I have installation with Controllino mini, it's essentially an ATmega328, and with 6 LDRs and 6 lasers, creating laser rays/barriers.

The system acts as a lasers labyrinth for entertainment purposes, and it's working for approximately a year. One of the LDR's - just generic one you can buy in every electronics component shop, stopped working as it should. Meaning - it outputs constant voltage no matter what are the light conditions. The installation of the sensor consists of +12V input on one pole, and connection to the ATmega328 based PLC analog input on the another pole with a 10K resistor to GND.


simulate this circuit – Schematic created using CircuitLab

After taking it apart, the measured resistance on the LDR was constant (don't remember the exact value) and not reacting to different light conditions. I did replace this sensor, but after a week or so, the same problem occurred again.

This, and the fact that it was working for a year without issues, made me believe there might be external factor affecting it/ taking it out of order.

Why did the LDR or circuit fail after a long period of time?

  • \$\begingroup\$ I'm not sure LDRs were ever really mean to be blasted with lasers... \$\endgroup\$
    – Trevor_G
    Apr 24, 2017 at 18:19
  • \$\begingroup\$ Its a 5v@25ma red dot 650 nM Laser. All sensors/LDRs are working for more then a year, without issues. Only this one stopped working normally after 12 months of operation. \$\endgroup\$
    – Eli
    Apr 25, 2017 at 18:11

1 Answer 1


Don't know if this is the cause of LDR failure, but your circuit has a basic flaw: A +12v supply can cause damage to the ATmega328, when the photo-resistor's resistance is low enough to raise the Analog input pin above ATmega328's Vdd (+5v?). Too much current can flow.
The solution is to lower the +12v supply to no more than ATmega328's Vdd voltage. You may have to lower the threshold where you decide if the laser beam has been blocked, because with a lower supply, your circuit will be less sensitive.

  • \$\begingroup\$ Hi Glen thanks for answering. Its an Atmega based PLC. Ive attached link above. Normal operating voltages for its IOs are 12/24V. \$\endgroup\$
    – Eli
    Apr 25, 2017 at 18:13
  • \$\begingroup\$ @Eli It would appear that the PLC analog input conditioning makes your circuit OK. I can only suggest that Trevor's suggestion of concentrated laser power might cause spot overheating. A de-focused beam might help. \$\endgroup\$
    – glen_geek
    Apr 25, 2017 at 18:53

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