Where best to cut the LED?

[m_charles]

This has to be a subject covered a million times, but I guess its a weird search to do because I couldn't pull anything up.
Assuming a 3PDT is being used... As far as noise, click, etc, does it make any difference whether you interrupt the GROUND or the 9V+ for the purpose of the LED coming on and off with the effect?
I've always done it with a lead coming from the limiting resistor connected to 9V+ on the PCB to one lug, then another 9V+ lead to the other lug. Pretty common.
Just had the thought wondering if it made a difference if I interrupted the -neg leg of the LED to ground instead, etc...
Thoughts?

[R.G.]

If the LED and resistor and the switch that turns them on/off are all in series from +V to ground, it makes no difference whether you interrupt the ground or power side.

The exact details of the power and ground wiring/traces and the details of the rest of the circuit make a difference as to whether the sudden change in current will make an audible noise.

To mitigate any noise, you can either slow down the transition from on to off, or be sneaky. Slowing down the transition is the subject of many schemes using various resistors, caps, diodes and what not. Sneaky is to change the game. What if you were able to dramatically reduce the amount the current in power and ground changed when the LED was brought on/off?

One simple sneaky way is with shunt switching. Use a resistor and LED from + to ground, but turn the LED off by using a transistor or FET in parallel with the LED. When the transistor eats all of the available current, the voltage across the LED drops to below the conduction threshold and it stops glowing. If you juggle transistors, drive current and resistors cleverly, the difference in current from LED on to LED off can be very low.

Even better, drive the LED from one collector of a differential amplifier. The current source in the diffamp emitters sets the current, and the difference in voltages on the bases sets which path the current flows in. The difference in current from LED on to LED off can be almost zero.

Sneaky, eh?   

[m_charles]

Wow, thx. This is similar to the "discussion" the other day that we had.
Do you have a link to an example of using the FET or BJT to do this?
This sounds very interesting and I appreciate your thorough reply.

[armdnrdy]

http://www.diystompboxes.com/smfforum/index.php?topic=106850.0

[R.G.]

Sorry, I don't know of any examples. I've done all of these, but not for pedals.

There are some obvious issues with these tricks, notably that the battery use is just like leaving the LED on all the time, and the LED may  be the biggest single user of current in a small pedal. However, the day of the battery in pedals is about over. It's still around, but costs too much and has other issues, giving you only isolation and guaranteed quiet power in return. If a pedal is not battery powered, the use of a continuous 3-10ma of current for an LED is not an issue.

[m_charles]

Sorry, I don't know of any examples. I've done all of these, but not for pedals.

There are some obvious issues with these tricks, notably that the battery use is just like leaving the LED on all the time, and the LED may  be the biggest single user of current in a small pedal. However, the day of the battery in pedals is about over. It's still around, but costs too much and has other issues, giving you only isolation and guaranteed quiet power in return. If a pedal is not battery powered, the use of a continuous 3-10ma of current for an LED is not an issue.

Ahhh. Damn laws of electricity. Always gotta be a catch. Haha
Don't know if I agree with you about the batt. Guitarists are stubborn and don't like change.
Also, there's nothing easier than grabbing a pedal and play. No power cable needed.
However, my theory isn't that the power supply will go away, my theory is that the batteries will eventually change.
Like, one battery lasts 5 years. Something like that. Like a watch battery.
Or perhaps there won't be a detachable battery, but you just recharge the pedal.
However, this wouldn't be much good until the batts can hold juice a lot longer than lithium.
Making a pedal that charges like an iphone wouldn't be that difficult (as u know RG), but making it not a total pain in the a$$ would be!
?

[armdnrdy]

there's nothing easier than grabbing a pedal and play.

And there's nothing more annoying than finding out that you forgot to get fresh batteries right before you're about to go on stage!

one battery lasts 5 years.

I don't see that happening very soon. I don't believe that Energizer, Duracell, Rayovac, etc, would like to take a big slice out of their profits to sell you one battery every five years. 

[nosamiam]

I don't see that happening very soon. I don't believe that Energizer, Duracell, Rayovac, etc, would like to take a big slice out of their profits to sell you one battery every five years. 

[/quote]

No, but it seems like pressure is being exerted to come up with longer lasting in other related industries such as cell phones. It may not be the big players in alkaline/lithium/etc technology who are pushing the envelope. If that tech crosses somehow crosses over into our field, we could be accidental winners. Rechargeables that can hold a charge for a decent length of time with pedals drawing minimal current seems like a good combination. Seems like limitations in voltage are what's holding us back, but I wouldn't think that will last forever.

[jubal81]

Better super caps - recharge in about 10 seconds.

[PRR]

> Better super caps

A 0.12F (not uF) 9V super-cap is $36.

Assume the 9V pedal sucks 9mA. Then the equivalent resistance is 1,000 Ohms.

1,000 Ohms times 0.12F is 120 Seconds. At the end of this time the original 9V is down to 63% or 5.6V.

Two minute run-time is disappointing.

Some pedals (without LED) can be 0.9mA or 10K equivalent resistance. 20 Minutes at least lets you get a few songs done. But you could be re-filling your power more often than you re-fill your beer (depending on your thirst).

This performance is about 10 X better than the last time I took a good look, 10 or 20 years back. So they ARE getting better, but not in a big hurry.

[Lurco]

Better super caps - recharge in about 10 seconds.

http://www.maxwell.com/285v/ ?