Light Dependent Resistor LDR controlled 555 Inverted Schmitt Trigger Circuit

Light intensity can easily determine the state of a 555 timer. You just need to add a light dependent resistor based voltage divider.

Light Dependent Resistor LDR controlled dark high 555 timer schmitt trigger demo circuit by electronzap electronzapdotcom
Light Dependent Resistor LDR controlled dark high 555 timer schmitt trigger demo circuit by electronzap electronzapdotcom

The light dependent resistor (LDR) voltage divider output is connected to both pins 6 and 2.

  • 2 = Trigger.
  • 6 = Threshold.

With the LDR on the high side of the voltage divider, the voltage will go up as brighter light falls on it.

  • Brighter light = higher signal voltage.
  • Less light = Lower signal voltage.

The output will state (either high or low) depends on the input signal voltage.

  • More than 2/3 of the supply voltage = a low output.
  • Less than 1/3 of the supply voltage = a high output.
  • 1/3 to 2/3 supply voltage *hysteresis range for this circuit) = Output stays high or low. Whichever one it was last set to.

I used the 5528 LDR in the video below. Other LDR’s will work, they just will probably have a different resistance at a given light level. So, you will have to change the fixed resistor accordingly. You will need to change the fixed resistor if you want to adjust sensitivity anyways.



Kit I got my 5528, and other light dependent resistors from. An affiliate link ad that supports this site.


Assorted integrated circuits (ICs) kit. Included is the NE555 timers and other commonly known ICs. I have previously covered some of the other ICs in YouTube videos. It is an Affiliate link ad that supports this channel.

Video:

To support this site, check out the following links:

Home page

  • Information on this site is not guaranteed to be accurate. Always consult the manufacturer info/datasheet of parts you use. Research the proper safety precautions for everything you do.
  • Electronzap is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com.