A Switching Scheme [documentation]

[trixdropd]

You'll all know my hatred of the crappy TPDT and similar footswitches.

I still have customers who insist on "true bypass" and a "solid copper path bypass". Buffered bypass is almost always better, but just to satisfy these (very wealthy / influential) musicians, I frequently use bistable relays. Many players are deeply distrustful of mains powered effects, and (perversely) demand batteries that last for many hours. I also have a large supply of really sturdy SPDT footswitches (like the old "Carling" ones). To overcome the deficiencies of the switches (they crackle and can't do proper bypass), I've been using the following circuit. The relay is easily available from all the major suppliers (including many Ebay ones) and if used in this circuit, draws micro-amps!

Ultra-low_current_relay.png

The relay is slightly unusual - it has two stable positions, and is set and reset by small current pulses sent in one direction then the other through the coil. The transistors I used in the originals were BC183L for the NPN and BC213L for the PNP. You can also use 2SC1815 and 2SA733 or BC550 and BC560. The last batch of them had blue LEDs wired across the anode of the diode to ground through a 8k2 resistor, and used 2N3703 (PNP) and 2N3706 (NPN), and were made to fit on to the back of the footswitch.

The circuit gives six big advantages:

1. Hermetically sealed gold contacts - clean and quiet switching.
2. Additional relays can be put in parallel with the first one. You might have to increase the 100µF to 220 µF, but the switching will be entirely reliable.
3. Draws minute current - the whole of the battery power will be used to power your effect and the indicator LED!
4. Passes the "disconnected battery" "True Bypass" test.
5. Costs less than a good quality DPDT footswitch.
6. Allows simple connection of an indicator LED.

A couple questions...

How can this circuit be "reversed" in that instead of the ground being the switch, i want the ground always connected and the +v to be my switch, this way I can make a 2 loop box with a trs connector with a common ground. In this current iteration I end up with a common + on the sleeve which is less than ideal. I'd also like the led indicator.

If that can be achieved, how about my "remote" switch having an led as well and being powered only with the 2 switch wires. Can it be done with this setup somehow??

What I mean is I want The POSITIVE to stay connected in the circuit. I want to use 2 of these circuits independently. I want to use a TRS jack with the sleeve ground, and the tip and ring completing ground to their respective circuits, all with an led, both on the relay board and preferably on the remote switch board.

[trixdropd]

Anyone??

[mictester]

What I mean is I want The POSITIVE to stay connected in the circuit. I want to use 2 of these circuits independently. I want to use a TRS jack with the sleeve ground, and the tip and ring completing ground to their respective circuits, all with an led, both on the relay board and preferably on the remote switch board.

Build the original two transistor circuit. Connect the "switched" input to the positive supply permanently (ie: not through a switch). Connect the ground side of the circuit through a switch. Job done!

Ultra-low_current_relay_-_negative.png (1.49 KiB) Viewed 3923 times

The LED must be a low current type.

If you need further help, please draw up a quick sketch of what you're trying to achieve, and I'll see what I can do.

[trixdropd]

Thank you. I'll give it a go and see what happens. I just want two independent circuits in a box that share a ground on the sleeve of a jack. Then I'll trigger it with another box, hopefully with an led powered by box A.
I think you have it sorted for me. To the breadboard I go. Thanks!

[nr372]

I've built a couple versions of this circuit using both a 555 and a 40106. It's absolutely stellar!
My current work with it is to add polarity protection and refine my LED choice for low current. I've also decided to drive the capacitor discharge circuit with a transistor rather than a gate of the 40106. I'd like some quick verification on this schematic; is this bistable/transistor arrangement correct?

[trixdropd]

What I mean is I want The POSITIVE to stay connected in the circuit. I want to use 2 of these circuits independently. I want to use a TRS jack with the sleeve ground, and the tip and ring completing ground to their respective circuits, all with an led, both on the relay board and preferably on the remote switch board.

Build the original two transistor circuit. Connect the "switched" input to the positive supply permanently (ie: not through a switch). Connect the ground side of the circuit through a switch. Job done!

Ultra-low_current_relay_-_negative.png

The LED must be a low current type.

If you need further help, please draw up a quick sketch of what you're trying to achieve, and I'll see what I can do.

Alright, I built this and also a footswitch to "toggle" between two of these circuits. On the footswitch, I used red leds with no resistor. One of them is a champ and fine, the other 5 burnt out. I don"t have on hand less than a 100 ohm resistor, which is too large.

What leds and or resistor will be compatible?

[trixdropd]

Bump.. Anybody?

[mictester]

Alright, I built this and also a footswitch to "toggle" between two of these circuits. On the footswitch, I used red leds with no resistor. One of them is a champ and fine, the other 5 burnt out. I don"t have on hand less than a 100 ohm resistor, which is too large.

You can't use an LED without a series current limiting resistor - you'll destroy it!

So - you want two of these circuits operated by a single switch? When one is on, the other is off? Why not just use one circuit with two paralleled relays? There's enough drive for a pair of them. That's the way I've made QPDT switching.

Do the indicator LEDs have to be near the switches at the end of your remote control lead, or an they be on the main box?

There are several simple approaches, but please bear in mind that you're moving away from the original idea of the circuit - I wanted a "solid copper" bypass that would draw much less current than the rest of the pedal, which made relay operation possible using batteries. Recently I've been using very high efficiency LEDs that are visible on stage with 22k series resistors (so minute current drawn), but I find that most players are well aware when an effect is on!

[trixdropd]

Alright, I built this and also a footswitch to "toggle" between two of these circuits. On the footswitch, I used red leds with no resistor. One of them is a champ and fine, the other 5 burnt out. I don"t have on hand less than a 100 ohm resistor, which is too large.

You can't use an LED without a series current limiting resistor - you'll destroy it!

So - you want two of these circuits operated by a single switch? When one is on, the other is off? Why not just use one circuit with two paralleled relays? There's enough drive for a pair of them. That's the way I've made QPDT switching.

Do the indicator LEDs have to be near the switches at the end of your remote control lead, or an they be on the main box?

There are several simple approaches, but please bear in mind that you're moving away from the original idea of the circuit - I wanted a "solid copper" bypass that would draw much less current than the rest of the pedal, which made relay operation possible using batteries. Recently I've been using very high efficiency LEDs that are visible on stage with 22k series resistors (so minute current drawn), but I find that most players are well aware when an effect is on!

I do want a single switch to toggle between the two circuits. The reason I don't parallel the relays is so in the future there can be two independent channels as the rig changes.
I already put leds on the main box, and I also want them in the remote pedals.

I went ahead and ordered about 5 different low current leds and a bunch of resistors of small value to test.

[trixdropd]

I got one set of low current leds. What I've found is that channel 1 works fine with my old leds and these, channel 2 burns them quick. very strange, being that they are 2 identical circuits. I guess the tolerances are at play here.

[trixdropd]

I ended up with a workaround tor the toggle pedal. What I did is take an led and resistor and when 1 loop is active, the led is connected to the power from loop 2. It lights the light, but it's not enough current to activate loop 2. When I want loop 2, power is "stolen" from loop 1. Not ideal, but it works in this case.

[MartinFreeman]

I ended up with a workaround tor the toggle pedal. What I did is take an led and resistor and when 1 loop is active, the led is connected to the power from loop 2. It lights the light, but it's not enough current to activate loop 2. When I want loop 2, power is "stolen" from loop 1. Not ideal, but it works in this case.

Thanks for sharing scheme in detail.. I will surely try to apply it and hope it works for me

[MartinFreeman]

I ended up with a workaround tor the toggle pedal. What I did is take an led lightand resistor and when 1 loop is active, the led is connected to the power from loop 2. It lights the light, but it's not enough current to activate loop 2. When I want loop 2, power is "stolen" from loop 1. Not ideal, but it works in this case.

Thanks for sharing scheme in detail.. I will surely try to apply it and hope it works for me

[trixdropd]

I ended up with a workaround tor the toggle pedal. What I did is take an led lightand resistor and when 1 loop is active, the led is connected to the power from loop 2. It lights the light, but it's not enough current to activate loop 2. When I want loop 2, power is "stolen" from loop 1. Not ideal, but it works in this case.

Thanks for sharing scheme in detail.. I will surely try to apply it and hope it works for me

Did it work for you?

[galmar]

From general comments, I am leaning towards implementing the switch using a flip flop plus a momentary tactile switch.

The other option of course is just using an SPST switch.

In terms of reliability, which is actually better? As I said, general comments suggest the first one.

And what about future availability for replacement parts?

[mictester]

The little bistable relay has been available since the late 1980s and is widely used in Panasonic products, often for audio and video switching. There's no reason to believe that they're going to discontinue the device. The rest of the components aren't critical, and alternatives are plentiful.

In terms of reliability: the small tactile switch driving a logic bistable is very reliable. I use a Carling SPST switch and drive the very first circuit (page 1 above) directly - the switches are poor for their original use (they were the ones in "Cry Baby" wah pedals), but they're great for switching a couple of milliamps of DC.

A further advantage of using these little bistable relays is that it's easy to add extra relays if you need more switched contacts. One ultra high gain compression / distortion circuit I built had a bit of breakthrough in "bypass" mode, so I used an additional relay contact to mute the input to the high gain stage, and another to reduce the gain of the high gain amplifier by (effectively) shorting out the gain pot. It just needed a bit of extra track routing on the PCB and an extra relay!

[galmar]

The little bistable relay has been available since the late 1980s and is widely used in Panasonic products, often for audio and video switching. There's no reason to believe that they're going to discontinue the device. The rest of the components aren't critical, and alternatives are plentiful.

In terms of reliability: the small tactile switch driving a logic bistable is very reliable. I use a Carling SPST switch and drive the very first circuit (page 1 above) directly - the switches are poor for their original use (they were the ones in "Cry Baby" wah pedals), but they're great for switching a couple of milliamps of DC.

A further advantage of using these little bistable relays is that it's easy to add extra relays if you need more switched contacts. One ultra high gain compression / distortion circuit I built had a bit of breakthrough in "bypass" mode, so I used an additional relay contact to mute the input to the high gain stage, and another to reduce the gain of the high gain amplifier by (effectively) shorting out the gain pot. It just needed a bit of extra track routing on the PCB and an extra relay!

Thanks mictester! I am actually talking about the "switch" reliability - mechanical contact vs tactile switch. I do also hope that these relays don't go out of production.

Have you ever had any problems with these Carling switches?

My plan was to use the typical Boss flip flop arrangement to actuate the base of a BC550C, which will be the switch - emitter follower arrangement to drive the relay circuit. If tactile switches are so reliable, I prefer it.

Oh by the way, congrats on this little beauty.

[trixdropd]

I've built this circuit a few times. When I leave it engaged for a while the heat builds up quite a bit. What's up with that?

[mictester]

I've built this circuit a few times. When I leave it engaged for a while the heat builds up quite a bit. What's up with that?

You're using the wrong kind of relay or you've wired something up wrongly. The circuit runs completely cold if built correctly and it draws just a couple of hundred µA!

[trixdropd]

I've built this circuit a few times. When I leave it engaged for a while the heat builds up quite a bit. What's up with that?

You're using the wrong kind of relay or you've wired something up wrongly. The circuit runs completely cold if built correctly and it draws just a couple of hundred µA!

You were right. I was missing a trace. The one from + to the transistor and the 150k resistor. Circuit works, just gets hot after a while of being on.

Thanks again.