rfid door control retrofit

Forum 2005-2010 (read only) Hardware Interfacing
1

I currently have a system that controls a solenoid that will unlock a door via the latch in the door frame. This system is 24vdc and has a keypad. This was purchased and installed professionally many years ago. I want to add an arduino rfid reader to the system to allow the system to either be opened with the keypad or an rfid card.

So I have a few questions
-I have read about relays quite a bit, and I think I have it figured out. As The system is 24vdc and~30ma (according to the manual). Do I need a relay to control this or can I just use a tip120?
-Assuming I get the relay working correctly, I am going to have the arduino possibly energizing the door solenoid, and then (not likely) the keypad will also energize the door solenoid. What will happen if both are energizing it simultaneously? What do I have to do to keep it safe?

thanks

2

Do I need a relay to control this or can I just use a tip120?

You can just use a transistor.

What will happen if both are energizing it simultaneously

It depends on how your system is wired up, it could be it will be fine if the switch is just grounding a point and your transistor is doing the same.