Directly driving reed switches

Greets,

I'm looking to drive up to four 5 volt reed switches. The switch I've selected requires 25ma, which I believe is within the limits of a digital I/O output.

What says the collective wisdom? Go with that, or add the complexity of a 2N2222 to drive the reed switch directly? I picked reed switches because they have low current -- I'm hoping to avoid adding a transistor to the circuit.

jfhaugh:
Greets,

I’m looking to drive up to four 5 volt reed switches. The switch I’ve selected requires 25ma, which I believe is within the limits of a digital I/O output.

What says the collective wisdom? Go with that, or add the complexity of a 2N2222 to drive the reed switch directly? I picked reed switches because they have low current – I’m hoping to avoid adding a transistor to the circuit.

It’s doable without using switching transistors. Be sure to wire reversed biased diodes right across the relay coils. Also be aware that as well as individual pin current limits, there are total current drawn from a 8 bit port and total current drawn from Vcc and ground pins. The later is I think 200ma, you will have to look up the port limitation in a datasheet or just spread the output pins between ports. I’ve driven such reed relays directly from output pins without issues, but not five, but again I think it can still be done and stay within limits. Keep track of what other ‘loads’ you may be driving with other output pins. There are 16 pin DIP packaged driver ICs that can be used to drive your five relays without any concern on pin loading if you want to go that way.

Lefty

Are you a lefty, Lefty?

Sinistrality rules :)

I'll probably go to a relay driver IC whenever I re-layout the PCB after removing the Arduino that's soldered smack-dab in the middle of the board. I have about 10 square inches and if I can get another inch or two back, I'll have all sorts of room. I think a four layer board will help some as well -- lots of stuff to do before I add yet another IC to the board -- the four reed switches and terminal blocks are gobbling up space as it is.

Are you a lefty, Lefty?

Sinistrality rules

Yep, southpaw all the way.

Good luck on your project.

Lefty