LED Indicator

[Phend]

Hello,
What effect does adding a LED / resistor combo to a circuit have other than needing power and lighting up to indicate "something ". ?

[antonis]

To act as a part of a vactrol with a photoresistor..
https://en.wikipedia.org/wiki/Resistive_opto-isolator
(see Applications..)

[anotherjim]

Depends on the Vf forward voltage of the LED and the required CLR current limiting resistance. You can't always just hang it anywhere on a circuit unless you're sure the current it takes won't change that circuit. That said, some high-efficiency LEDs light up enough for very little current which allows a CLR of 20k or a little higher but that still affects many audio circuits other than opamp or buffer transistor outputs. And of course, the Vf of the LED means the voltage at the circuit must exceed the Vf to work at all.

[Kevin Mitchell]

Depends on the Vf forward voltage of the LED and the required CLR current limiting resistance. You can't always just hang it anywhere on a circuit unless you're sure the current it takes won't change that circuit. That said, some high-efficiency LEDs light up enough for very little current which allows a CLR of 20k or a little higher but that still affects many audio circuits other than opamp or buffer transistor outputs. And of course, the Vf of the LED means the voltage at the circuit must exceed the Vf to work at all.

Well said!
Reminds me to the trouble of implanting an LED into the small-stone circuit, where the draw is allocated away from the OTAs the lfo is driving - causing trouble unless compensated.

So really it all depends where the "indicator" is pulling current from. Supply voltage, no problem... Controlled current junction... more consideration is needed unless there's a buffer there.

[Phend]

Supply voltage, no problem

Which is where it is most often, connected to supply.
Is it benifical having a LED, connected to supply, in the circuit, other than "Indicating it's On" ?

[marcelomd]

Is it benifical having a LED, connected to supply, in the circuit, other than "Indicating it's On" ?

"It is on AND supply voltage is more or less correct, fluctuating, etc"

Ask me how I know =)

[Phend]

^ How do you know?

[mozz]

It's a diode, they make noise. Only joking. They do make noise but it's so small you would need some good test equipment to measure it.

[ElectricDruid]

Supply voltage, no problem

Which is where it is most often, connected to supply.
Is it benifical having a LED, connected to supply, in the circuit, other than "Indicating it's On" ?

No, there's no other benefit to the typical "on light" LED connected across the supply when the effect is switched in.

In fact, sometimes they cause pops or clicks when you switch the pedal because the LED is a current draw that kicks in instantly - hence mitigation measures like using higher efficiency LEDs (less current for the same light output), increasing the series resistor (less current) or adding caps (slows down the current spike so you can't hear it).  So in some cases there are benefits to *not* having the LED, but I've never heard of other benefits of *having* it.

It's useful to show the effect is on is all. Like you don't know when you just kicked on the latest Fuzzgasmatron 4000!

[PRR]

You can't see electricity. It is not always convenient to put your tongue on it. So a pilot light is basic trouble-shooting: "Is it plugged in? Is it turned on??"

The pedal could not care. It is a small drain on a battery. The noise should be quite insignificant if you are tapping a power rail (not a signal point; even then no big deal).

[marcelomd]

^ How do you know?

Where I live we have two mains voltages. 127V and 220V. Power supplies other than SMPS usually have a switch that I always forget about.

When you plug a 220V equipment in a 127V outlet, the LED warns you by being faint

When you plug a 127V equipment in a 220V outlet, the LED warns you by flashing VERY bright once, and then turning off.

Sometimes the LED oscillates, indicating bad power.

[antonis]

Where I live we have two mains voltages. 127V and 220V.

And pluralistic constitutionalism, may I suppose..??

P.S.
Never heard of a country with dual mains..

[idy]

Brazil is like that. State to state and sometimes city to city. Got to travel with dual voltage adapters. Diverse.

[amptramp]

One more function of the LED is a power supply voltage indicator.  Some pedals have a zener diode in series with the LED.  If the supply voltage is too low, the LED doesn't go on regardless of whether it is in operational or bypass state.

[marcelomd]

Dual mains is inconvenient, but not unique.

Canada has 120V and 240V. China, 220V and 330V. And I know Japan has 100V and 200V at 50Hz and 60Hz.

I heard that SMPS were invented because of this (but I cannot find a reference).

And our constitution is... in need of an overhaul, frankly.

[Phend]

Some pedals have a zener diode in series with the LED.  If the supply voltage is too low, the LED doesn't go on regardless of whether it is in operational or bypass state.

Hum, like a battery "low" indicator.
What low voltage value does a typical effect stop working, or is this to vague of a question  ?
I read that some people like low voltage to add an effect to an effect.

[ElectricDruid]

What low voltage value does a typical effect stop working, or is this to vague of a question  ?

It totally depends on the circuit in question.

However, that said, if you want a "typical" number, I'd say "around 7V or so". That's pretty low for a "9V" supply, and it's about the voltage that some op-amps start to shut down, although TL072 will go lower. It's also about the minimum voltage you need to run a 5V regulator, which is significant for any circuit that includes 5V digital elements.

It really does completely depend on the circuit though. If I used a AS2164 quad VCA chip in the circuit, it has a *minimum* voltage of +/-4V, so you really need a 9V supply that stays at 9V to use it reliably. On the other hand, the AS3080E OTA can go as low as +/-2V and has been used effectively even below that datasheet specification.

[antonis]

It also depends on power supply reservoir capacity and load..

A big cap and a light load might withstand voltage sag (to an acceptable level) according to formula: Vc = Vs x e^-t/RC, where Vc is the voltage across the capacitor, Vs is the supply voltage, t is the elapsed time since the removal of the supply voltage and RC is the time constant of the discharging circuit..

Unless, Phend talks about "for ever" low voltage working..

[PRR]

Canada has 120V and 240V. China, 220V and 330V. And I know Japan has 100V and 200V at 50Hz and 60Hz.

US copies Canada(?). I have 240V in a few places but mostly 120V. Japan has several networks, 100V and nominal 110V, in 50 and 60Hz flavors.

Brazil seems to be distributing 3-phase power, which shows a square-root-three step between voltages. In my old school there was neutral and three hots. 117V from any hot to neutral, but 203V (nominal 208) from one hot to another. We ran a 230V range/oven on the 208. Took a little longer to come to temp but once the thermostat was in control the oven worked fine. Brazil seems to feed 220V across two (of three) hots which makes 127V hot to neutral.

Switchers are not really about multi-voltage. Transformers can have multiple taps. But when you need -regulated- voltage, as for computers, the difference in size and heat became compelling before 1979. I had a Diablo "printer" (really a specialized computer) with a massive transformer and heatsink; and a 5150 IBM PC with the switcher. The Diablo PS was full-width and hot, the PC PS was third-width and warm. Costs were similar then (before you factored a larger box) today mass-produced switchers are stupidly cheaper because less guts.

[marcelomd]

Yup. 3-phase at home. Weirdest thing is when one phase is down and only half the house has power.