Using a 5v relay as a momentary switch

[Bloodbath]

My basic need is to actuate a 5v DPDT relay with a spst momentary switch so when the spst is pressed, the relay is in one state and when the spst is let go the relay returns to the other state.
I know very little about relays but something tells me it can't be as simple as supplying voltage across the coil. Is there any particular type of relay I would need for this. I'm assuming it would be non-latching. Supply voltage would be 9v so I would probably need a 5v regulator.

I know I could use a momentary DPDT switch but size is an issue and the spst switch is the right size.

[Jazz P Bass]

From what you have described, go for it.
A relay is nothing more than a coil with switches.
The simplest circuit is as you have laid out.
Send the voltage to the coil & groung the other side for it to activate.
Release the groung & the relay will 'drop out'.
If you want to get picky, you could add a diode across the relay coil to suck up any back emf that the coil will produce when it drops out.
The pushbutton switch that you use must be able to handle the current that will flow through it.
That will depend on the resistance of the relay coil & the supply voltage. (I=V/R)

[eschertron]

I wouldn't bother with a solid-state regulator, use a couple of resistors to drop the voltage to the coil (be sure to add the resistance of the coil itself into the calculation). If your voltage supply is a 9v battery, size the circuit to supply a *high enough* current to the coil to ensure it picks up and a *low enough* current to keep the battery from discharging any faster than it needs to. The rating for the relay coil should be available from the supplier or manufacturer. Now that I think about it, selecting the relay based on the coil's DC resistance or current requirements might be an integral part of the design process. ...now that I'm really thinkjing about it, if you haven't already decided on a relay, why not find a 9v model?

If you want help with the design, publish the specs for the relay (and any other tech data you think is relevant) here in this thread.

[Bloodbath]

Thanks guys. I haven't picked out a relay yet as I'm not 100% sure what is needed yet. Not really sure why I picked 5v. I guess I just didn't want to pick something so close to the main voltage supply for fear of it not working if the voltage fell much below 9v as the battery dies off.

Ok. I'll start looking at relays and try to decide on one and order it. So would you guys agree that I should get a single coil, non-latching relay?

[Enzo]

Why don't you tell us exactly what you are doing, and in what. Then we can make more meaningful suggestions.

Some thoughts:
If you want momentary action, you don;t remotely want a latching relay. Besides, latching relays can be made by controlling regular momentary ones with a latching circuit. But you don't need that anyway.

You want the lowest current coil you can find, if you use a relay.

WHAT are you switching? If it is signal path, you can probably do the same thing a LOT cheaper with a few JFETs, or a CMOS IC like 4051/2/3 or 4066. If it is an LED, a simple transistor works.


If your relay drew 30ma and your circuit drew 60ma, consider the relay would increase battery draw by 50%, or another way, it reduces battery life by 1/3.

[Bloodbath]

I'm certainly open to other switching. The most important part is that I need the a momentary spst switch to activate it.
The circuit is for a wah pedal. I'm trying to help a friend of mine work on this weird switching thing he wants to do.
He wants to make it so when you step on the wah treadle, the wah is activated by stepping on the spst switch. When he takes his foot off, the wah goes into bypass. He's complicating things more by wanting a toggle to use the normal wah bypass switch also but I can deal with that. I just need to get the "step on" part of it figured out. I worked it out using a dpdt for the step on part but finding a momentary dpdt that fits on the treadle is a little hard to come by and finding one that I don't think would break is near impossible.

Edit:
Well I've been looking at relays and what I think I need if I go the relay route is a single side stable non-latching relay. I'm still up for other suggestions though. The relay, if I can just run voltage to it and use a spst switch to actuate it, just seems the simple way to go.

[dgb1958]

Why not a pressure sensitive mat on wahwah tread to do your switching ?

[Bloodbath]

I've seen pressure plates/pads as Sparkfun or Adafruit and have always wanted to do a project with something like that. I've only seen a resistive pad. Are there other kinds? I don't want to complicate things more by having to convert signals to activate a switch though.

I did order some 9v single side stable/non latching relays to play around with. I'm assuming since I got the non latching, when I apply voltage to it, it should switch and when I remove voltage, it should switch back to it's initial state. Right? I shouldn't have to have anything else to make it work?

[Jazz P Bass]

Yup.
Apply the voltage & the relay will turn on.
Remove the voltage & the relay will turn off.

[Bloodbath]

Thanks. I kind of figured. I am interested in any other ideas for doing this though. The relay just seems the easiest/simplest.

[SHORT EDDY]

Are SSR's (solid state relay) used any in audio applications?
Trying to learn where and why they wouldn't be a preferred replacement for mechanical relays.

[Steve Conner]

Yes. Special MOSFET SSRs are required to handle analog signals, though. They are close cousins to analog switches like the 4066, except optoisolated. (The device consists of a pair of MOSFETs or a JFET, with the gate charged by a tiny solar cell illuminated by a LED.)

These SSRs are somewhat expensive, and consumer audio gadgets are very cost sensitive. So it's more common to use a regular relay where isolation between coil and contacts is required, and an analog switch (either IC or JFET) where it isn't. For example, Boss and Ibanez pedals use JFET switching for the bypass mode. http://www.geofex.com/Article_Folders/bosstech.pdf Snobs look down their noses at this as it's not "true bypass", but it has its advantages.

[SHORT EDDY]

Yes. Special MOSFET SSRs are required to handle analog signals, though. They are close cousins to analog switches like the 4066, except optoisolated. (The device consists of a pair of MOSFETs or a JFET, with the gate charged by a tiny solar cell illuminated by a LED.)

These SSRs are somewhat expensive, and consumer audio gadgets are very cost sensitive. So it's more common to use a regular relay where isolation between coil and contacts is required, and an analog switch (either IC or JFET) where it isn't. For example, Boss and Ibanez pedals use JFET switching for the bypass mode. http://www.geofex.com/Article_Folders/bosstech.pdf Snobs look down their noses at this as it's not "true bypass", but it has its advantages.

Thank you for the valuable info sir! I would assume that you would always want to use dual contacts per channel when using a standard relay in an audio path?

[Enzo]

Ahem... Hey Blood, I myself am not happy wanting foot pressure on a unit that requires also foot motion. COnsider an optocoupler? Mount an LED off of one side, and a photocell on the other. Then a basic circuit would detect the presence of a foot blocking the light, and control your relay or whatever.

[Bloodbath]

Yeah, you are probably right. I have some samples coming of some sensitive pressure pads (only require 2-20lbs of pressure) to mess around with but I probably won't use them for this project. I thought about an LDR-type setup but I'm really thinking the relay and spst switch are the easiest so far. I have some relays on the way so I'm going to mess around with them and see how that goes.