Driving Ms Diode (LED)

I’m working on a module idea for a friend which is basically a collection of attenuverters, each with a button. Each one can be dialed to a different voltage, and when the button is pressed it delivers the voltage it’s set to, to a single output. When the button is pressed and the voltage delivered, an LED lights up. Right now the circuit in my head looks like this:

Voltage in > button > attenuverter > voltage out

I’m just having trouble figuring out where in the circuit to drive the LED. So far I’ve considered:

  1. Driving the LED with a transistor after the button but before the op-amp
  2. Driving the LED with a transistor after the op-amp but before the output impedance resistor (prob won’t do this since the output from the op-amp could be positive or negative)
  3. Putting the LED in the (negative) feedback loop of the op-amp [ dunno if this will work, if I need a PNP transistor driving it here or what ]

The module will be running on standard eurorack power (+/-12v) and I’ll be regulating a 5v supply as well, which is what will be going to to the attenuverters, to give a +/-5v output. Right now my intention is to have the LED light at a constant brightness, though an option to have it light relative to the level of the attenuverter would also be welcome (I figure this would go on pin 2 of the pot, between the pot and the +input of the op-amp?).

Anyway, it seems a pretty straightforward circuit, but the LED is what’s tripping me up. Any help is greatly appreciated.

Editing to add: I’m basing the attenuverter circuit on this one from Skull & Circuits. It’ll be taking 5v at the input (where the jack is in the diagram, obv) so I may need to change the resistor values, I know. I’ll figure that out in due time.

I think I figured out my solution. The LED needs to go between the button and the attenuverter. It looks like I can just do an LED and a resistor to ground without a driver circuit (maybe even in series? I haven’t simulated that yet). When I drove it with a transistor tied to that junction the transistor just sank all the incoming current and there was no output from the op-amp, which I’m sure isn’t good for the transistor in the long run. I’ll do some further simulations and see what I can come up with.

You could maybe add this to the options:

  1. Use a button that makes 2 connections. One for the signal, one for the led.

If there’s no input jack, just a connection to a voltage source, that should work. If there’s a resistance in the path between the voltage source and the button (or after the button and before the LED) then that’ll potentially sag that voltage, if the LED resistor isn’t large compared to that other resistance.

Right now, when the button is pressed it sends the source voltage (5v) to the attenuverter circuit and also to the base of an NPN, which connects 5v at the collector to the emitter and connected LED + 470R resistor to ground. I like that this gives a consistent bright level regardless of the draw from the rest of the circuit.

I’m working on finding a way to do a soft latch circuit for each button of several–no more than 8-- (probably involving a CD4044). The ideal situation is an array of momentary buttons:

  1. The circuit starts up with all button circuits in their off state.

  2. Push a momentary switch to connect the output of its associated attenuverter to the output jack. The button circuit latches on and the output remains connected.

(nb: I’ve got the button placed before the rest of the circuit and then am buffering/mixing the outputs at the end.)

  1. Push again (a simple push, not anything like a 3 second long press) to turn off the circuit which is latched on. This should be relatively simple.

  2. When a button is pressed, any other latch circuit which is currently on will turn off, and the most recently pressed button will latch on.

  3. A master switch (a physical toggle) which disables the soft latches and makes each button work as a normal momentary push button/manual gate.

So, I’m thinking: CD4044 quad S/R latch–press the button, it takes the SET pin high for that latch, turning it on. Easy.

When a button is pressed, it activates a trigger generator which pulses all the RESET pins high, turning all the latches off. I think I would have to have a trigger gen for each button to do this next trick, but when the button is pressed it sends the reset trigger, but also sends a copy (?) of the same trigger through a very brief delay before it hits the SET pin for the latch, turning the circuit on.

The latch/momentary selector would probably just pull all the RESET pins high and hold them there, though I’m uncertain if the output of the CD4044 will go high at all if the RESET pin is high (EDIT: probably not, it appears to generate an “invalid” state).

On button-press, maybe I should set it up so ENABLE gets pulled low, all the latches are reset, then the SET pin for the activation latch goes high, and then ENABLE is brought high again?

Maybe I can use a different SR latch IC with both Q and Q-not outputs and use Q-not to reset the other latches?

Thoughts, oh genius hive-mind? I know this was a lot. I’ve been rolling this around in my head all day and now it’s late and I’m rambling. Sorry.

is this a signal LED or a status LED?

I guess I would call it a status LED, though the distinction seems very fine. It lights up when the circuit is active.

Did you make any progress with this? I’m looking for a similar latching circuit but haven’t find or come up with a solution yet.