Quantcast

A Guide to Soft-Latch Relay Bypass

classic Classic list List threaded Threaded
4 messages Options
Reply | Threaded
Open this post in threaded view
|  
Report Content as Inappropriate

A Guide to Soft-Latch Relay Bypass

Silver Blues
The big blue 3PDT has been the go-to actuator for true bypass switching for who knows how long, and it has served its purpose well with a high success rate and reliable operation. Nevertheless, a popular upgrade and increasingly frequent inclusion in high-end stompboxes is the so-called “soft latch” true bypass system, which uses an electronic circuit actuated by a momentary switch to toggle a small relay that does the bypassing. The momentary switch doesn’t produce the characteristic mechanical “thunk” that the 3PDT does; rather, all you hear is the tick of the relay, giving rise to the “soft-latch” moniker. It’s a great system, hardly ever pops because there is no substantial mechanical component, and the smooth, detent-free actuation feels amazing. I’m a big soft-latch nerd, and have tried most of the common circuits readily available today, so I thought I’d write a little guide to help people get into it if they so desire.

Soft-latch has traditionally been a bit expensive, due to the far larger number of components necessary to make it work versus a purely mechanical system or even something like the Millennium Bypass. My main goal was to find a circuit cheap enough such that I could put a soft-latch in every pedal I built. A short overview of what’s available follows.

AMZ FX DPDT Switch Board: When I began my journey into soft-latch, I began by using Jack Orman’s “True Bypass with Relay PCB”s, which were an inch-square PCB designed and dedicated to true bypass switching. He has since discontinued this product and instead sells a general-purpose DPDT relay switch PCB which can be configured for true bypass switching quite easily (I have used both). The circuit is AVR microcontroller-based, uses single-coil latching relays like the ubiquitous Takamisawa AL5WN-K, and has a low parts count. Advantages to this circuit are that it is very physically small and can fit virtually anywhere, the current draw is low and the switching is reliable and crisp. The circuit flashes the indicator LED on power-up, so you know everything is working as intended, which is a nice feature. Disadvantages are that there is a flywire required for the true-bypass configuration, which doesn’t look great and is a bit difficult to keep out of the way given its location. The unit cost is also expensive, at least for me, at $7 for the board and AVR plus $7 shipping to Canada (although that will obviously vary with your location). This is not including the other components necessary to make the thing work; Jack sells this as a complete kit for $14. Other systems like this exist (the Monomonster system, or the one from 1776 Effects) although these are essentially functionally identical. Though there’s nothing negative to say about the functionality, the sky-high unit price is a bit of a roadblock to being able to include one in every build.  

Mictester’s CMOS module: Developed iteratively from a circuit by Mictester on FreeStompBoxes, this circuit uses a 40106 Schmitt trigger to actuate a single-coil latching relay like the AL5WN-K. The parts count is moderate, and there are PCB layouts available that are nice and small (1.2” x 1.4”, or 30x36mm), however the nature of the parts involved makes it awkward to lay out on vero and thus a bit on the large side. Nevertheless, it will fit inside a 1590B so long as low-profile parts are used. The actuation on this circuit is also nice and defect-free, however there are some minor issues with it.  Firstly, the power-on state is not guaranteed and there isn’t any way to know what it will be unless you extend the circuit to include it (this is possible, and is discussed in the original thread on FSB). The one that I built defaults to “engaged” when power is applied (seemingly a common problem), which may not be acceptable for some implementations. Secondly, the circuit doesn’t much like rapid engage/bypass cycles; there is a (short, but noticeable) minimum time required between button presses to get it to cycle properly. In most cases it should not be an issue, but still something to note. Cost is low, I estimate $3-4 based on Tayda and eBay prices.

The Andy Relay: Developed by the famous R.G. Keen, this circuit uses a 4069 hex inverter to actuate a dual-coil latching relay. OSH Park sells extremely high-quality PCBs for this circuit, at a unit cost of around $3 (total cost is $5-6 for a complete system). One of the best circuits of this type I have used; the switching is very reliable and there is no extraneous noise to speak of. The board is 1.8” x 0.94” or 46x24mm, and so is easy to fit in 1590B and up sized boxes at various locations. It also includes wire-routing holes for strain relief, which is very welcome. It uses very common components with a moderate parts count. The only disadvantage to speak of is that it uses dual-coil latching relays rather than single-coil ones, and the relay “of choice” is the Panasonic TQ2-L2-5V; this part cannot be found at a discount, and the big-box retailers (e.g. Mouser) want to charge $8 apiece for it, which would double the overall cost of the system. Other similar relays could be used, although most are equally difficult to find, not to mention at a good price. The NEC EA2-5TNFG work brilliantly in this circuit, and they can be found for a very reasonable $2.50ea from eBay (not quite as cheap as the AL5WN-K, but manageable nonetheless), and are what I employ in my boards. I have used four of these to date and continue to use them; the system cost is slightly higher than something like the Mictester module but the quality of switching and of construction is beyond worthwhile. This is my recommended system. Do note the wiring of the LED if you are to build this; it has caused much confusion. Additionally,  and very importantly, the silkscreen on the OSH Park boards have an error in them; the 10K resistor immediately to the right of the top-right corner of the 4069 must be 100K. If a 10K is placed in this position, the relay will not toggle (although the LED will still work correctly).

Pruttelherrie’s “Generic I/O Board”: Introduced to me by Pruttelherrie on FreeStompBoxes, this very simple system uses an ATTiny85 microcontroller to drive a single-coil latching relay. The code is available freely on the original FSB thread. The parts count is very low, and so even on vero the board can be made small enough to fit just about anywhere. The switching quality is very good. The default power-up state can be toggled, which I think is a cool feature. I just finished testing this circuit and I like it a lot as well. The major disadvantage is the up-front entry cost and relative complexity; you’ll need something to program the microcontrollers and a computer of course, and there are a number of solutions of varying price to accomplish this. I chose to use an Arduino Uno as an in-system programmer, which cost me all of $7 to set up. From there you can load the code onto the ATTiny85s, and you’re good to go. System cost is similar to the Andy Relay. Also recommended.

Paulinthelab Soft Latch Daughterboard: One of the first DIY soft-latch systems to surface if I remember correctly, but not one without its issues. This one uses a transistor network to actuate a single-coil non-latching DPCO relay, and people have reported significant popping and poor isolation issues with this circuit. Although it is not included, this circuit needs a flyback diode across the relay coil, but even when this is added issues are still reported. Another major disadvantage is that since the relay it uses doesn’t latch, the coil must be energized as long as the pedal is engaged, making this unsuitable for a battery-powered build and just generally inferior to the other options I have discussed, especially when considering the comparatively gigantic board (although Paul himself has laid one out that is about the size of a 9V battery). Unit cost looks to be quite low. I haven’t used this one and don’t plan to; in fact, I recommend just avoiding it entirely.

For those looking to build,

Takamisawa AL5WN-K Single Coil Latching Relays for the AMZ, Mictester and Pruttelherrie systems. (Also available in 10s or 20s from the same seller.)
NEC EA2-5TNFG Dual Coil Latching Relays for the Andy Relay. (Also available individually or in 10s from the same seller.)
SPST NO Momentary switches for actuating all listed circuits. You won’t find them cheaper than this.

Thanks for reading!
I stand watching the steam-liners roll by...
Reply | Threaded
Open this post in threaded view
|  
Report Content as Inappropriate

Re: A Guide to Soft-Latch Relay Bypass

Snare227
Thanks @Silver Blues !  Looks like you have spent a lot of time compiling and testing this information, so thank you for passing it along.  I've ordered some boards for the Andy Relay since it seemed to be the best option for something low cost and still working well.  It is my first order from that company and it looks like it may be a few weeks before the boards show up.  Has anyone made a compact vero layout of this relay system?  I saw the PCB layout by JohnK, but I'm strictly vero as of now.  Looking forward to trying this out once the boards show.
Reply | Threaded
Open this post in threaded view
|  
Report Content as Inappropriate

Re: A Guide to Soft-Latch Relay Bypass

Silver Blues
No problem at all! I'm always glad to help. For sure, the Andy Relay is definitely the easiest to get going i think, you shouldn't be disappointed. OSH Park is a fantastic company. I'm not sure on the vero layout, but it probably wouldn't be that hard to make.
I stand watching the steam-liners roll by...
Reply | Threaded
Open this post in threaded view
|  
Report Content as Inappropriate

Re: A Guide to Soft-Latch Relay Bypass

dpm309
Great summary of soft switching. Been looking all day for information on this subject. Have a Mr. Black Downward Spiral that a friend wants to eliminate the pop when he turns it off and on. Will have to look into these options a little more to see if one of these will work. Haven't opened this up yet to see if there is enough room.
Loading...