Weirdest Thing Ever

gez · December 31, 2007, 05:51:48 PM

Quote from: gez on December 31, 2007, 05:43:11 PM
Because red LEDs have a forward drop of around 2V or so, the gate is held at approx 7V, ensuring that the FET is fully turned on.

Well, because of the huge resistance of the reverse-bias diode, the gate isn't really at 7V, but the diode is being pulled up to that voltage, in turn pulling up the gate to whatever (hopefully enough to turn it on).

ayayay! · December 31, 2007, 06:00:08 PM

I got it now, thanks guys. I didn't start the thread, but it intrigued me. Hopefully kurtlives got his answer.

TELEFUNKON · January 02, 2008, 04:06:43 AM

Quote from: gez on December 25, 2007, 03:42:45 AM
If you don't want to go down the route of using a DPDT, the following might work out OK (depends how much 'creative soldering' you have to do). I think this is what Sock Puppet was mooting.

Disconnect the 10K (R25) from the junction of the 4.7M (R24) and the switch, and wire in your LED between them: cathode to upper leg of the 10k and anode to the switch/R24 junction. Keep the red LED soldered in as shown (for the moment), but use the blue/new one as your status indicator.

Ideally, you'd want to remove the red LED so that it can be used for another project, but pulling everything up to Vcc might introduce switching noise. You could short the LED with croc-clips to see what happens. If you get switch glitch, then making C16 slightly larger (or sticking a cap in parallel with it) might sort things out. Alternatively, pull everything up via a resistor instead of the red LED. 5k1/10k?? Whatever option you take, you'll probably need to increase the value of R25 to adjust for brightness (and increase the value of any pullup resistor if used).

Easiest option, though, is to stick your new LED in, socket the FET and try different ones until it works (hopefully!)

PS, I didn't really pay much attention to what effect this was for, but if it's one of the new EH jobbies, don't bother posting the whole schemo (the fragment shown is enough, anyway).

Maybe someone ought to warn Sock Puppet that hosting copyrighted material can lead to nasty consequences,
as has been proven in the past?

kurtlives · February 01, 2008, 02:51:37 PM

Sorry to bump this up again..........I tried a resistor on the cathode....All it did was dim the LED......still no chorus.

Help?

gez · February 01, 2008, 05:50:02 PM

Quote from: kurtlives on February 01, 2008, 02:51:37 PM
Sorry to bump this up again..........I tried a resistor on the cathode....All it did was dim the LED......still no chorus.

Help?

What do you mean by 'resistor on the cathode'?

The solution was outlined by Sock Puppet

Quote

http://s273.photobucket.com/albums/jj219/designacryl/?action=view&current=led.jpg

This should work:

Replace R25 with a higher value, say 47k/100k. Connect your LED with current limiting resistor added across this.
Short out the original LED so the footswitch common connects directly to Vcc.

By 'added across this', he means in parallel with R25. It might not be a good idea to short out the original LED, you'll have to see whether this introduces switching noise.

kurtlives · February 01, 2008, 06:25:24 PM

Ok I will try this next

kurtlives · February 01, 2008, 07:14:09 PM

Ok I replaced R25 with a 51K resistor...I solder a 5mm diffused red 5mm LED parallel to it...No limiting resistor...

I also left the wires that connected the LED prior....You know the two wires that connected to the LED when it was stock....

The result...the LED does not light up and still no chorus....Does it matter the configuration of the LED? Ex. cathode this side

Sack Puppet · February 01, 2008, 11:27:50 PM

Picture worth 1000 words:

http://i273.photobucket.com/albums/jj219/designacryl/Switch.jpg

When I said short out the original LED I meant bypass/remove it.

Hope you win soon.

S.

kurtlives · February 01, 2008, 11:45:43 PM

Im sorry but I didnt get that...

Sack Puppet · February 02, 2008, 05:40:16 AM

The diode under R in the diagram is of course your LED, I just drew it wrongly.

I see no reason why this wouldn't work and don't see an issue with noise.

S.

gez · February 02, 2008, 07:32:07 AM

Quote from: Sack Puppet on February 02, 2008, 05:40:16 AM
I see no reason why this wouldn't work and don't see an issue with noise.

It will definitely work. However, by pulling everything up directly to V+, as opposed to a couple of volts short due the original LED's drop, there's potential for increased 'charge injection' into the gate of the FET. This can translate as switching noise.

I doubt it will happen here, but it's something to be aware of, and I'd definitely advise to A/B test before removing the original LED (keep in circuit and short with a crock clip)

kurtlives · February 02, 2008, 10:58:59 AM

So what did I do wrong?

Sack Puppet · February 02, 2008, 12:48:54 PM

However, by pulling everything up directly to V+, as opposed to a couple of volts short due the original LED's drop, there's potential for increased 'charge injection' into the gate of the FET.

Take another look at the diode on the gate of the FET Gez.

So what did I do wrong?

S.

gez · February 02, 2008, 12:57:52 PM

Quote from: Sack Puppet on February 02, 2008, 12:48:54 PM
Take another look at the diode on the gate of the FET Gez.

The reverse diode is there to minimise charge injection, but that doesn't mean it prevents it entirely. One fix I used to do for LFO tick in chopper trems using FET switches (wired up in a similar way as bypass switching) was to divide down the square wave using a trimmer, and then feeding the wiper to the diode. It cures the problem, at the expense of possibly creating a little more distortion of the signal, and you'd be amazed at how much a reduction in noise it makes (or looked at another way, how much a slight increase in voltage can create tick/noise).

If all it took was a reverse bias diode to prevent switch glitch, they'd be no need to 'slowly' ramp up the gates of FETs as well. I stand by my words...

kurtlives · February 02, 2008, 01:18:55 PM

So the cathode needs to be going to the ground....

kurtlives · February 02, 2008, 06:53:42 PM

AHHHHHHHHHHHHHHHHHHHHHHHHHHHH!!!!!!!!!!!!!!!!!

sh its goin fly

This thing wont work...cathode is going to the ground....R25 is 51K, the LED is parallel.

Please tell me I over looked something super simple.

gez · February 03, 2008, 03:38:31 AM

Engage the switch so that the effect is supposed to be on and test with a multimeter. Check that everything is being pulled up to V+, or a couple of volts short if you've left the original LED in.

Are you sure you've got the LED orientated the right way? A quick out of circuit test might be an idea.

kurtlives · February 03, 2008, 10:36:49 AM

I tired the LED both ways....should I try with it completley out?

How do I meausre if everything is being pulled to V+?

gez · February 03, 2008, 12:06:22 PM

Quote from: kurtlives on February 03, 2008, 10:36:49 AM
I tired the LED both ways....should I try with it completley out?

How do I meausre if everything is being pulled to V+?

Using Sack Puppet's hand-drawn schematic above, the junction of the 4M7, the 51K and the LED's current-limiting resistor connect to one lug of the switch. With the contact switched to the other lug as shown ('Bypass'), you should measure 0V at this junction. When you hit the switch (effect mode), you should now see 9V at said junction (or, if you've left the original LED in place, just less than 7V).

kurtlives · February 03, 2008, 12:18:02 PM

So do I need to rewire the swich?

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