Connect Gate Remote Board Directly to Arduino...?

Hi All,

When it comes to specifics of electronics I'm a bit of a noob, and as such, looking for a little help / guidance.

I've got an arduino uno with ethernet shield, reed sensor, and relay module all connected up and working great for opening / closing my garage door, detecting open/close status, etc.

What I would like to do next, is, given there is also a security gate just outside my complex, I would like to take an extra remote that I have, and connect the remote's board directly to my arduino uno to mimic the button press, so that I can issue a gate toggle programtically. This seems to be the most "eloquent" way for me to pull this off, i.e, I can't monkey with the gate operator to install another arduino with wireless connectivity to trip a relay to open the gate, and I think it is kludgy to try and rig up a mechanical device to actually press the button on the remote, so this is the direction I would like to head.

However, given my electronics noobiness, I'm not 100% clear how I would go about interfacing the arduino with the remote board. The board has the standard momentary switch on it you typically see in arduino starter kits, and I hooked up a multimeter to the varying 2 points on the board where the button is connected, and when selecting various alternate points on the board, w/o pushing the button, I get what appears to be about 4V DC (I didn't expect to see anything until I actually pushed the bottom, btw).

Any kickstarter tips, points in the right direction, etc, would be greatly appreciated.


I've done this before open the case of the remote and find if the buttons are high going low most time that the case I then soldered a NPN most any work I had 2n3904

solder the collector to the high side and emitter to the ground side put a 1k resistor on the base tie the ground of the remote to the ground of your arduino and hook the 1K to a digital pin.

Thanks be80be for the reply, this helps IMMENSELY!

I think, based on my multimeter readings that it is definitely going high to low, correct? I.E, I see the current when testing 2 pins w/o a button press, and it goes away when holding the button down.

Keeping in mind my noob status, how did I determine which side where the button connects is high versus ground? Once I can figure that out, I think I'm good to go with your instructions!

You got her right just use the transistor as a switch you shouldn't need a resistor on the collector but I would look see what's on your button

Okay, now I'm confused.

Originally you said to put a 1k resistor between the base of the NPN and the digital i/o on the arduino. But in the most recent reply you said I shouldn't need a resistor on the collector...?

And I'm still hoping for some guidance on understanding which is high side side versus ground side on the remote board.


Let me show you what i’m saying


So, to confirm, in your diagram, your positive from the board is connected to the NPN collector.

Ground from the board goes to the NPN emitter.

1k resistor between base and digital i/o of arduino.

What about ground from board to arduino? I’m realizing in your original reply you mentioned “tie ground of remote to ground of arduino”, but you also mentioned “solder emitter to ground side”. I don’t see this in the drawing.

Sorry for so many questions. Just want to make sure I actually understand what I’m doing before I do it :slight_smile:

EDIT: Attached, for your reference, is a photo of the remote board with the 4 pins for the momentary button circled.

Thanks for your help.

Yours is a lot easier to figure out then My board you can see thee parts. lol Just place the NPN pins collector and emitter to ground and hook the base to the arduino with a resistor 1k or higher and you should be able to make it work just as the button did.

Your button is easy if you test it one side is high and one side should be ground just solder the collector to the high side of the button and solder the emitter to the low side of the button and you'll be good to go

Okay, I've essentially re-drawn on my own board what you provided previously, but I'm just covering my bases here.

Here's my proposed setup:

If I hold the board to the light, I can see the red lead headed to the bottom right pin in the photo above, so this should tell me (even w/o a multimeter, which I've tested as well), that this is my "high" and therefore, my solder point for the NPN collector.

One of the other pins that doesn't read out any volts is going to be my low (ground), and the NPN Emitter gets soldered to it.

NPN base is getting connected to one of the Arduino I/O's with a resistor between.

All good so far, right (hopefully)?

Under such a setup, I'm not using the Arduino itself to supply any power (I think where some of my initial confusion came from between us), and as such, the 9V battery remains in my setup to supply the needed power. Correct?

Yep you need power Just make sure you tie the grounds to the remote Ground to the ground on the arduino need to be tied.

And you don't hook the emitter to any thing but the switch low side and it will work

Here I made a video showing the hack. I'm toggling the npn with the battery for now

Another easy way to do this is to use an opto-isolator like 4N35 or PC817.

Take a look on the ArduinoInfo.Info WIKI: and scroll to OPTO-ELECTRONIC IC'S

These are less than 50 cents each and you don't have to worry about "What is grounded"...


So, looking at the PDF for the 4N25, I would just use connectors 4,5,6, with 6, base, going to my Arduino I/O. Do I still need a resistor between base and the Arduino?

Connections 1,2,3 would not be used for anything?


The way the 4N35 works is you turn the LED inside it on and that turns on the transistor.

So pins 1 and 2 are an LED. You need a series current limiting resistor, say 180 to 330 ohms or so and drive it from arduino just like any LED.

That turns the transistor ON.. So you can use pin 5 (+) and pin 4 (-) to connect where the switch is / was.

Make sense??

Thanks. I’m getting there. Bear with me, I’m very “green” with the electronics side.

In this setup, then, I was clearly off on my thinking, as pin 6 is unused. I’m good, per your reply on 4 & 5.

The only thing that confused me in your statement is that pins 1 AND 2 are for the LED. Is that 1 for the I/O slot, the other for ground from the Arduino? And, the resistor is between the Arduino I/O slot and Pin 1, right?

Assuming I’m on the same page, it would seem to make sense to just go with the PC817, since it has just the 4 pins I need and nothing else.

Easiest way is:

220 ohm (approx) resistor from Arduino I/O Pin to Pin 1 4N35 4N35 Pin 2 to ground. ..just like any LED...

PC817 should be fine... this is not a critical application.

Perfect. Thanks for the reply. If I have any troubles, in the words of Arnold, I'll be back... :stuck_out_tongue_closed_eyes:

Stupid question. Are the pins on the opto-isolator marked so you know which is which?

Look in the PDF that is linked on the WIKI. Pin 1 is marked.

Pin numbers go Counter-Clockwise from pin 1. As seen from the TOP.

This is because people are used to going clockwise with pin numbers. On vacuum tubes. From the BOTTOM of the socket. Inside the chassis where you are soldering the filament transformer wires etc etc. In 1949. Everybody knows that 8)

terryking228: Look in the PDF that is linked on the WIKI. Pin 1 is marked.

Thanks, I saw that in the PDF, but didn't know where to go from there.

terryking228: Everybody knows that 8)

Everybody but me :grin:

What I don't under stand is why you changed from transistor to Logic Output Optoisolators

If you think it works better you can still hook it up backward and 2 I showed you clearly how this worked with a transistor.

be80be: What I don't under stand is why you changed from transistor to Logic Output Optoisolators

If you think it works better you can still hook it up backward and 2 I showed you clearly how this worked with a transistor.

Actually, I'll probably try it both ways just to learn. I like the simplicity of the opt-isolator though, as it appears I don't have to worry but any other ground connections, etc. It just seems simpler to me. Certainly appreciate your help as well. I already have 2 NPN transistors (1 for backup, lol), so they will probably get first shot.