LED power for feedback looper - smaller batteries

[joeychickenskin]

Hi

I'd like to build a feedback looper with a stutter switch  and only really need power for LEDs.

I've only powered pedals with 9v batteries before.

For 2-3 x 5mm LEDs could I use a couple of 3v coin batteries? Could I still use a 2-5k CLR or would I need to adjust them?

Thank you

[hymenoptera]

a couple (2?) 3v batteries would give you 6v, and typical LEDs voltage drops can be well over a volt. 5 LEDs in series with 6v might be cutting it close.

Try this calulator though. Measure the forward drop (with your multimeter) for the LED you want to use. Enter the dope in the calculator and it gives you a schematic and what resistor to use.
http://ledcalc.com/

Really, a 9v should last a LONG time powering only an LED, especially if you use a single super bright at like 2mA or something.

edit: there's buttons at the top of that page you can click that will give you parallel calculations. You might be able to work out something that way with only 6v and 5 LEDs.

Also, I wanted to add that those button cells aren't known for stellar capacity, meaning they probably wont last as long as a standard 9v cell. And they might cost more. Something to think about.

[hymenoptera]

Sorry, I misread you said 2 to 3 LEDs, not 5. Still above all applies.

[davent]

If you're dropping the supply voltage to the led's the current limiting resistors will need to dropped as well for the led's to be of similar brightness. Ohms law is your friend.

dave

[deadastronaut]

I,m guessing your getting/squeezing  this in a 1590a maybe?

I always fancied using a joule thief for just switching leds in passive boxes..

1.5v AA/AAA batt...3 leds easily......and more...breadboard up your leds choice to test..

Hmmmm...just thinking aloud...

[hymenoptera]

I always fancied using a joule thief for just switching leds in passive boxes..

1.5v AA/AAA batt...3 leds easily......and more...breadboard up your leds choice to test..

Hmmmm...just thinking aloud...

You'd risk picking up some serious interference. The emissions from even a switched mode PSU can wreck havoc on high impedance signals if certain precautions aren't taken. A joule thief would almost certainly raise hell, especially inside an enclosure like that.

But I'm just thinking aloud too, so I could be wrong. Maybe right size little cap across the jack terminals could shunt the dirty dirty to ground.

[joeychickenskin]

Thanks everyone, all great advice. That LED calculator is really useful

Thank you

[Transmogrifox]

Apologies if this seems irrelevant to the OP.  It looks like the original post has been adequately answered....but if somebody later reading this has interest in the joule thief...

I always fancied using a joule thief for just switching leds in passive boxes..

I think this is an under-used circuit in DIY stompbox LED indication.  It always feels like a shame to make something ultra-low power (like a lo-waht) and realize the LED burns 3x as much power as the stompbox effect.

A joule thief would almost certainly raise hell, especially inside an enclosure like that.

This is a good point.  However, why not try it?  Sometimes it's just plain fun to try something even if it doesn't turn out as expected.  Every once in a while we get surprised.

It is perfectly possible to include a joule thief in a stompbox design without bad noise effects.  You just need to pay attention to layout and decoupling.

In a passive box, decoupling is easy:  just don't connect it to the audio ground and there's no place for it to push and pull currents on the audio system.  I am doubtful that radiated noise will be a problem in the frequency range where the JT operates (edges and transients usually don't go above 30 MHz).  It's all low power to begin with.  It's not like we're delivering an amp at 9V.

[hymenoptera]

So true. One would have to try it out to be sure. After all, transformers are used in pedal enclosures, and with proper considerations they don't cause problems. I'm no inductor expert, so I really couldn't say for sure. I shouldn't have been so absolute about a joule thief causing problems. Thanks for setting me straight

[Transmogrifox]

I shouldn't have been so absolute about a joule thief causing problems.

It was good to mention it because if used without some attention to layout and decoupling it is almost absolutely a problem.

The perverse thing about these loosely controlled switcher circuits is they almost always have audio frequency jitter or other similarly correlated noise.  It's a good point that we can't simply assume if it's above 50 kHz it's inaudible.  These switchers almost always contain audible noise patterns unless something has been done specifically to prevent it...we also need to give specific attention to keeping it decoupled from our audio circuits since this is usually easier than preventing audible patterns from existing on the ultrasonic carrier.

[hymenoptera]

It's also true about stuff above AF. Good example would be opamp oscillation which is often above audio band but swings practically rail to rail making that stage hot and unusable, etc. Your signal just stops there, until you put a scope on the output and realize it's screaming at RF! We always try to care about out-of-band stuff, especially when it's unpredictable.