I would like to build a circuit with two blinking LEDs using a NE555 timer and a photoresistor. In the schematic below the blinking frequency becomes higher when more light shines on the photoresistor (represented by the photodiode D1 in the schematic; there was no symbol for a photoresistor) and lower when there is less light. What I want, however, is the opposite: I want the blinking frequency to become higher when there is less light and lower when there is more light.

Can anybody tell me how to change the schematic so that it does what I want? I have to add that I'm not really advanced in electronics yet, so please understand if I did not provide information you need.

enter image description here

  • \$\begingroup\$ This circuit won't give you what you want. \$\endgroup\$ – JIm Dearden Apr 21 '17 at 20:41
  • 1
    \$\begingroup\$ The voltage on pin 5 (CON) normally sits at 2/3 of the supply voltage, but you can feed a larger or smaller voltage into it to alter the frequency of oscillation. That's probably a better place to experiment with attaching your LDR. \$\endgroup\$ – Finbarr Apr 21 '17 at 21:04

Use the 555 in Voltage Controlled Oscillator (VCO) mode.

Something like this where the saw-tooth current source is your Photo-Diode.

Some tweaking of values will be required.


simulate this circuit – Schematic created using CircuitLab


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