True bypass has been an unspoken must-have in DIY pedals world and sometimes buffered switching is looked down upon (for whatever reason). I personally hate wiring up 3PDT switches … soooo many wires.
What if there was an option to do all the switching with just 2 wires and add a bit of high-end feel to my pedal for an extra bonus? Well, I say sign me up! Turns out – relays might do just that.
Today, I’ll cover couple of different types of relays and a simple control circuit. As usual, you can watch video straight away, and come back later if you wish, for diagrams etc.
Make sure to visit additional reading section, I list out all the relay related documentation I used to conjure the diagrams.
Relay, say what?
Relays are really just an electrically operated switches. Instead of using a latching stomp switch (like we usually do), we send current to a relay to operate it – flipping the switch on and off.
How is this useful though? We still need to stomp on a switch to operate the pedal, relay or no relay, right? Well, it could be really useful if you want to remotely control them, for example. Using MIDI pedal or some other way. Or, like me, you want to use just 2 wires ?. Other than that, I feel it’s just a gimmick.
For example, I know that my DigiTech Whammy uses a relay. There’s a distinct click when I press the button and it does not come from the button itself. And it can also be controlled using MIDI. Another one is Polytune 3 – I believe a similar relay I’ll cover here is used in it. At any rate, it makes sense using it when we already have a lots of electronics in a pedal.
There are few different types of relays out there. I will be using a signal relay. Solid state relays can’t be considered as true bypass because they use something like MOSFETs in them. Other types of relays could probably used, but signal relays are perfect for what we need them for.
IMPORTANT: Relays are used a lot to control high voltage circuits/devices using low voltage control circuits. I will not cover here using high voltage. Please assume I have no clue what I’m talking about here, I’m just a guy on the internet. Using high voltage not knowing what you’re doing is dangerous and can cause serious harm.
Here, I’ll be covering using relays with 9V power supply/battery, so I’ll be perfectly safe.
Basic Relay Circuit
Let’s look at a basic relay circuit.
Here, I’m using AXICOM IM01TS. That’s the little one on the photo ?.
IM01 is a monostable relay with two switches (so DPDT) and with 3V operating voltage. Monostable just means that the switch is normally in one position (NC – normally closed). It switches to the other position (NO – normally open) only while voltage is applied to the coil. As soon as coil is not energized anymore, relay switches go back to the initial position.
R4-R6 and D2-D4 are just LEDs. They are there just so we see something is happening. D3 and D4 are wired so +9V is applied to one or the other depending on whether the coil is energized or not. Green (D4) should be lit up when coil is not energized, and Red (D3) when the coil is energized.
D2 is lit when Q1 is on and at that point, the coil is energized, so D3 (Red) diode should be lit at the same time.
The MOSFET will be turned on when the positive control signal is applied (for my 2N7000 this should be any voltage over 3V). The transistor is off when the control signal is 0V. We need a transistor to drive current because our coil consumes about 40-45mA of current.
I highlighted above my relay (in red). Lower the operating voltage, higher the current consumption. For 9V rated coil (which I should’ve gotten in the first place), current consumption would have been about 15mA.
To put this into perspective – if we wanted to use 9V alkaline battery (roughly 500mAh capacity), just driving our relay would deplete it in about 11 hours. Using 9V coil, our battery would last 3 times longer.
R1 and R2 are normally used in these kind of circuits. R1 to mitigate ringing of the control signal, R2 to bleed any MOSFET’s stray charge so it does not turn on accidentally. I am not sure if any of this is needed, the advices around using MOSFETs normally involve way more complicated situations. It really depends on the control circuit I suppose and what are we driving, but I’m really not competent enough to say.
The relay needs that resistor R3 so we don’t burn the coil. The coil is rated at 3V, we’re using 9V supply. R3 drives that voltage/current down so we don’t destroy the coil. I put 150ohm resistor because I use 3V rated relay. The resistor would not be needed or 9V relay, and a smaller resistor is needed for 5V for example.
This means that with appropriate resistor (and some contribution to the global warming) we can drive any relay rated for under 9V.
D1 diode is there for protection. When the power is suddenly cut to the coil a voltage spike is seen across it. This can be several times the voltage of the power supply. D1 is there to protect our MOSFET from this spike.
Testing the Circuit
Have a look at the video. I’m using a different power supply to drive MOSFET and the relay coil – I use my 9V power supply. For driving diodes, I use the 9V battery. This is what relay enables you. We can make two circuits independent of each other – the control circuit and the load. Not that we need it, since I’m just going to be switching pedal on and off, but good to know.
To turn the transistor on, I just connect the gate to 9V, and to turn it off I just connect the gate to the ground. Works like a charm, but not really useful ?. Let’s look into how to practically control this.
How do we control this? There are lots of options. Normally some sort of a microcontroller is used. Something miniature like ATTiny13 works great (8-pin DIP package with 6 usable I/O pins). Or if you prefer development boards – something like Arduino Nano or Raspberry Pi Pico could be used.
But we also have a bunch of options that do not require a controller. Boss pedals, or something like Tube Screamer uses buffered switches made from just a few transistors and passive components. A good starting point might be Tube Screamer analysis by Electrosmash (look it up on Google, site’s SSL certificate expired though so a bit of a bother to access). The article covers JFET bypass switching in some detail.
Or look up a service manual for your favourite Boss pedal (or really any Boss pedal). I looked up Metal Zone pedal, and here’s the excerpt:
Here’s my version of it:
Total rip-off, I know 🙂
R1, R2 and C1 are there for switch de-bouncing I guess. The rest of it is the toggle circuit. Essentially, the circuit is symmetrical. The principle of it’s working is simple (in theory) – only one of the Q1 or Q2 can be ON at the same time.
If Q2 is ON, it drives its collector to ground and Control is low. If Q2 is OFF, its collector is high (voltage depends on R6 and whatever is connected to Control). If the switch is briefly pressed (momentary switch), it turns off whichever transistor that is ON at the time and in doing that activates the other transistor.
Actually, great resource for this is geofex site – just search for “Technology of Boss and Ibanez Bypassing” on the page. The pdf covers pretty much everything.
Some of the components are different from the original Boss schematic. I used BC550C transistors. 2N3904 did not work reliably for me. When I looked it up, the original transistors used have quite a high gain: 200+, so something like BC550C will work better.
I found that playing with C2-C5 and R7 and R8 values gives some mixed results. While not wholly critical, messing around with the values may disturb biasing and also timing of the Q1 and Q2 toggling, so straying away too much from the original values may cause toggling to be unreliable. Boss has been around a while, they have figured it out I guess.
The main change really is the value for R5 and R6. I needed a higher voltage for active Control signal. This works just fine.
Driving the Relay
Well, it really is a matter of connecting Control to input of the relay circuit. That’s it. We use a momentary switch to toggle state of the control circuit, and that drives the MOSFET and thus the relay.
Wow, we just used a footswitch, 12 resistors, 5 capacitors, 3 transistors, a diode and a relay to replace a single footswitch!!!! Speaking of high-end feel, I’m positively tingling with excitement!!!
Hmmmm … I’m definitely revisiting this with some fine microcontroller action. I have to note though, the toggle circuit cannot feed relay directly, and if relay circuit loads the control circuit too much, the toggling will just stop working. MOSFET is there for that reason, providing sufficient current and not loading control circuit.
We concluded that the pedal is drawing a bit much power for battery operation. It will work for a gig or two. So if you use something like this in your pedal, and want to use battery, make sure you have a few spare at all times.
For those more climate change conscious, let’s do a power saver option! For this, we need a different type of relay: bistable (or latching). Monostable has one stable state, bistable has … two.
Because monostable has one stable state, to move it to the 2nd state we need to provide constant current. Bistable needs a pulse of current to switch state and then it does not consume any more current at all. Frugal indeed.
Let’s look at the basic relay circuit:
I’m using another 3V rated relay, this time Omron G6SU-2. The schematic symbol is slightly different if you pay close attention. IM01TS has only an indicator where the positive end is. G6SU indicates that there is a Set and Reset polarity to it.
There are 2 types of these bistable relays – single-winding and double-winding. Mine is using single-winding which means, positive impulse between pins 1 and 12 is Set operation, and sets switches to the first/set position. Negative impulse on the other hand is Reset operation and sets switches to the second/reset position. Double winding option has separate pins for Set/Reset operations.
How Does this Work?
Control circuit we used before will work here too. I deliberately used Control, to indicate that for monostable version high level of Control turns on the driving transistor, in this case, low level turns on Q1. In reality, you need only to worry when your LED indicator is turned on, the switching will occur if you use either Control or Control signal.
Low level of Control signal because that’s P-type MOSFET. High to turn it off, and that’s why I had to use 4.7K resistors in the control circuit, to make my control signal even higher to be able to turn off the transistor.
Well, if Q1 is turned on, it charges C1 through D1, R4, coil, D2 and R3. This sudden charging current for C1 Sets the latching relay. Control signal has to be still kept low, thus our control circuit is still consuming some current. We are talking about microamps in this case, but our relay is not consuming anything! Our Earth thanks you!
When the Q1 is off, C1 discharges through Q2, R4 and the coil and resets the latching relay. Wow, there’s no way this will work ?. I mean, it definitely does, make sure you watch the video.
R4 is there just for voltage control so I don’t burn the coil since I’m using 9V power. It just happens that 150 ohm works in both circuits, but there’s no guarantee so make sure to check datasheet of whatever you’re using.
Some Extra Reading
For datasheets of the relays I used google “IM01 datasheet” (use the one from te.com site – the link works today but who knows if it will work in future). Likewise search for “omron G6S datasheet”, again the link is here, but who knows if it will always work.
Obviously, use datasheet for whatever relay you are using, but make sure to take notes of nominal coil voltage, current, resistance etc. That’s all important to know how to size the rest of the components and not damage your relay.
Now, I have to admit, I’m not that clever. Shocking I know ?. The above circuits are all from various manufacturer’s guides and application notes.
Good source for more detailed information on how to use relays are documents from Panasonic (search for “panasonic relay general application guidelines” and “panasonic applications of relays in electronic circuits”). I mean, I may have picked up a thing or two from there ?.
Also, look up “omron relay application K337”. Here’s the link of the pdf. The title comes up in search as “Safety Precautions for All Relays”, but that’s just a warning at the very beginning and google incorrectly uses it as title. The circuit I used for bistable relay is taken from page 12, I only adapted it for my relay.
It is good idea to look up various application notes before you start working on something completely new. I had no idea about relays before today … and now I know more than I wanted ?. It’s a bit of a rabbit hole once you get hooked. There’s no lack of application notes.
Some Parting Thoughts
I kind of avoided mentioning that documentation recommends sending reset signal to latching relay on start-up so we have it in a known position. This is a major pain in the back, especially if you are driving LED indicator independently … I forgot to add that indicator for that bistable option … doh! If we use microcontroller, then this is an easy problem to solve.
I’d personally go with monostable version, it’s way easier to use. Let’s save Earth instead by walking more and using car less … and playing the guitar more ?, of course.
There’s a lot of components if we want to implement relays as I did in this post. While this is clever and all, I’m not sure if this is very practical. Microcontroller option might be better way to go. In the end, it’s also good to have some extra options in your pocket.
And, btw, microcontroller just removes toggling circuit … might also offer some extra options, but in the end of the day, may or may not be simpler solution after all.
Btw, while all of the above will work just fine, it has to be said that I don’t have much experience with relays. We use it in pedals, and most of the time we can get away with murder. If anyone has better knowledge or/and more experience, please share your thoughts in the comments. I would be interested to know if there are some practical considerations that I may have missed.
Of course, an option is also to forget about all of the above and just use our trusted latching 3PDT footswitch!