Door Entry - Using Arduino as an electronic switch

Hi guys,

I've very new to Arduino, and although I have a coding background, hardware is a bit of mystery to me...

I've completed some small projects to light LEDs, start motors, or read keypad input, but now I want to tackle something a little bigger and more interesting. I've therefore decided to hack my apartment door entry system!

The existing phone input system has a button that buzzes people in the main door, by simply closing an existing circuit. I can plug wires in either side of the button and when I touch them the door opens...

What I would like to do is use Arduino to close the circuit and need some help with the fundamentals before I use my ethernet shield, bluetooth or RFIDs to enhance it from there... See photo below (it's sideways but you get the idea!)

So I have some questions....

1.) I'm guessing the door entry is pretty low power, but do I need a relay? How can I test this? I have a multimeter and I touched either side of the button at the DCV 20 setting and the display fluctuates between 3 and 10...

2.) How to I actually use Arduino to close the circuit? Because the system already has it's own power I'm not exactly sure how to wire it up. I guess I want a simple button to begin with to close/open the circuit, but I will eventually replace this.

Any help getting started would be much appreciated!

Cheers,

K

Edited title to better indicate contents, and added photo...

I've added a photo to my post to illustrate.... This setup works when I push the button on the breadboard. Now I'd like to use the Arduino to control that switch.

I'm thinking I simply do it like this only without wiring the power to/from the Arduino? Would that work without blowing anything up? :o

If so, how could then I control the button state electronically from the Arduino - from serial input for example?

Would this work (where the loose ends go to the external power of the door entry system)?

I'm itching to get started on this, so thanks in advance!

Well, since you have already handled a project with LEDs, it would probably be best to repalce the LED with the base of a transistor, and drive a relay with the transistor. A separate power source for the arduino, (USB is fine), and a small 5V relay with a coil current of less then 100mA would allow you to use a general purpose transistor such as a 2n2222. (don't forget a diode across the relay coil) This would provide complete isolation from the existing door control system, and allow you to play without damaging infrastructure in the building. You would use the N.O. contact on the relay and common. DO NOT try to drive a relay directly from an arduino pin!

Google driving a relay from arduino and all will be well.

If you are going to add wi-fi or bluetooth shields, be aware you may need to provide a stronger power supply than built onto the arduino itself.

-fab

Cheers Fab!

So I do need a relay - and I can switch the relay on and off from the Arduino and that will effectively open and close the door circuit. The Arduino will power control of the relay, and the door entry system will be isolated and will continue to use it's own power.

I have a relay on the way anyway... I will order the transistor now.

I'll do some more research and will get back to you if I need more help if that's ok!

Thanks again for your reply... :slight_smile: