Photoresistor in parallel

When this circuit is built, the LED dims according to how much light is cast on the photoresistor. Why doesn't the LED glow brighter, as it normally would in series with the photoresistor?

by allspice
January 09, 2013

Assume that:

i) no light from the LED reaches the LDR (Light Dependent Resistor), i.e. the light source is entirely external to this circuit;

ii) the resistance of the LDR in darkness, Rdark >> R1;

iii) the resistance of the LDR in bright light, Rbright << R1;

iv) the forward drop of the LED Vled = 1.8V

Now, ask the circuit two questions:

a) what happens to the currents flowing in the circuit in darkness?

b) what happens to the currents flowing in the circuit in bright light?

If you can't work it out from that, post back.

:)

by signality
January 09, 2013

Thank you, I figured out what I was missing :)

by allspice
January 09, 2013

Well done.

FYI:

by signality
January 10, 2013

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