Add latching LEDs to momentary footswitch. Modify guitar tuner

Shoe-Laze

Joined Jul 19, 2013
6
Hello all!
In my search for some answers I found this board, and I'm super impressed by all of your knowledge!

I'm a novice when it comes to this, but I'm eager to learn in new fields. My search found several similar threads on this board, but I'm not sure how to apply them to my project. I would be super appreciative for any help or directions you could give me.

What I have is one TC Electronics Polytune guitar tuner pedal that functions as follows:
1, Pedal is bypassed - signal goes to guitar amp
Press the pedal once and release
2, Pedal is active - signal to amp is muted and tuner is active.
Press again and we're back to step 1...

What I want to achieve is to add 2 LED's that are lit one at the time. Green light when pedal is bypassed and a Red light when the pedal is active.

From what I can tell the footswitch is a DPDT which made me think I could change it to a 3PDT and add the LED's and external 9V to the 3rd pole. However, the switch a momentary switch and my LED's would not be latching doing that. So I believe a flip flop/schmitt trigger would be the way to go? The pedal itself is powered with 9V, and I'd rather add another 9V source to power the LEDs than to try and hijack power from the pcb.

So my questions would be if anyone know of a specific circuit that would work for this specific need (flip flop/schitt/or other)? Also, I wonder what kind of switch I would have to use to service the needs of the tuner itself as well a using it to trigger the latching LED's, and how it would be connected to this new circuit?

I saw that Bill Marsden, to name one, had some fantastic illustrations and schmatics, but I'm not sure if they completely apply?
I'm incredibly thankful for all the help I can get.

Thanks!
Niclas, Sweden

Attachments

• 293.1 KB Views: 66
• 298.3 KB Views: 56
• 284.7 KB Views: 54

wayneh

Joined Sep 9, 2010
17,189
So my questions would be if anyone know of a specific circuit that would work for this specific need (flip flop/schitt/or other)?
That reminds me - now that I've had my morning coffee, it's time to go Schmitt.

Dodgydave

Joined Jun 22, 2012
10,591
you will need a 3pole change over switch, or you can use one of the poles and use that to switch a dpdt 6V relay.

absf

Joined Dec 29, 2010
1,968
a 3-pole switch would have 9 pins. How are you going to mount it onto your PCB? Drilling extra 3 holes is not an option as the PCB is double sided as you might drill through the trace.

Are you able to trace where all the 6 pins of the DPDT go to?

Allen

Ron H

Joined Apr 14, 2005
7,014
Somewhere, that switch controls some sort of toggling latch or flip-flop. If you try to add another toggling latch for your LEDs, won't it be difficult to keep it in sync with the existing one?
It might be better if you could locate the existing toggling circuit, and go from there.
I suppose your circuit always comes up in the same state when you power it up. You could also initialize the new circuit to come up in the same state each time you power up, but if you are running it for several hours, I would worry about a glitch causing sync to be lost.

tubeguy

Joined Nov 3, 2012
1,157
One method for switching audio signals on/off is to use one or more Jfet's.
To help track down the toggle circuit, look for some Jfet's. They are frequently labeled with a J designator.

Last edited:

Shoe-Laze

Joined Jul 19, 2013
6
a 3-pole switch would have 9 pins. How are you going to mount it onto your PCB?

Are you able to trace where all the 6 pins of the DPDT go to?
Allen
Forgot to mention! The switch will be removed from to the pcb and attached back with 1 feet wires so I can press it "remote" from the pedal itself.

Unfortunately I can't see where a single pin of the DPDT goes, and the manufacturer won't release the schematics.

Somewhere, that switch controls some sort of toggling latch or flip-flop. If you try to add another toggling latch for your LEDs, won't it be difficult to keep it in sync with the existing one?
It might be better if you could locate the existing toggling circuit, and go from there.
I suppose your circuit always comes up in the same state when you power it up. You could also initialize the new circuit to come up in the same state each time you power up, but if you are running it for several hours, I would worry about a glitch causing sync to be lost.
You are correct. The circuit always comes on in "Pedal bypassed" when power is applied. The best thing would indeed be to find where the switching happens on the board and start there, but the components are really tiny and I would rather stay away from changes on the pcb.

I'll be running it for max 3 hours, and the lost sync can be dealt with occasionally as it's not a crucial circuit, only light turning red/green.

Any ideas of which circuit to build?

absf

Joined Dec 29, 2010
1,968
Assuming that your 3PDT switch is non-locking. I designed a simple circuit that might work.

C1,R2 is a power up reset that makes 4013 Q always low at power up (thus lighting the Green LED). U2 4011 is used for contact debouncing. You can try it on a breadboard and see if works and see if improvements can be made. After all the circuit would take less than $5 to make. Allen  Or use one chip 74HC74 and a 78L05 voltage regulator -------- schematic #2 Attachments • 129.1 KB Views: 84 • 32.6 KB Views: 57 Last edited: Ron H Joined Apr 14, 2005 7,014 Assuming that your 3PDT switch is non-locking. I designed a simple circuit that might work. C1,R2 is a power up reset that makes 4013 Q always low at power up (thus lighting the Green LED). U2 4011 is used for contact debouncing. You can try it on a breadboard and see if works and see if improvements can be made. After all the circuit would take less than$5 to make.

Allen

 Or use one chip 74HC74 and a 78L05 voltage regulator -------- schematic #2
Why not take the best of each circuit? Use half of the 4013 to debounce the switch, and the other half as the toggle FF. That way, you won't need the voltage regulator and its associated capacitors. A 100nF cap should still be connected across the power pins of the 4013.

Shoe-Laze

Joined Jul 19, 2013
6
Thank you for your replies! I will try to build this and let you know how it goes. But first, I will ask what Ron's suggetion of combining the two circuits would do? And how would it look in the schematics?

Ron H

Joined Apr 14, 2005
7,014
Thank you for your replies! I will try to build this and let you know how it goes. But first, I will ask what Ron's suggetion of combining the two circuits would do? And how would it look in the schematics?
It does the same thing as either of absf's circuits would do. It looks like this.

Attachments

• 25.4 KB Views: 67

Shoe-Laze

Joined Jul 19, 2013
6
I'm constructing this circuit right now, so I can soon let you know how it works out! But where does Q on U1a go?

Ron H

Joined Apr 14, 2005
7,014
I'm constructing this circuit right now, so I can soon let you know how it works out! But where does Q on U1a go?
Leave Q1 disconnected.

Shoe-Laze

Joined Jul 19, 2013
6
Alright!
I can start to say that this is probably my first attempt on anything like this, and I made some worse-than-rookie-mistakes apparently. But bear with me, and I'll pick up all I can and learn When reading about schematics I was confused with the various symbols for + and - so when reading Ron's schematic I used Vcc for - and Arrows for + regardless of what I previously knew. Please tell me it should be the other way around?

Other than that mistake, I re-did the circuit in three stages for verification from the experts. Am I way off, or do I just need some tweaking before applying power?

Thanks!

Attachments

• 305.2 KB Views: 51
• 280.8 KB Views: 50
• 293.1 KB Views: 44

absf

Joined Dec 29, 2010
1,968
Alright!
I can start to say that this is probably my first attempt on anything like this, and I made some worse-than-rookie-mistakes apparently. But bear with me, and I'll pick up all I can and learn When reading about schematics I was confused with the various symbols for + and - so when reading Ron's schematic I used Vcc for - and Arrows for + regardless of what I previously knew. Please tell me it should be the other way around?
I checked the wiring of the 3 stages and they seem to be correct except the battery plus and minus are backward. The "arrow" sign is negative (black) and Vcc is actually positive (red)

I hope you did order extra 4013 just in case you accidentally kill one.

Allen

Last edited:

Ron H

Joined Apr 14, 2005
7,014
If you look closely, you will see that battery "+" connects to vcc (through the switch), and battery "-" connects to ground (the little arrows).