Using a single dimmed LED for metering.

Has anyone ever used a dimmable LED bulb to express voltage / signal presence / G.R.? I'm toying with a 500 series build which would benefit from some basic / not-super-crucial metering, but (cough, 500 series) space is a consideration.

So I was thinking of maybe just using one 3mm LED with a good range of luminescence that would react differently depending on voltage present.... a faint green for receiving a bare minimum threshold all the way up to "JFC my EYES" if it's overloaded.

If anyone's tried this or has any reco's? I really would like to have in/gr/out metered in a simple, tactile way - but it's more for a quick visual reference than actual measurement (i mean, this is a 'one knob comp' with a switchable soft clipper.... it's not exactly mastering grade.

gnarmageddon said:

Has anyone ever used a dimmable LED bulb to express voltage / signal presence / G.R.? I'm toying with a 500 series build which would benefit from some basic / not-super-crucial metering, but (cough, 500 series) space is a consideration.

So I was thinking of maybe just using one 3mm LED with a good range of luminescence that would react differently depending on voltage present.... a faint green for receiving a bare minimum threshold all the way up to "JFC my EYES" if it's overloaded.

If anyone's tried this or has any reco's? I really would like to have in/gr/out metered in a simple, tactile way - but it's more for a quick visual reference than actual measurement (i mean, this is a 'one knob comp' with a switchable soft clipper.... it's not exactly mastering grade.

Click to expand...

I've done something along this line, but with 3Led's. The first LED (green) follows the signal intensity and the others gradually set in. I don't remember any of the resistor values but it's not very difficult to experiment and find out.

You can get LEDs that can do multiple colors in the same package which would save on space. Having a separate color that provides clear overload indication is preferred imo.

gnarmageddon said:

Has anyone ever used a dimmable LED bulb to express voltage / signal presence / G.R.? I'm toying with a 500 series build which would benefit from some basic / not-super-crucial metering, but (cough, 500 series) space is a consideration.

So I was thinking of maybe just using one 3mm LED with a good range of luminescence that would react differently depending on voltage present.... a faint green for receiving a bare minimum threshold all the way up to "JFC my EYES" if it's overloaded.

If anyone's tried this or has any reco's? I really would like to have in/gr/out metered in a simple, tactile way - but it's more for a quick visual reference than actual measurement (i mean, this is a 'one knob comp' with a switchable soft clipper.... it's not exactly mastering grade.

Click to expand...

I have not had success modulating the brightness of leds to indicate loudness...  For simple signal display two color, green for signal present and red for OL warning seems to work well... I was unable to get anything useful from both red/green on at same time...

JR

john12ax7 said:

You can get LEDs that can do multiple colors in the same package which would save on space. Having a separate color that provides clear overload indication is preferred imo.

Click to expand...

I was considering that as well - just trying to consider the space I'd take up on a daughter card to create the network for 3 leds. (The current design calls for three of these - one for input gain, one for gain reduction and one for output gain). But again, these don't are more for quick visual reference, not for critical readout.  (The comp is intended to be more for the smashy-smashy than critical application)

Looking right now at some diagrams for the LA-2a and 4a for the send to the photocell - that's about all I'm really trying to do with these. More signal, more light. More GR, more light.

Hi gnarmageddon

Douglas Self came up with something called a 'Log Law Level Led' that uses a log converter to give a sensitivity range of about 10dB and the LED on-off ratio gives an indication of where the signal is within that 10dB.

It is described in his Electronics for Vinyl book and give a circuit diagram as well.  It is a bit complicated just to drive just one LED, but does something along the lines you mentioned.

Cheers

Mike

Awesome - will check out. Cheers!

Couldn't you start with a simple LM339 comparator circuit to give you 4 discrete levels.
The LM339 is an open collector output, meaning, it's output shorts to GND when triggered by a threshold voltage.
Use a single LED, which goes to 4 transistors in parallell.
When 1 (minimum signal) Collector goes to GND, then you'll have minimum current.
When 2 (little more signal) collectors go to ground, you'll have 2 parallel paths to ground - more current.
When 3 (Decent signa) collectors go to ground, you'll have 3 parallel paths to ground - WAYHEY!
The 4th could either drive a different LED for Red VS Green, or you just have max current then.

Not sure if this makes much sense in text.

Not sure you need the extra transistors, just use the ones in the 339. What you are describing is a simple 4 bit DtoA with a current output that drives the LED. Connect LED to +ve supply. Cathode goes to 4 resistors the other ends of which are grounded by the 339 collectors as required. You can alter the 339 thresholds to indicate different signal levels and weight the four resistors to  obtain a sensible range of brightnesses.

Cheers

Ian

ruffrecords said:

Not sure you need the extra transistors, just use the ones in the 339. What you are describing is a simple 4 bit DtoA with a current output that drives the LED. Connect LED to +ve supply. Cathode goes to 4 resistors the other ends of which are grounded by the 339 collectors as required. You can alter the 339 thresholds to indicate different signal levels and weight the four resistors to  obtain a sensible range of brightnesses.

Cheers

Ian

Click to expand...

If you invest in a quad comparator you could just as easily light 4 different LEDs... 

In console indicator design there are different economies to consider. Front panel space is dear, so a bicolor LED has attraction for small panel footprint, while multiple LEDs can provide a lot of level information... Behind the panel circuit complexity for channels that get repeated nX is undesirable for cost/PCB real estate reasons.

JR 

On the theme of minimising front panel space, an interesting variant would be to use a multi-colour  LED driven by a set of window comparators so just one of them operates in each level range.

Cheers

Ian

ruffrecords said:

On the theme of minimising front panel space, an interesting variant would be to use a multi-colour  LED driven by a set of window comparators so just one of them operates in each level range.

Click to expand...

The sketches and cocktail napkins currently call for three different indicators (input, GR and out), so it'd definitely all need to inhabit a 2nd pcb on standoffs for metering. I could conceivably get 3 comparators on there without breaking a sweat - but economy of space and available power is the usual 500 series curse. I'm swimming a bit beyond my depth with the RGB LED's, doing some reading on how to make that work in this instance. But being able to have an arc of comparative values giving -36db green, 0db yellow, +18 db red to one emitter, especially if it was a continuous mix of colors.

It's officially into head-scratcher territory, but I'm reading up. Of course, it might come out distracting and butt ugly, except to that one guy in the band who likes taking mushrooms during the mix.

I looked at some three colour LEDs and it looks like they are all common cathode types. So I have come up with a scheme that uses a single LM339 to light one of them depending on the level. It uses two window comparators for the green and yellow plus some diodes and an inverting transistor to get the Red one to work as a peak indicator. The input voltage is assumed to be an already rectified/smoothed version of the audio.

Cheers

Ian

Almost all bargraph LED displays used in mixing desks utilise a constant current feed to drive the series chain of LED's, ensuring that the current taken from the supply (normally the same as feeding the audio path circuitry) - remains the same whether the LED's are on or off.

If you switch the LED's individually - you are likely to produce clicks in the audio when  each one turns on and off.........

AudioJohn said:

If you switch the LED's individually - you are likely to produce clicks in the audio when  each one turns on and off.........

Click to expand...

That can be avoided in a number of ways. You can source from the positive rail and sink to the negative rail and not source / sink from ground at all. I like to just use a separate ground for LEDs and relays and then connect the grounds together at the PS filter cap. Depending on the circuit it might be possible to connect grounds through a smallish resistor. Then, even if your LED / relay driver switches are a little too fast, the resistor can limit transients.

AudioJohn said:

Almost all bargraph LED displays used in mixing desks utilise a constant current feed to drive the series chain of LED's, ensuring that the current taken from the supply (normally the same as feeding the audio path circuitry) - remains the same whether the LED's are on or off.

If you switch the LED's individually - you are likely to produce clicks in the audio when  each one turns on and off.........

Click to expand...

Over several decades I probably did it as many ways as you can imagine... managing noise in consoles is what design engineers get paid to do (some better than others).

JR

AudioJohn said:

Almost all bargraph LED displays used in mixing desks utilise a constant current feed to drive the series chain of LED's, ensuring that the current taken from the supply (normally the same as feeding the audio path circuitry) - remains the same whether the LED's are on or off.

If you switch the LED's individually - you are likely to produce clicks in the audio when  each one turns on and off.........

Click to expand...

The constant current strings used in many audio bar graph displays can be applied equally well to small numbers of individually switched LEDs. Indeed there are low cost TO92 programmable current sources ideal for this. However, this may not be necessary. Instead of turning the LED on and off by connecting or disconnecting it, you can achieve the same thing by shorting the LED or not. In this way, when driven for example from a 15V supply, the change in current is much smaller.

Cheers

Ian

The biggest benefit from driving strings of LEDs from only a handful of current sources is saving power... Inside a large console the heat difference and current draw difference can be significant, times tens of strips...

JR

To add to the various possibilities, by chance I ran into an old, simple & elegant signal-driven duty-cycle modulator by Thomas Henry.

Can dig up & post.

One quad-opamp, 1 bi-colour LED and some R & C.