Can anyone recommend some DPDT relays for an arduino based programmable looper?

[soggybag]

I got the bug to build a programmable looper box. I figure I need some DPDT relays that will work from 5v. There is a lot of these out there. Can recommend a suitable part?

Has anyone done this here?

[ElectricDruid]

I've done some experiments along these lines, and I gathered up a little pile of possible candidate relays to test.

One thing I discovered was not to get too hung up about "5V" relays. Quite a few of them need so much coil current that you can't drive them directly from a uP, or it's at least rather borderline. And if you're going to need a driver transistor anyway, then the 5V switching is no longer so important.



One other thing to note is that these relays are all small, but some are significantly smaller.

[soggybag]

So latching relays would hold their state after powering down? If that's the case these might be a problem with a micro controller? Better for pedal switching?

[ElectricDruid]

So latching relays would hold their state after powering down?

Yes, exactly.

If that's the case these might be a problem with a micro controller?

No, not really. Obviously the uP won't know what state the relay is in at power-on, so it'll have to send it an initialisation pulse to put it in a known state. From then on, it can keep track.

The advantage of latching relays is that they don't use power *except* when being switched, so the overall current draw is probably much lower. For non-latching relays, any that are on are going to be drawing the full coil current, which adds up pretty fast. Imagine an 8-channel looper with everything switched on: 8 x (say) 25mA = 200mA. That's a big current draw in pedalworld! Clearly it's nothing you can't handle, but you need to plan for it.

The advanatge of the non-latching relays is the simplicity - the coil only needs a single pin to control it, and the state of that pin reflects the state, so you don't need another pin to give that indication. A latching relay might need three pins, a Set pin, a Reset pin, and another to indicate the state.

[soggybag]

Jameco has many relays, some of the items on the list above. What should I be looking for to find a suitable part?

- DPDT
- latching or non-latching, sounds like it could be either based on design decisions
- coil voltage 5v?

[ElectricDruid]

Jameco has many relays, some of the items on the list above. What should I be looking for to find a suitable part?

- DPDT
- latching or non-latching, sounds like it could be either based on design decisions
- coil voltage 5v?

DPDT - Yes.
Latching or non-latching - could be either based on design decisions, that's up to you.
Coil voltage 5V - only worthwhile if it's significantly under 20/25mA in my view, which might allow you to avoid driver transistors. Otherwise, you could equally well use 9V coil, it wouldn't matter. Or 12V if that's what power supply you have available.

In short, *any* of these choices can be made to work. What relays can you get cheaply, and how many uP pins have you got available to drive how many relays?

[R.G.]

I've used all of the ones E.D. recommends. My preference is the Panasonic TQ series if you can get them. I've used the Fujitsu types in a production run, and it worked fine.

E.D.'s comments on latching/non are on target. I prefer the two-coil latching types.

I also like the ULN2004 or ULN2804 darlington arrays that give you 7 or 8 darlington drivers in one chip for about 0.50.

On nice thing about latching types is that you can cut down on switching noise by providing a resistor-electrolytic "power bucket" for the relay and driver when it's switched. This can be relatively isolated from the power on the PCB by resistors and charge slowly (200-500ms) between switching, making the pulse loading on power and ground so slow that switching transients are almost nonexistent.

[PRR]

I also like the ULN2004

Golly. That's another chip that won't die. I blew-up one by 1980 (maybe 2003) and it wasn't new then.

[soggybag]

Thanks everyone, that was super helpful. I order some TQ2-5v relays.