Fuzz Face bias indicator LED?

[Morocotopo]

Hi gang, I recently finished a Ge Fuzz Face with some mods, a buffer, a tone control, etc etc. Now I´m about to box it, and I´m planning to put an external bias pot to save me the aggravation of opening it and fiddling with a trimpot every time the weather changes... the idea is, if it sounds bad, tweak the pot till it sounds good. But I got to thinking (danger!) and an idea came to me: wouldn´t it be nice to have a LED that lights when you have exactly 4,5V on the collector of Q2? Sort of how a tuner works, but for voltage...

Am I clever or what!



Actually the idea came partly from a guy in another forum that made a fuzztone and put a digital voltmeter in the box!

http://www.elmusiquiatra.com.ar/viewtopic.php?f=36&t=24143

That´s too much for me.

So, that´s the idea. Anyone with a clue about how to implement it?

Thanks for reading.

[zyxwyvu]

You can try a window discriminator circuit like this:



It's based on a circuit on page 669 of the Art of Electronics, with a few modifications for your pedal. Just connect the bias voltage to "IN1", and the LED should light up whenever it's close to 4.5V. VR1 controls the sensitivity. Low resistance = narrow range, High resistance = large range. Make sure to use comparators, not opamps.

[.Mike]

You could probably do something pretty slick for not a lot of money with an LM3914, a couple of resistors, a capacitor, and a 10 LED bargraph display. If you order from Futurlec, you can get the LM3914 for $1.75, the bargraph display for $0.30 (bottom of page), and pennies for the rest.

There is even a calculator to figure out the appropriate resistor values to use.

Mike

[Gus]

IIRC Joe installed a panel meter in a fuzz.
IIRC  Some years ago there was a thread here about installing an indicator or meter to a FF type circuit.

Then there is this circuit that has been on the web for years
http://www.hollis.co.uk/john/rockface.jpg

[lvs]

http://g.landry.myweb.absamail.co.za/Smileyface.htm

[petemoore]

  Joe Davisson had a 1 transistor jobby for low battery indicator, transistor LED and a few peices including trimpot [to set 'on voltage], at least that would make a lighted indication where ''on = Vx'' is.
  A Jfet [high impedance input] buffer?..getting a reading = tapping FF circuit = might load it down.

[Morocotopo]

Guys, thanks for the suggestions.
Zyxwyvu, that looks simple and easy to do, I´ll try it. Too bad I already removed the parts from the protoboard, to mount them in the PCB that I haven´t etched yet...



I don´t think I´ll go with the bargraph meter, seems to me too much... but we´ll see.

When I have news I´ll report back.

[Gus]

http://www.diystompboxes.com/smfforum/index.php?topic=67988.0

http://www.diystompboxes.com/smfforum/index.php?topic=10987.0

[joegagan]

thanks gus, yes that was the nitroburner. interesting, i never think about looking in the archives.

[zombiwoof]

I was thinking you could just have external bias points/jacks that you could plug your meter probes into to easily check the bias.  Are there jacks that would accept the standard probes on a meter?.

Al

[John Lyons]

Yes, those jacks are cap tip jacks. For plugging meter probes into.
They come in red and black even.

[zombiwoof]

Yes, those jacks are cap tip jacks. For plugging meter probes into.
They come in red and black even.

Thanks, I now have a new mod to do to my Fuzz Face!

Al

[phector2004]

Just a random idea:

Can this be done with a dual-gang pot, with one set connected to bias and the other "harnessing" fwd voltage drop, using multiple diodes in series with the LED to get it to turn on only above 4.5V?

not very fancy, especially since it wont turn off above 4.5V, but it's pretty simple IMO

[petemoore]

  Stick the cheap meter in with the FF.
  Put a switch on it's power supply, the output, and the bias tap [at least the bias tap and the power supply for the meter, save batteries and allow 'FF' to be when not in bias test mode.
  Then you can have a meter reading by stepping on or flipping a switch, momentary or other, only real drawback being the large box to house the circuit and the meter. Try search for Voltmeter kit, perhaps there's a super simple PCB that can be made to do the part of this you want [volt reading in 20v range] without the whole case/multiswitch/battery box/lead jack-mounts = DMM size deal.

[kupervaser]

Something like this:

http://store.qkits.com/moreinfo.cfm/FK924

[Morocotopo]

Guys, great info. Too much options!   

I´m a little short on time currently, but I´ll try all the possibilities as soon as possible. I´ve been fiddling with this Fuzz Face since the beginning of the year! Time to finish it already 

John, those connectors are nice. I´ll see if I can get them here.

[WangoFett]

Hey why not just automatically adjust the bias so that there is 1/2 voltage supply on the collector?
Actually now I remember coming across a circuit by John Hollis called the Rock Face that does that: http://www.hollis.co.uk/john/rockface.jpg
The target voltage could even be user tweakable by smashing a pot in somewhere near the op-amp's inverting input.

[Morocotopo]

OK, news report: I tried zyxwyvu´s schem with a variable power supply, ´cause my Fuzz Face is in pieces awaiting it´s PCB.  Works like a charm! Thanks sooooo much Z (don´t expect me to write your nick more than once...)!



I think I´ll go with this because it´s the simplest option: just a LED and a pot, tweak the pot till the LED lights up, done. And it can be added easily to any pre made PCB or pedal.
The only thing left to test is if this addition will have any impact on the sound. What do you think guys?

[zyxwyvu]

OK, news report: I tried zyxwyvu´s schem with a variable power supply, ´cause my Fuzz Face is in pieces awaiting it´s PCB.  Works like a charm! Thanks sooooo much Z (don´t expect me to write your nick more than once...)!



I think I´ll go with this because it´s the simplest option: just a LED and a pot, tweak the pot till the LED lights up, done. And it can be added easily to any pre made PCB or pedal.
The only thing left to test is if this addition will have any impact on the sound. What do you think guys?

Glad to see it worked out well! (and z is fine; you can also use my real name, Josh)

[zombiwoof]

The only thing I would say about having an LED that lights when the voltage is at 4.5 volts is that many guys report that their FF's sound best at a slightly higher voltage (like 4.75, etc.).  At least the LED would tell you when you are at the 4.5v point, and you could still adjust slightly from there, but that points out one advantage of a digital readout, you could adjust to exactly whatever setting you find the best.

Al