Wiring diagram for Arduino to prevent relay firing on reboot

Question

I'm starting to explore using an Arduino to control a couple of switches programatically. I have read somewhere about relays firing during reboot due to floating inputs/outputs during the reboot phase.

What is the best way to ensure that a relay will not fire on a reboot?

Is there a simple wiring diagram to explain it? I am using an Arduino Uno and a Relay Module.

Any assistance would be greatly appreciated.

Answer 1

Two things:

1. If that relay uses a FET input with no pulldown then you should add your own. If it uses an opto-isolator there is no floating problem to deal with.
2. Don't use pin 13 for a relay - that is connected to the LED which blinks during reset and will "flash" your relay.

Answer 2

You can add a pull down/up resistor between the wire and ground/vcc so that when the arduino pin is set to high impedance during reboot the wire is pulled low/high as desired.

Answer 3

What is the best way to ensure that a relay will not fire on a reboot?

put pull-up/down, as appropriate, on those pins.