1U Effect Rack With Relay Control

[Aleksi]

Hi guys,

after posting some pics of my 1U rack with four effects in the Pictures thread I got questions about the relay switching I used. So, as suggested, it's best to post about it in a separate thread.



The rack holds four effects (BYOC overdrive, triboost, compressor and envelope filter). The switching is done with DPDT relays (one for each effect) and the relays are controlled by regular DPDT switches in the grey stompbox.



Here's a schematic of two relays (please note, that the series resistors R1, R3 and so on should be omitted). Basically when a switch is used to connect TO_SWITCH and FROM_SWITCH the relay will change state and the signal is then sent effect board and back from it to the next relay. There's also an option for a LED when this is done. I didn't use these LEDS, but they can be used in the rack front panel as indicators.




Here's a quick idea of what's in the foot controller. Each effect/relay has it's own foot switch that simply connects the two signal mentioned above and lights either a green or red LED (depending on state). I used bicolor LEDs, but that doesn't change basically anything.




Overall the schematics are a bit crappy (as I added a bunch of GND and Vcc pads in the last minute), but the board layouts are fine. The regulator board is nothing special, just two 7812 regulators. One feeds the relay and the other one is used for the effect boards. I didn't want both the relays and the effects behind the same regulator, as I had my concerns about the switching noise it might cause. So far the dual regulators have worked well.

These were the first boards I had etched and to be honest the layout has too thin nets and isolation distances for etching. But I will redraw both boards into a single PCB as I will have them redone into a properly manufactured PCB. I'll try to make it more etchable while I'm at it.

I uploaded the Eagle 5.2.0 project file here => http://www.youshare.com/Guest/0d92b3425747d0fe.zip.html

Any questions, just ask!

BR,
Aleksi

[arma61]

what to say ?  THANKS


Good job m8
Armando

[Aleksi]

Thank you and my pleasure! 

BR,
Aleksi

[Aleksi]

Hmmmm.. something's wrong 

Was playing around with that thing, worked OK and plugged the transformer out of the wall socket. Went to have few hours of sleep and when I plug power to thing it powers on OK. However when I connected the control voltage to the FET gate the power supply sagged, indicating a short or similar. Couldn't find the cause for the problem by measuring, so I replaced one of the FETs. Bang, that relay was working again. It seems that all four FETs had "burned" so that they were conducting constantly (even with the gate voltage at 0V) and also seemed to draw the gate to near GND (that might explain the sagging, as the control signals don't have a series resistor at the moment).

I don't get it.... The relay has "discharge" diodes, the BS170 FET should handle the current and the gate voltage is within the datasheet range. And how the hell can all four FETs break down at the same time.

Off to do some thinking.

EDIT: The relay diodes are 4148, although 400x would be better. But all the diodes are OK, so I doubt that is the cause. Although the damage would indicate a problem with relay diode.

[flo]

The BS170 is a MOSFET. It is very sensitive for static voltages. Because of the high impedance of the gate, the FET is easily destroyed by touching the gate for instance via the "from switch" input. Add some protection diodes (or a zener), one from the gate to ground and one from the gate to the power supply. (search SHO booster where the same is done.)
A good alternative is to use a ULN2803 relay driver chip. These are low cost, use darlington transistors and already have snubber diodes for discharging the relays.

[Aleksi]

Hi Flo,

I honestly didn't even think about that, thank you very much! Will add the zeners to schematics and layouts (along with substituting the 4148s with 400x in the layout)

For the current board it might be easiest / cleanest to look if we have any MOSFETs at work that have the builtin protection zener.

BR,
Aleksi

[Ripthorn]

Aleksi, where did you get the empty 1U enclosure?  Was is something you salvaged or a new enclosure that you found somehwere?  This looks great, I think I am going to have to do this.

[Aleksi]

Thanks Ripthorn,

I got it from the german pedal parts supplier Banzai Music (along with other stuff I need - some odd components, LED bezels etc etc)

Here's a link to their rack page http://www.banzaieffects.com/19-Inch-Rack-Enclosures-c-810.html

It actually seems they don't have the 1U rack in black, the price was about 30€ which I considered quite cheap. They're available here in Finland for something like 50-60€.

But I think your best bet is an electronics supplier or eBay.

BR,
Aleksi

[Aleksi]

update relay schematic and layout (includes the BS170 zener protection diode from GND to Gate, snubber diodes changed to 1N400x and added correct series resistors for the gate signals)

http://www.youshare.com/Guest/7fba70990f147f25.zip.html

BR,
Aleksi

[cpnyc23]

Wow - Aleksi - this is exactly the kind of switching that I have been looking for!!!   I'll be the first to admit that I don't have the knowledge to piece together the various relay switching articles that R.G. has been kind enough to post.
I get the concepts but it wasn't the exact schem/layout I was looking to incorporate into my setup.  Modifying it to fit my needs a skill set that I don't have at this point.

Your schem fits what I need and I think it has opened up a few mental doors for me... thank you!!

-chris

[Aleksi]

Thanks Chris! I'm really happy people have found the effect rack helpful with their own projects.

[deaconque]

Aleksi- thanks a lot for this info.  i've been wanting to do a rackmounted effects unit for a while but honestly it's a little over my head.  you just put me one step closer

[turkey101]

+1 on the thanks, Aleksi.   

Good work - please keep us updated on your progress.  This is just at the right level for me - I was (and still am) into R.G.'s stuff, but it was just a little bit of a stretch for my abilities.  This solution appears simple and repeatable - I even breadboarded a basic version to test it out the other day and was happy with the result.

One question - in your completed system do you hear any pop/click when switching effects in and out, or is it totally silent?  My breadboard version was OK, but it was not mounted in an RF-free enclosure and I was only using a couple of hastily rigged up noisy power supplies.  I suppose I'm really asking how does it sound at performance volume? 

Thanks again for sharing,

turkey101

[burningman]

Very cool project ! I was wondering where you got your rack chassis from?

[turkey101]

"Very cool project ! I was wondering where you got your rack chassis from?"

See reply #7 from Aleksi:

http://www.banzaieffects.com/19-Inch-Rack-Enclosures-c-810.html

turkey101

[Aleksi]

Turkey, Burningman and Deaconque: Thank you!

Well, I would say that thing makes about the same noise as a normal 3PDT with proper pull down resistors at the effect inputs/outputs and proper power supply filtering (practically none?). I play at home with quite low volumes, but I can't hear any pops.

Although, I think there might be a chance of hearing something if you have the guitar signal cables physically close to the relays... I think the signal wire could pick up some noise from the relay's coil when you change states. But I haven't that problem and my signal cables are about ~2cm on the PCB from the relays.

My next job is to slighly mod the caps on the Tubescreamer (change the lowpass filter higher after the clipping op-amp) and maybe change some freq caps on the envelope filter too. Other than that it's done. So trying to decide what's next. I kind of have my eyes set on those Weber tube amp kits, but will see. Although a 1U rack with all tube based effects would be nice 

BR,
Aleksi

[Aleksi]

Now it's done. Update the TS and Envelope Filter caps. I thought the TS808 sounded really muddy with the stock low pass filter after the clipping, really needed the tone pot to be set to fully clockwise. Now with the low pass filter corner frequency higher the tone pot is usable through the entire range. Altered the sound quite a bit too, now it sounds like I expected a Tubescreamer to sound.

[.Mike]

Hi Aleksi. I have a few questions about relay switching, and thought that maybe you or someone else could help. I've read the switching and relay articles at GEO, and I've read quite a bit at other sites. I think I just need clarification.

My question is: Why use a transistor to switch the relay on and off instead of just using a switch?

From what I have gathered, the reason is to slow down the activation/deactivation of the relay a bit. This slowdown is necessary to prevent capacitance between the relay coil and the actual switched contacts on the relay, and that doing this helps prevent clicks or pops when the relay is switched. Is this correct?

Last, why use a Mosfet? I've read a few documents like this one [pdf] that use regular NPN transistors. Is there any benefit in using a Mosfet, or will a regular transistor do the job just as well?


The reason I am asking is because I'm building a multi-effects unit kind of like yours, but with 8-10 effects, and I'm putting it in a Hammond 1456 sloped enclosure. Each effect will have it's own in/out jacks wired up like the Jimi-In-A-Box design so I can repatch the order of the effects. The switches will be towards the front of the enclosure, and the pots and circuit boards will be mounted near the back. That means that for each effect, the signal would come in the jack and go to the switch (11 inches), and return from the switch to the output jack (11 more inches). If the effect is engaged, the signal has to travel to the circuit board (8 inches) and back to the switch (8 more inches). So I'm looking at 38 inches of cabling per effect, and if I have 10 effects I am looking at 32 feet of cabling. I could easily mount the circuit boards right by the switches to shave 16 inches of cabling off each effect, but since my potentiometers are near the back, I would then have to add 16 inches of cabling for each potentiometer-- not a good idea. So the plan is to have dpdt switches up front run the LEDs and the relays that are near the back, between the jacks and the circuit boards. It also comes out cheaper to use relays and DPDT switches than to use 3PDT switches, especially when you consider the cost for all that shielded wire.

Thanks for any input you can provide!

Mike

[flo]

You can control the relay directly via a 1PDT (or 2PDT) switch, no need for a transistor.
But dont' ommit the snubber diode!

[cpnyc23]

Slightly off topic but what mods did you do to the Tube Screamer?  I'm assuming you changed the cap values - just wondering what you changed.

Thanks!
-chris