturation and the LED should be lit and when there is not sufficient light falling on the LDR then that transistor is in cutoff and the LED is not lit and the buzzer should sound. How would I go about designing this circuit? Any help is greatly appreciated
 

dannyf

Joined Sep 13, 2015
2,197
Typically it is a divider. Your transistor will have a threshold voltage to turn on / off. You just to pick a resistor so that

LDRlight/(R+LDRlight) << Vthres, and LDRdark/(R+LDRdark) >> Vthres.

In addition to that, you need to make sure that the base current isn't too light.

A few trial and error will do.
 

Thread Starter

Rip

Joined Apr 28, 2016
12
Where would I put the buzzer and the LED for this circuit? I'm just not sure how to lay it out to make it work

Any help please?
 
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