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Topic: Relay activating without signal (Read 977 times) previous topic - next topic


Greetings all.

Need help to understand why a relay is activating / energising without output from the Arduino.

I have made a drawing ( attached ) of the section where I am experiencing the problem. It is for a small house alarm system.

All the opto-coupler inputs are working fine. Problem is with the relays.

In the drawing, the third relay powers a Radio Link ( linked to an armed response company ). Their radio transmitter, powered from the same 12V DC as the circuit, has a trigger port.

To trigger the radio signal, the port requires a 12V pulse.

However, when I use the Arduino output pins to activate a different relay ( eg.. Gate 1 or especially the Siren relay ), the radio triggers.

I have many Serial.println statements in my code, so I am certain that the Radio Link relay is not being triggered by code.

Unfortunately, the triggering of the radio is quite noisy, so I can't be certain that the radio trigger is as a result of the Radio Link relay being energised, but can't see any other possibility.

What could be causing this ?  I would be extremely grateful for a solution.


Do you have any capacitor after the 5V regulator? You are getting some noise and it might well be the current in the circuit when you release on of the other relays is causing the radio relay to trigger. As it only requires a pulse, that might just be where the noise shows up, the others require an physical action and they may not have a long enough pulse for you to hear the siren or see the flash. Relays are noisy devices and you will probably need a capacitor to take care of that noise.

A good place to  start would be a 10 micro farad, 25 V electrolytic on the input and output of the regulator.


Jan 27, 2012, 06:06 am Last Edit: Jan 27, 2012, 01:17 pm by DaveO Reason: 1
The 5V regulator has a 0.33uF on the input and a 0.1uF on the output.

If it is caused by noise on the pulse trigger line to the radio, would a resistor on the line ( positioned at the radio ) not reduce any slight noise so that it does not trigger the radio ? If so, what size resistor would you suggest ?

I am assuming that if a resistor can kill any stray noise ( and I assume we're talking about very small voltages here ) , it would not kill a full 12V signal when the radio link relay is activated.

Alternatively, would a tie down resistor ( say 10K ) to Ground keep the trigger line tied down until it gets a big 12V signal from the radio link relay ?



I tried adding a 10K resistor to the radio trigger port. It worked fine when triggering from the Arduino, so I can't see any harm done.

However, it started to affect other relays / inputs when triggered. My thought was that the radio was next to the conduit that all the system wires are in ( ie.. a few centimeters from the wires coming from the remote receivers ).  I moved the radio unit further away ( 1 meter ) and have had a stable system since.

Does this sound logical ?


It could be one of the following:

1. You have some earth wiring in common between the siren and the Arduino supply (so the siren current is disturbing the Arduino);
2. The current surge when the siren is turned on is dropping the Vin to the Arduino sufficiently to cause to to malfunction;
3. Whatever input device you have that causes the radio to be activated is picking up noise due to (1) or (2).

Need to see your sketch and a photo of the wiring to say more.
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