A simple Relay Bypass

[Chuck D. Bones]

How many leads on the LED?

 

[Chuck D. Bones]

If your plan is to have the LED glow one color when the relay is engaged and another color when the relay is disengaged, I can think of two ways to do it.
1. Add a transistor and two resistors to drive one of the LEDs to the opposite state of the other one.
2. Illuminate one of the LEDs constantly. A Red/Green LED will glow Red/Yellow or Green/Yellow, depending on which LED is on constantly.

If you had a 2-lead bidirectional LED, then there is another option.

 

[Brett]

How would #1 integrate into your existing simple bypass?

Unverified, but maybe something like this would work for your application:

*Updated drawing per @Chuck D. Bones suggestion in post #113 below.

 

[Chuck D. Bones]

Unverified, but maybe something like this would work for your application:

Wow, great minds. That gate stopper resistor (RX5) should be larger to guarantee it does the job. I like 10K for that.

 

[Chuck D. Bones]

Quick question as I try and do some design work for the Forum Univibe:

If I were to use more than one DPDT relay for the Speed switching, am I correct in this assumption:

If I place two relay coils in parallel, would I use their parallel resistances in the coil resistance calc to get my proper coil resistor?

Yes, but the 555 does not have infinite drive capability. When powered by 9V, we cannot expect the 555 to sink more than 30mA. If the sum of the coil currents exceed that, then you need a MOSFET to drive the coils. This is an example "requirements creep" where a circuit works fine until someone changes the requirements.

 

[Brett]

...That gate stopper resistor (RX5) should be larger...I like 10K for that.

I figured the values may need to be adjusted some, but thank you for confirming! I've updated my post above for posterity.

 

[Paul.Ruby]

This is just FYI. I made a discrete soft-touch toggle circuit for my TPA3118 power amp pedals that would also work for all this stuff, if anyone want such a thing. This link jumps to the post about it in the TPA3118 amp thread.

TPA3118 60W Power Amp Module

EDIT #2: I've decided this is not worth the effort. The only way to really avoid pop is with proper behavior. Don't plug in a speaker cab until everything is powered. Unplug speaker cab first for power down. A mute button or stand-by on the amp might be a nice-to-have, but still requires proper...

forum.pedalpcb.com

 

[Brett]

This is just FYI. I made a discrete soft-touch toggle circuit for my TPA3118 power amp pedals that would also work for all this stuff, if anyone want such a thing. This link jumps to the post about it in the TPA3118 amp thread.

TPA3118 60W Power Amp Module

EDIT #2: I've decided this is not worth the effort. The only way to really avoid pop is with proper behavior. Don't plug in a speaker cab until everything is powered. Unplug speaker cab first for power down. A mute button or stand-by on the amp might be a nice-to-have, but still requires proper...

forum.pedalpcb.com

This is cool. You may want to consider posting a new thread for this since it’s not really related to this circuit.

 

[sammo303]

I have a few questions that I haven't seen on this thread. Apologies if I'm covering old ground.

- Looking through relay datasheets it seems like current draw reduces with increasing coil voltage, which is to be expected. Given the current passes through the 555 which can't tolerate too much, is there any downside to simply using a relay with a 9v coil to remove R4 and minimise current draw?
- If we use a latching relay instead, besides other modifications, would I be right in thinking the limit on current through the 555 could be increased due to the short duration during state change? If so, perhaps a simple circuit could still be used to drive more than one relay if that was ever needed.
- Any idea what modifications would be required to make the circuit work with a latching relay? Pedalpcb's basic relay switching schematic is a fair bit more complicated for the latching relay option so I suspect modifications will end up sacrificing the simplicity but keen to hear if anyone thinks a similarly bare bones schematic may work.

Thanks,
Sam

 

[Chuck D. Bones]

Yes
No
More mods than are practical IMHO. Without getting into the gory details, if you want to reliably drive a latching relay, use a microcontroller.

 

[sammo303]

OK, thank you for the swift reply. Looks like I got 0/3 there, sorry for littering the thread.

 

[Chuck D. Bones]

No worries, you get credit for being on-topic and not posting any memes.

 

[MattG]

if you want to reliably drive a latching relay, use a microcontroller.

Here is one such example.

PedalPCB has a latching relay bypass board that uses an NE555 (instead of a microcontroller) to maintain state and react to momentary switch presses. I think the NE555 could be replaced with a couple inverters (e.g. cd40106 or 74hc14) with a circuit like this. I have breadboarded this, and it certainly works for toggling between two states with a momentary switch. The advantage is dramatically reduced power consumption (the disadvantage is a physically larger chip).

With the single-coil latching relay, you need the ability to produce a bi-directional current pulses (pulse one way for set, pulse the other for reset). That is easily done with two pins of a microcontroller (that have the appropriate current sourcing/sinking capability and builtin protection from coil flyback). The way it's done with the PedalPCB bypass is to use a capacitor in series with the relay coil. While DC won't pass through the capacitor, there will be a current pulse generated by it being charged or discharged depending on which transistor is open (either the PNP to ground or the NPN to VCC).

My question: assume that Q2's collector is attached to the relay coil's design voltage (5v in the case of the TQ2-L-5V, which also happens to be a good supply voltage for inverter ICs). Could we then remove the series capacitor C3, and replace R4 with a series capacitor (i.e. in series with the base of Q1)? And additionally add a resistor to ground between transistor gate and the series capacitor. The goals being (1) to activate the relay coil with it's design voltage, instead of the approximately-correct voltage from the cap charge/discharge current, and (2) get away with a smaller capacitor (hopefully small enough to use film or ceramic) since we only need to drive the transistor base?

I have bread-boarded something conceptually similar, though I'm using a two coil latching relay, and MOSFETs instead of BJTs - but the main idea from my question is the same: using a series capacitors to generate a pulse for the FET gates (and the FETS do the "heavy lifting" of current sinking). It works as expected on the breadboard. Assuming this isn't a flawed design for some reasons that I'm unaware of, for simple on/off relay bypass, it has a lot of advantages: micro-amp current consumption (except when switching of course), simple circuit, uses cheap, commodity, current-production components. Schematic attached.

 

[Chuck D. Bones]

You might want to try a CD4013 type D flip-flop. Saves some parts. But at about a buck each and free firmware, a μC is hard to beat.
We used latching relays a LOT in spacecraft because the coil current is zero 99.9% of the time and they remember their state when the power is off. 4PDT relays were a favorite because one set of contacts could be dedicated to feedback to the controller.

One more thing... you don't have to run the 40106 or 4013 from regulated +5V. They work fine on +9V. You don't need to run the relay off of regulated voltage either.

 

[szukalski]

For anyone following this relay, I uploaded the v2.1 of my layout:

GitHub - szukalski/pedal-simple-relay

Contribute to szukalski/pedal-simple-relay development by creating an account on GitHub.

github.com

It's the same as v2.0 but with a smaller footprint, less height.

I have a v3.0 drawn up which lets you choose to use the LED of the main board or the relay, but it's not smaller than this one.

 

[drgonzo1969]

For anyone following this relay, I uploaded the v2.1 of my layout:

GitHub - szukalski/pedal-simple-relay

Contribute to szukalski/pedal-simple-relay development by creating an account on GitHub.

github.com

It's the same as v2.0 but with a smaller footprint, less height.

I have a v3.0 drawn up which lets you choose to use the LED of the main board or the relay, but it's not smaller than this one.

Is the SPST momentary?

 

[szukalski]

Is the SPST momentary?

Yes. You can use any mechanism to make a complete circuit.

 

[Chuck D. Bones]

Stomp switch should be normally open.

 

[aj53202]

For anyone following this relay, I uploaded the v2.1 of my layout:

GitHub - szukalski/pedal-simple-relay

Contribute to szukalski/pedal-simple-relay development by creating an account on GitHub.

github.com

It's the same as v2.0 but with a smaller footprint, less height.

I have a v3.0 drawn up which lets you choose to use the LED of the main board or the relay, but it's not smaller than this one.

Hi, could you please help with the attributes of U1(when using FTR-B4CA) on relay-v2.1-cpl.csv? Is the rotation =269.99 correct? Seems like it should be 180 based on visual inspection on JLCPCB.

My recent Paragon build has LED pop and after all the usual trials ( LED change, Tantalum output caps, AMZ method etc) I was wondering if using Relay Bypass would eliminate/reduce it as removing the LED makes the switching dead silent.

 

[Chuck D. Bones]

LED pop could be caused by grounding issues. The relay bypass might fix it, but without understanding why the LED pops, there is no guarantee. Maybe you want to post something in the Troubleshooting forum.