Sonoff dry contact switch

(Flyerhub) #1

Can you power a Sonoff board with 5v without damaging the board, and why won’t the Sonoff relay, switch, when powering the board with 3.3v? It sounds like the relay is closing, but there is no continuity across the line/load. I am not connecting this to mains, I just want to power the board with a wall wort and use the relay to trigger a garage door.


No, the sonoff board is an ESP8266 chip which runs at 3.3V ONLY. If you give it 5V you will fry it very quickly.

(Flyerhub) #3

Have you had any experience powering the board across the capacitor B9 with 5 volts. I have it working, but not sure if I should trust it. Diagram below.

(Eric) #4

Did you try it? I have.

I doubt any damage will be done. I have wired several 3.x VDC devices to 5vdc (USB adapters) and NONE have cooked.

If you can’t afford to damage one then sure, don’t do it.

(Flyerhub) #5

Yes, I have one working right now, but want to build up several. I have many uses for a dry contact switch. I just was hoping someone had been doing this for a while and had some history.


Read the specs on the ESP 8266 chip. It cannot take 5v. If you wire 5v to the exposed VCC pin it will cook your board and possibly start a fire. I’ve not cooked a Sonoff but I have cooked an ESP 01 which is basically just the exposed board. There’s a reason you have to use a UART adapter that supports 3.3 V. But hey, go for it. You asked, I told you. Don’t say you weren’t warned.

(Eric) #7

I think you are right - they look to be touchy and lots of anecdotes of 5v problems. 2 diodes will fix it.

I will try it to replace some dry contact sensors. Why are these things so cheap.


Or just a voltage regulator.

(Flyerhub) #9

Do you think this simple voltage divider would work?


No, because your current draw is not going to be constant across both resistors. You’re going to be drawing more through R1 than R2. That’s why you can’t use voltage dividers on electrical circuits, only for reference voltages. The calculation you used to come up with R1 and R2 assumes that the current is going to be the same across both, which it won’t be. I would use a regulator instead.

(Flyerhub) #11

Another good point, but can I ask another stupid question. I’m not actually applying voltage at the VCC pins. The suggestion I got was to apply power to the board across one of the capacitors within the Sonoffs power supply. As I’m not an EE and I don’t actually have a schematic I assumed this was part of the internal power supply that possibly was regulating the voltage down stream. I found this post on the internet, so it must be true.:grin:


I’ve never tried wiring it this way so I can’t speak to that. But in this case you wouldn’t need the voltage divider either. Just make sure you don’t blow the capacitor by applying too much current.

(Flyerhub) #13

I actually tried to run it this way on 3.3v, but it wouldn’t work at all. Seems to be working fine at 5v. It has been on cycling for two days now.


Good to know it can be wired that way. Can you tell me how hard it was to solder the wires on? The capacitor looks pretty small.

(Flyerhub) #15

Actually pretty easy. If you look close it’s a pretty big space of copper. Just use solid wire and tin it pretty good. There is usually enough solder there to use.