Relay Bypass?

[PeterTheRed]

Greetings,

I'm embarking on my first 'guitar-oriented-analog-signal-processor' project (an envelope-controlled filter following Anderton and Keen's published designs), and came across this forum while googling for info.  Seems like a lot of wisdom in these posts! 

So, I thought I'd ask a noob question about using relays for bypass.  Picturing a SPST switch to control current through the coil of a DPDT relay, wired in the traditional true-bypass configuration, with an indicator LED driven by the coil current somehow.  DPDT Stomp switches seem expensive and hard to find, plus I'm considering a future remote-switching scheme.  I'd prefer to send a control signal from rack-to-floorboard-and-back to control the relay coil, rather than send single-ended, high-Z audio through a remote bypass switch.

In looking around online for examples of bypass switching circuits, I've seen no mention of relays though.  I figure I'm either not using the right search queries, or there's a good reason why people aren't doing it.  So, I welcome any thoughts you can offer!

[R.G.]

Read the series of articles on bypassing, audio relays, latching relays, and programmable footswitchers at geofex.com. It's covered in some depth.

Relays suffer from needing either constant power to hold the coil in one position, or modestly complex circuits to provide the pulses for latching relays, and passing noise from the high voltage changes on the coil to the audio path.

However, DPDT and even 3PDT footswitches are not all that expensive or hard to find these days, even though they once were.

[smallbearelec]

Relays or stomp switches?

You can go either way, and with either of various hardware/firmware approaches. The simplest, of course are DPDT or 3PDT latching stomp switches. Refer to any DIY site for pricing and choices. Whether you want to cobble your own hardware setup using R. G.'s ideas or use pre-built modules, you'll want a reliable momentary stomp switch. This one is commonly used:

http://www.smallbearelec.com/servlet/Detail?no=672

If you want to look at hardware/firmware modules that are purpose-designed for signal-switching to and from a pedal array, check out the offerings from Molten Voltage:

http://www.smallbearelec.com/servlet/Categories?category=Molten+Voltage%2FPedalSync

Regards
SD

[PeterTheRed]

Thanks R.G. and smallbear. 

I take it geofex is your brainchild, R.G.?  Impressive collection, well explained.  You apparently designed and documented ten years ago many of the ideas I'm just now putting together in my head  .  The circuit page called "Remote Footswitch Bypass with Relays" captures what I had in mind for remote-switched relay bypass, including the control logic that I didn't plan to start thinking about until after I had one or more functional FX boards to rack up. 

Maybe I'll try to design the board to accomodate either a relay or stomp switch...

[blackieNYC]

Different approach for a pedal with too many stomp switches already. This ought to work, yes?
A pedal with multiple stomps (I have one with four, and no room for a TB switch amongst them). Diodes come from each of the 4 LEDs of the stompswitches to a relay coil. If any LED is on, the relay is pulled to the Normal Open and the effects are in the path.  If all LEDs are off, the relay closes the NC and bypassed audio passes.
http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/image.jpg.html?g2_imageViewsIndex=0