LED Dimming

Thread Starter

dlad87

Joined Nov 9, 2020
27
Hello,

I’m looking to dim a standard 5mm led based on the position of a 100k volume pot.

Not sure if it’s possible to do so using a single gang pot that is already controlling the volume of a mono audio circuit?

I am also not opposed to using a 100k double gang pot (one side for volume and one for the led) but am still not sure how I would supply the right voltage range to the led with the 100k value pot. Not sure if there’s some kind of led driver that could do the job or some kind of CCS.

The supply voltage is 9V.

Thanks
 

wayneh

Joined Sep 9, 2010
17,153
I’m looking to dim a standard 5mm led based on the position of a 100k volume pot.

Not sure if it’s possible to do so using a single gang pot that is already controlling the volume of a mono audio circuit?
I don't think that's likely to work. The double gang approach can work, but an audio taper will not be as satisfying as a linear taper. There are tweaks to "linearize" an audio pot though, and that might be good enough.

I think you'll want to use PWM based dimming. That's a pretty standard way to go. There are simpler ways but I think PWM is more compatible with your needs. Your pot will provide a reference voltage to the PWM generating circuit and that will provide a proportional brightness.

Let us know if you want to pursue.

See this, "The Secret Life of Pots", too.
 

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Thread Starter

dlad87

Joined Nov 9, 2020
27
I was looking up some PWM methods earlier and that seems to be a popular choice. Saw one using a 555. Not sure if there is a better device for my application though.
 

wayneh

Joined Sep 9, 2010
17,153
I was looking up some PWM methods earlier and that seems to be a popular choice. Saw one using a 555. Not sure if there is a better device for my application though.
There are many ways to accomplish PWM. I think the 555 is a fine approach and you'll find tons of examples online. You can do it with op-amp circuits if that helps avoid a parts order but if you have to order anyway, a 555 is fine way to go.
 

MisterBill2

Joined Jan 23, 2018
9,235
Adding a PWM circuit right next to a low level audio circuit is not a good idea at all. What will be the purpose of the dimmed LED addition? And if the only power is 9 volts I am guessing from a battery, the additional batter drain will be a problem.
So we need more information to be able to provide useful assistance.
 

Thread Starter

dlad87

Joined Nov 9, 2020
27
Yes, it will be next to a sensitive vacuum tube circuit.
The 9V is from a 9V supply, not a battery.
I was hoping the led brightness would be able to track with the volume control.
 

MisterBill2

Joined Jan 23, 2018
9,235
Switching that LED load off and on will certainly couple a bit of the PWM frequency back into the 9 volt power circuit and then it will be audible in the speakers. Why not have a variable density optical filter in front of the LED?
While an indicator that changes brightness with the control position is "cool", that alone is certainly not worth the effort of both adding the circuit and then battling with the noise added by the PWM driver. And there is no certainty that the brightness variation will match the sound level variation.
And we still have no explanation for what the intended benefit of this would be.
 

Tonyr1084

Joined Sep 24, 2015
6,243
The perception of "brightness" is not linear for the human eye. Though you may taper the brightness via whatever means, what you actually see - or perceive - will not turn out to be what you expect.
 

MisterBill2

Joined Jan 23, 2018
9,235
No matter what, the current through the LED is switching on and off at some frequency, and so the current draw is a square wave of some pulse width and frequency. And I think that is a lot of complexity just to show a volume control position. And we still have no explanation about why do it.
 

Thread Starter

dlad87

Joined Nov 9, 2020
27
The led dimming would be for backlighting aesthetics, so perfect tracking with the control is not critical.

I’ll do a little research on the emitter follower circuit
 
Thoughts:
The volume control is likely a log taper. What you could do is use a voltage controlled volume control which is normally linear for a log response and use the same control signal to dim the LED.
 

MisterBill2

Joined Jan 23, 2018
9,235
Emitter follower circuit is linear, so no switching on and off.
If the driving signal is a square wave then the emitter follower signal will be a square wave. And square waves have been known to not filter out of power supply circuits very well. Thus switching an LED with a square wave will produce square wave current variations on the supply line. Hence my remark.
 

Sensacell

Joined Jun 19, 2012
2,890
I think he's talking about the idea of using an emitter follower after the wiper of a plain-old pot connected across the supply.
The transistor acts as a unity gain buffer with high input impedance.

This isn't actually a terrible idea, as long as the LED current is only a few mA.
The dimming will not be super linear, but it will work.
 

Thread Starter

dlad87

Joined Nov 9, 2020
27
I think he's talking about the idea of using an emitter follower after the wiper of a plain-old pot connected across the supply.
The transistor acts as a unity gain buffer with high input impedance.

This isn't actually a terrible idea, as long as the LED current is only a few mA.
The dimming will not be super linear, but it will work.
Once I find out what pot works best. I’m going to bread board up the emitter follower and see how it works
 
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