Using 3.3v Arduino pin "output" to ground 5v

I am aiming to operate my automatic garage door using an Arduino pro mini, 3.3v.

The existing hard wired switch to “pulse” the garage door open/close operates on 5v. The switch grounds the signal wire to make it happen.

I am wondering if there is any issues in connecting the 5v signal wire to a digital pin, and then pulsing the output to low. I can see there are two potential approaches of doing this.

Approach 1:

// in setup
digitalWrite(PINx, HIGH);
pinMode(PINx, OUTPUT)

// when I need to pulse the door
digitalWrite(PINx, LOW);
delay(250);
digitalWrite(PINx, HIGH);

Approach 2:

// in setup
pinMode(PINx, INPUT);
digitalWrite(PINx, LOW);

// when I need to pulse the door
pinMode(PINx, OUTPUT);
delay(250);
pinMode(PINx, INPUT);

Some of the issues that may be encountered: • 5v going through 3.3v Arduino causes damage. • Too much current - I believe there are already resistors in the existing door circuitry but will measure the current passing through the switch to check. • Arduino grounds input during boot up, which could inadvertently open the door.

The alternatives would be to use either transistors or relays, however I am trying to avoid additional components if possible.

HI,

I have a couple of thoughts:

  • You cannot apply 5V to the I/O pins of a 3.3V Nano. The 5v will feed through some internal protection diodes and raise the power to the processor to 5V. This may or may not damage your Nano (not sure). I have a pro mini that runs on either 5 or 3.3 V.
  • Some older garage door openers use AC voltage for the controls. Perhaps the newer one are DC. You should check.
  • When connecting two systems together you must [u]know[/u] the grounds are common. If not you should use some isolation, Relay, Opto Isolator, maybe just a Mosfet.
  • I believe it is best (required) to have the drive to the garage switch remain Off when the arduino is not powered and when powering up.

Another thought, if you are automating your door in some way, it might be useful if the arduino knew if the door was open or closed. I used a simple alarm type magnetic switch, mounted at the top of the door.

The safest thing would be a relay. I assume your garage door opener has a wired/manual momentary pushbutton switch? A relay is just an electrically-isolated electrically-operated switch and you can wire the normally-open relay contacts in parallel with the (normally open) switch contacts. Then you have “or logic” where it can be operated with the relay or the switch.

You will need a driver for the relay coil.

Or, [u]here[/u] is an example of a transistor being used as “pull down” switch. You can probably leave out RL, since you say the voltage is already pulled-up to 5V. Rin can be somewhere around 1K. Any NPN transistor should work since you’ll have low voltage and (hopefully) low current. The transistor will invert the signal so writing a logic-high will turn-on the transistor, pulling the collector to ground.

For operating 5V interfaces a 5V Arduino is the right tool to use.

Thanks for your responses. To add a bit more detail to this project, I am using the Arduino combined with RFM69 (wireless comms) and BME280 (temperature) chips, which both operate at 3.3v, hence the 3.3v Arduino choice. The aim is to monitor the garage temperature, light (on or off) and doors, and also control the garage door.

The garage door electrical panel operates on numerous voltages: 33v AC, 5v DC and 2.5v DC. I have found the microcontroller pins that are linked to the limit switches (2.5v), which I have hacked to to input into the Arduino and determine if the door is open or closed (I have tested this and it works). I am powering the Arduino off the 5v circuit so it will only operate when the door is powered, which is good (and also means that there is a common ground).

Fortunately the switch (momentary) only pulls 1mA current so no need for additional resistors. However it is clear that I cannot input 5v directly to the 3.3v Arduino pins.

I’m leaning towards using transistors, I will just need to find one (although this shouldn’t be too hard from what I understand).

Connecting a 3.3V powered arduino to a 5V signal could blow the pin in question, or even fry the whole chip. Adding a current limiting resistor can reduce/eliminate the risk, but here the circuit you are controlling probably needs a certain minimum current. An opto-isolator is an easy way to do things if the current is small anyway, then you don't need common grounds. Or an NPN as an open-collector inverter. For low current a 1k resistor from arduino pin to transistor base, emitter to ground, collector to your door opener signal.

MOSFET as a switch

The BME280 can interface to a 5V Arduino with no issues, so long as it’s powered from 3.3V and the pullup resistors on SDA and SCL are also powered from 3.3V. I don’t know about the RFM69 though. I have yet to use one of those.