GPIO 02 and 03 seem always high when used as an input (with pull down resistors attached)

Question

Hopefully this question isn't too silly on my part... but I am very new to this - so I am very likely missing something obvious :)

So, the background... I have relays which I want to control them with switch inputs... I am able to control the relays... no problem... I am able to pick up most of the switch inputs - but not all...

For some reason GPIO2 (pin3) and GPIO3 (pin5) always show high...

I do have pull-down resistors installed on my breadboard... and if I move my testing switches to any of the other pins - I can see switch state on / off.

With GPIO2 and GPIO3, the switch always shows high - even when the switch is open...

Is there anything else I can check / set / look out for?

Answer 1

GPIO 2 and 3 are special in that they have hard wired 1k8 pull-up resistors to 3V3 fitted on board. That's to allow their use as I2C bus 1 (I2C requires pull-ups for SDA/SCL for proper bus operation).

Use any other GPIO.

Answer 2

For some reason GPIO2 (pin3) and GPIO3 (pin5) always show high...

Those pins have pull-ups built into the PCB. you must use pull-down resistors less than about 1k6 in order to overcome these. Two options:

1) Don't use them. You have about 30 others to chose from.

2) Configure your circuit for inverted logic such that the closed switch grounds the input (logic 0 = closed logic 1 = open).

I've skipped the obvious questions like: Have you actually specifically configured them as inputs?

Incidentally, it can be useful to know what language you're using. Then i could give examples.