switch with long wires not working

[jlra2fael]

Hello,

I’m trying to use a door magnetic switch wired between a GPIO pin and the ground but it’s not working.
I tried to connect the 2 wires connected to the pins near the switch but it doesn’t work.
If I connect the 2 pins on the board it works successfully.

I tried with and without pull-up activated.

I’m using phone twisted pairs wire of around 10m, I checked the continuity of the wires and also tried another pair of wires.
Could it a problem of length or because the wires are running along the house 230V wiring and may catch some noise from it.
What could be the solution to fix this as I can't move closer the box containing the board?
Thanks

[bobbybeans]

most likely the cable is too long you would have to much voltage drop to do anything
2 things you could try just for diagnosing
if you have a multimeter measure the voltage at the source then again at the destination. the difference is the voltage drop. if you have crappy cable the resistance is high causing more of a drop and the longer it is the more v drop there is
if you don't have a multimeter just try it with a short cable , try it with a 5 cm cable if that works try it with 2 meter then 5 etc, then you can figure out the max length

a solution is either shorter cable most likely
or a booster so it takes the input voltage stores it then amplifies it to go farther not sure how that would interact negatively or positively with a switch

[toffel969]

You could try to use a level shifter to 5v. That voltage might suffice as logic level should be high down to 2v on both 3,3 or 5v afaik

[Shardan]

Hello,

A somewhat simple solution might be an opto coupler.
The 817 (EL-817, KB 817, LTV 817 or similiar) is cheap, just some cents.

opto.jpg (61.46 KiB) Viewed 2860 times

It might be possible that the 680 Ohm resistor is a bit too much, depends on your cabling.
The current through the opto coupler LED should be 5..6 mA, check with a standard digital meter.

If you want to use 3,3 V for the LED the resistor should start with 360 Ohms, maybe less but i'd advise to use 5V if possible.
Higher voltages up to 12 V (on the LED side ONLY!) are possible. Calculate the resistor with (voltage -1,4) / 0,005 = resistor value (Ohm)

This keeps disturbances from the 230V mains out of your GPIO circuit, a "current loop" is more reliable then simply voltage high or low.
On top the opto coupler is less sensitive then a level shifter so spikes from the line won't damage anything.

Anyways, for better reliability you should put a double capacitor between 3,3V and ground, an electrolytic cap of about 220...470 µF
and a ceramic type of about 100... 470 nF.

The internal pull up should be deactivated with this circuit, or you may ommit the 4K7 resistor.

Regards
Shardan