First time asking a question here.

I am curious why the TSOP4838 ir sensor is acting erratically when my Google Pixel 2's screen is facing it.

As you can see from the schematic, I have a 555 arranged as a Schmitt trigger, with the output going to the CLK of a 4013 configured as a latch, which ultimately activates the NPN relay driver. The circuit works as expected when facing a 38 kHz IR LED at the sensor, but also falsely triggers when facing my cell phone's screen towards the sensor.

Any ideas?

enter image description here

  • 2
    \$\begingroup\$ Does your phone have an infrared based proximity sensor? \$\endgroup\$
    – Justme
    Apr 21, 2021 at 21:21
  • \$\begingroup\$ You have no frequency discrimination - so it's working correctly. \$\endgroup\$ Apr 21, 2021 at 21:38
  • \$\begingroup\$ @KevinWhite Thank you for answering so swiftly. So despite the fact that the sensor is tuned to only detect signals at 38k, there is additional frequency discrimination I should use? What kind of design would this entail? \$\endgroup\$
    – Craftyjam
    Apr 21, 2021 at 21:51
  • 1
    \$\begingroup\$ @Craftyjam - Sorry, I was wrong. I misinterpreted your schematic. The symbol you are using is for a plain phototransistor as opposed to a module. The TSOP4838 should filter the signal acceptably. It is possible that the proximity sensor near the ear section of the phone is triggering it. I worked on the design of the iPhone proximity sensor and we used about 100kHz, not 38kHz but the Pixel2 may use a frequency close to 38kHz. \$\endgroup\$ Apr 21, 2021 at 22:01
  • 1
    \$\begingroup\$ @Craftyjam - By putting your finger over various parts of the phone screen (and bezel) you should be able to determine where the interference is coming from. \$\endgroup\$ Apr 21, 2021 at 22:25


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