Switching out a servo for a relay in a project

I am trying to take a project ( Security System and Access Control With Arduino and RFID : 6 Steps - Instructables ) and modify it so that rather than control a servo, it would close a completely external circuit. By closing the circuit, this will allow me to control my garage door. I think that I could do that with a relay, but I have no idea what type. If you know what relay I would need, or have an idea other than a relay, please let me know, and try to include a link. Thanks!

An Arduino I/I pin can only source or sink about 10mA safely and there are very few relays that can be operated with such small currents. In turn these tiny relays are only capable of switching very small currents. So I suggest that you use an electronic driver circuit to increase the current available to operate the relay. There are various relay and motor driver shields and add-on boards available, and you can also get Arduino clones with integral H-bridge driver circuits which are capable of providing an Amp or so. That's enough to operate quite a substantial relay.

I suggest you work back from the circuit you're trying to switch, understand the voltage and current that you're trying to switch and spec a relay capable of switching that. Then look for an Arduino driver circuit capable of operating the relay.

If the circuit you're switching involves anything over about 50V I suggest that rather that you consider buying a relay assembly which is designed to maintain safe separation between the high and low voltage parts of the circuit. There is a big difference between making something that works, and making something that is failsafe. If you ever get a hardware failure which exposes the low voltage circuit to high voltage it could be extremely dangerous.

First of all, thanks for the response Peter. The circuit that controls the garage door only operates at 12 Volts, so I dont know if that makes a difference. And also, the project had a power and ground running to the servo, and a 3rd wire going to the arduino for control; Do they make any relays that could use the 5V power cable running to it, and a different wire for controlling it? Thanks

you can easily power a transistor with the Arduino. an PN2222 or some such.

the Arduino needs 5V DC or 9-12VDC depending on how you feed it.
you can get 5V coils in relays, or 12V coils. all should work.

the Transistor takes the low power feed and controls the higher power.