A Switching Scheme

mictester · Post by **mictester** » 30 May 2011, 20:05

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!

: Really Tiny Current drives a DPDT Relay!; Ultra-low_current_relay.png (9.56 KiB) Viewed 16985 times

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.

Hides-His-Eyes · Post by **Hides-His-Eyes** » 30 May 2011, 22:47

Cool idea if you have to have batteries! (nasty things)

mictester · Post by **mictester** » 31 May 2011, 00:10

Hides-His-Eyes wrote:Cool idea if you have to have batteries! (nasty things)

Batteries are really good! Especially if you use rechargeable ones. They inherently have no AC ripple, so no hum!

Groovenut · Post by **Groovenut** » 31 May 2011, 05:59

Since the relay is 5V, do you do anything to drop the voltage before it connects to the switch? Is it possible to use the lowly 2N3904, 2N3906 combination?

mictester · Post by **mictester** » 31 May 2011, 12:02

Groovenut wrote:Since the relay is 5V, do you do anything to drop the voltage before it connects to the switch? Is it possible to use the lowly 2N3904, 2N3906 combination?

If you look closely, you'll see that the relay isn't directly connected to the switch. There is an electrolytic capacitor in the way. All that gets through is a brief pulse, which looks like about 4.6 Volts on my storage 'scope when the switch makes. The relay will reliably switch at 4 V. When the switch is opened again, the capacitor is discharged through the relay - the other way - so the relay resets. The peak on this is about 4.3 Volts. The specification of the relay is never exceeded. I've found that it's (usually) necessary to use a 220 uF cap if you're switching two relays in parallel (for QPDT!).

The 2N3904 and 3906 work fine in this circuit. I use pretty much any small NPN and PNP transistors I have lying around on the bench. You just need an hfe of (roughly) >100 to guarantee reliable operation. The currents involved are really small, so you're not working the transistors hard!

The resistors aren't too critical, either. The 150k can be 100k, and the 15k can be 10k - you just get slightly increased quiesent current (up from typically 100 uA to 150 uA!!). The 47R can be 39 to 68R without affecting operation. Use a good quality electrolytic (for low leakage) to minimise the quiescent current in the "on" state, but don't get suckered into buying an "audio" type!

The only addition I sometimes make to this circuit is to use a CMOS bistable to feed the anode of the diode - the "+" point. I use a 4013 configured with one side as a bistable (pin 1 or 13 feeds the switching circuit directly), and the other side as a power-on reset and debounce circuit, or I use a 4093 quad schmitt NAND with two gates for the bistable, one for debounce and one for switch-on reset. Either circuit then allows the use of a momentary switch.

Hides-His-Eyes · Post by **Hides-His-Eyes** » 31 May 2011, 13:13

I've never seen that method of making a pulse, but it seems so obvious when you put it like that!

Groovenut · Post by **Groovenut** » 31 May 2011, 15:30

mictester wrote: The only addition I sometimes make to this circuit is to use a CMOS bistable to feed the anode of the diode - the "+" point. I use a 4013 configured with one side as a bistable (pin 1 or 13 feeds the switching circuit directly), and the other side as a power-on reset and debounce circuit, or I use a 4093 quad schmitt NAND with two gates for the bistable, one for debounce and one for switch-on reset. Either circuit then allows the use of a momentary switch.

Great idea as momentary switches are less expensive and last much longer. I personally loathe putting signal through a switch that was designed for high voltage and have been using small signal relays for some time, but was having an issue or two with figuring out an elegant way to use the latching relays. Thanks for the great explanation!

coldcraft · Post by **coldcraft** » 31 May 2011, 18:08

how much of this is specific to that relay? I have some of these, but I haven't decide how best to drive them.

http://www.mammothelectronics.com/TQ2-L ... 0-1000.htm

earthtonesaudio · Post by **earthtonesaudio** » 01 Jun 2011, 01:47

coldcraft wrote:how much of this is specific to that relay? I have some of these, but I haven't decide how best to drive them.

http://www.mammothelectronics.com/TQ2-L ... 0-1000.htm

Just glancing at the datasheets it looks like you can use Mictester's circuit to drive that relay. Might need to tweak some values on the breadboard though, because the power required to flip relay A is probably slightly different from the power required for relay B. You might need to increase the capacitor or decrease the 47 ohm resistor (or do both) to get it working.

mictester · Post by **mictester** » 01 Jun 2011, 13:46

coldcraft wrote:how much of this is specific to that relay? I have some of these, but I haven't decide how best to drive them.

http://www.mammothelectronics.com/TQ2-L ... 0-1000.htm

It's going to take you ten minutes and a few cheap components to try it out! Looking at the spec of your relay, it should work OK.

I can't emphasise quite how much better this switching scheme is than the usual TPDT cheap switch. I know the switch is simple, but it really isn't reliable, and it always introduces switching noise.

Also, the current drain is minute - a well-known London session player has a FET Fuzz I built for him using this switching circuit. He uses the effect all day, every day. His pedal doesn't have an LED indicator (he doesn't need it). He is still using the original Ever Ready Alkaline 9V battery I put into it in 2003!

dukie · Post by **dukie** » 04 Jun 2011, 17:07

Hi.. i've build this relay switching using my own perfboard layout but it had a problem. It connect the input (jack input) to output (jack output) when disengaged but when engaged no effect sound, only the bypass sound is heard. So whats happen exactly?everytime i engaged the spst switch, the LED would dimmed.
I'm using RED LED with 4,7K resistor attach to main Effect board's +9v supply, not after the anode of the diode in the seperate relay board.
Here's the layout..

Can anybody tell me what's wrong with it?

Cheers!

Fender3D · Post by **Fender3D** » 04 Jun 2011, 21:44

Why did you connected Relay's pin 10 with pin 2?

Groovenut · Post by **Groovenut** » 05 Jun 2011, 00:03

Fender3D wrote:Why did you connected Relay's pin 10 with pin 2?

It grounds the effect out when the relay is in bypass mode.

Fender3D · Post by **Fender3D** » 05 Jun 2011, 02:44

I see...
I ground the input usually...

dukie · Post by **dukie** » 05 Jun 2011, 05:35

Fender3D wrote:I see...
I ground the input usually...

Yeah you can also ground the input, it's just a matter of arangement

Anyway, one thing that suprise me was, that this circuit drained my battery in just about an hour?So something is definetly wrong in my layout, since this circuit supposed to run with small voltage.Or maybe because i'm using A5W-K not the latching type AL5W-K relay?

Cheers!

Hides-His-Eyes · Post by **Hides-His-Eyes** » 05 Jun 2011, 10:11

If your relay is non latching then it obviously won't work!

Groovenut · Post by **Groovenut** » 05 Jun 2011, 15:48

dukie wrote:
Fender3D wrote:I see...
I ground the input usually...
Yeah you can also ground the input, it's just a matter of arangement
Anyway, one thing that suprise me was, that this circuit drained my battery in just about an hour?So something is definetly wrong in my layout, since this circuit supposed to run with small voltage.Or maybe because i'm using A5W-K not the latching type AL5W-K relay?

Cheers!

Yeah you are using the non-latching version of this relay. That is your issue. I have successfully tested this circuit with AL5W-K and 2N3904/2N3906 and it only draws ~60uA current when on. Try switching to the latching version. That should cure your problem.

dukie · Post by **dukie** » 05 Jun 2011, 16:49

Groovenut wrote:on-latching version of this relay. That is your issue. I have successfully tested this circuit with AL5W-K and 2N3904/2N3906 and it only draws ~60uA current when on. Try switching to the latching version. That should cure your problem.

Hmm..well I'll have to wait another weekend to buy the AL5W-K relay then. I can't find it in my nearest electronic component store, I had to travel quite far to buy one..and I'm not even sure I can find one, since this is the only 5v relay they gave me when I bought it 2 days ago. So maybe this A5W-K non latching type can be use in another switching arangement, perhaps?any tought guys?So I can tinker with it while I wait for the weekend

Btw, If I'm using the latching relay then should I use momentary switch or just ordinary SPST switch?

Cheers!

Hides-His-Eyes · Post by **Hides-His-Eyes** » 05 Jun 2011, 17:14

You won't be able to make a "low current" version with a non-latching relay afaik.

The coils tend to be hundreds of ohms, so from a 9V supply with a 5v relay you're looking at anything up to 40-50mA.

Groovenut · Post by **Groovenut** » 05 Jun 2011, 18:35

dukie wrote:
Groovenut wrote:on-latching version of this relay. That is your issue. I have successfully tested this circuit with AL5W-K and 2N3904/2N3906 and it only draws ~60uA current when on. Try switching to the latching version. That should cure your problem.
Btw, If I'm using the latching relay then should I use momentary switch or just ordinary SPST switch?

Cheers!

With the posted circuit you need to use a latching switch. If you want to use a momentary switch, you have to use an electronic latch of some sort.

A Switching Scheme [documentation]