I've just discovered a strange behaviour of a LED bulb (E27, 220v/AC) that I'm currently turning on/off via an 8-channel SSR relay. Such a relay module is powered with a switching power-supply, powering both the relay-module and an Arduino MEGA 2560. I also have a wall-push-button connected to an INPUT_PULLUP PIN of Arduino (one side) and to GND (the other side). Another OUTPUT PIN of Arduino is connected to the first channel of the SSR relay module (to handle the SSR connected to the LED bulb).

As for Arduino, I choose to power it trough the Vin.

As for GNDs, both Arduino and the relay module get the GND from the power-supply.

A very rough schema is reported below (please, don't blame me for missing details. As for the driller, please ignore it for a moment!):

enter image description here

Such infrasctructure worked flawlessy for weeks, being powered on and correctly responding to push-button clicks without any issues.

Recently I needed to make an hole to a wall, actually a couple of meters away from the LED bulb, and at least 5/7 meters away from Arduino (Arduino, the relay module and the power-supply are located close to each other, in a box, under a staircase).

To make the hole I used an AC-powered driller (not a battery-powered one). Actually, it's exactly the driller you can see in the image above).

What happened was that.... the LED bulb "flashed", with a real quick "turn_on"/"turn_off" cicle. A single quick cicle.

This phonomena seems to be related to the "start" of the driller engine, more or less when the brushes inside the driller fires up some sparks (BTW: I'm supposing my driller is using brushes, inside).

I've spent some time investigating EMI, as electrical motors are known to be source of EMIs.

What I learnt so far is that:

  1. my problem is different from the (quite common, indeed) one that see a LED bulb "flashing" at a very fixed frequency (once every 20 seconds, for example) despite the fact that the SSR-relay is open. My BULB keeps staying OFF and... flash only when the drill is working (actually it flash with a random behaviour, but only when the driller is running);

  2. my SSRs, as well as almost any SSR, include a "snubber circuit";

  3. if I unplug the power-supply, turning off Arduino as well as the "control-plane" of the SSR relay module... the phenomena PERSIST.

  4. if I unplug everything... the phenomena disappear (obviously?).

  5. if I use the driller very close to the SSR relay module (and Arduino, and the power supply), phenomena is exactly the same.

There are lots of things that I still need to investigate (starting from using "shielded" cables, as for the connections between Arduino's PIN, push-buttons and relay modules) but... point 3) above makes me think that I'm missing some bigger problems: can the 10 meters long electrical wires between the LED bulb and the AC power source, acts like an antenna so to catch so much EMIs to "flash" the bulb?

Can someone help me in understanding what's happening or, eventually, which is the proper "troubleshooting process" that I should put in place to (try to) solve this problem?

A final note: should you want to check by yourself the "phenomena", feel free to give a look to this MP4 video (22 seconds for 5.6MB), where I asked my son to "shout" as soon as he saw the "flash" (that I cannot see, as I was close to equipments, slight far away from the room). The driller engine is, also, clearly audible.

  • 1
    \$\begingroup\$ Your question seems basically an electrical engineering one about EMI interference. The fact that an Arduino is turning the circuit on and off is not the main point. I'll send this over to Electronics Stack Exchange for their insights. \$\endgroup\$ Jul 22, 2016 at 22:23
  • \$\begingroup\$ The drill is inducing emi, or it the drill is providing enough of a load to cause fluctuations on the line (my space heater and microwave did this to my lamps all the time) enough to cause the capacitors to charge and discharge in the led circuit. It doesn't take much power to cause an LED to flash. \$\endgroup\$
    – Passerby
    Jul 22, 2016 at 22:39
  • \$\begingroup\$ What snubber do you have on the SSR? Does it help if you connect more load, resistive like halogen bulbs, on parallel with the LEDs after the SSR? \$\endgroup\$
    – winny
    Jul 23, 2016 at 8:27

4 Answers 4


Try to use 1K pull-up resistor with button, internal pull-up might not be enough to hold HIGH level with stronger EMI nearby.

Also set unused pins to INPUT_PULLUP mode (or OUTPUT) - they're floating after reset and it catches all kinds of interferences (if it's not a problem here it usually causes much higher current consumption by MCU).

  • \$\begingroup\$ Thanks, KIIV, but I doubt that "tuning" pullups can help as.... I get the strange behaviour even when Arduino is not powered (aka: when power supply is shutdown and VCC/GND are phisically disconnected from Arduino), and even if I phisically disconnect the wire from the arduino output-pin to the relay-module as well! In other words, the strange "blink" happens with only the relay-module powered (but only on the 220AC side!). So... what do you suggest? \$\endgroup\$
    – Damiano Verzulli
    Jul 22, 2016 at 16:44
  • \$\begingroup\$ My suggestion is to try pull-ups first :) Then shielded and grounded wires as short as possible, grounded shield encapsulation for arduino and ssr. And eventually better power source. \$\endgroup\$
    – KIIV
    Jul 22, 2016 at 19:37
  • \$\begingroup\$ I vote for the pull-up resistor too ;) \$\endgroup\$ Aug 22, 2016 at 14:50

LED bulbs have internal circuitry especially if it is dimmable. A snubber works to limit back EMF. Try a snap on ferrite on the power supply to the bulb. Experiment with placement until you can isolate where the interference is originating from.

  • \$\begingroup\$ "Experiment with placement until you can isolate where the interference is originating from": please, can you elaborate on this? My guess is that EMIs are originating from the driller. Do you mean that such EMIs will travel all along power cables and, with a proper placement of a "ferrite bead" (...along the cables), I can (hopefully) lower them so to not "act" on the LED bulb? \$\endgroup\$
    – Damiano Verzulli
    Jul 22, 2016 at 16:56

On the line going to the bulb and then work backwards. Power supply lines (drill and SSR module), interconnect between the power supply and your aduino and peripheral board. This can be time consuming and daunting without the use of a spectrum analyzer to pinpoint the radiate emissions. LED's work on low voltage and current so I really suspect noise entering the bulb either through the power lines or as radiated RF.


I do not know what kind of electronic testing equipment you have. Basically you will want to put a low pass filter on the power line tuned between the primary frequency and the second harmonic. Better would be to put a band pass filter at the power line frequency, a tuned circuit at the power line frequency. But that is a real pain to get adjusted properly without a frequency counter and the lower frequencies are probably not causing the problem. Ferrite are fine, but put many turns of wire around and through the ferrite to increase the inductive reactance. If you know a ham radio operator, you can ask for assistance in choosing the components for a good filter. The ham may even have the diagnostic tools to help diagnose and fix the problem.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.