Representing AC sine wave with common anode/cathode RGB LED

Question

I have a AC sine/triangle wave I want to represent with single red/green LED. When positive side goes up then green light increases and opposite for negative/red light. This is pretty simple with two separate LED's, but I'm not sure at all how to go about this with RGB LED that has common anode/cathode. There are two big issues to solve:

1. There is dead zone area where voltage is too low to trigger the LED.
2. Perceived brightness curve is not linear, it takes more power to make it brighter each step. So it probably needs some kind of linear to exponential response.

I managed to make this with separate LED's:

Answer 1

Really cool problem :)

Regarding your first question, you would need a circuit which has a kind of bend when your signal input is around 0V. You can do this by adding diodes in the feedback of an operational amplifier. Something like this:

The output function of the circuit looks like this:

Note that R1 is needed to have a proper load for the diodes. By decreasing R1 you increase the forward voltage of the diodes and therefore the "height" of the bend region. You can also add aditional diodes to increase the bend region without increasing the diode current.

Regarding your second question. You need something which transforms your linear input voltage to an exponential output current. Fortunately a BJT does exactly this. Leading to a circuit like this:

Please note I replaced the diodes with the BJTs in a diode configuration to have a similiar UBE in the amplifier feedback.

The current through the LEDs looks like this:

It seems that the exponential behaviour of the currents becomes more linear with higher input voltages. So maybe this is only suited for small input voltages.

Hope this helps :) Have fun :D

EDIT:

@somerandomusername: I just reread your problem and I totally overlooked that you wanted to use a common-anode or common-cathode LED. To solve this you can simply add an inverting amplifier with a gain of -1. And as user69795 suggested: if you only need a linear increasing LED current you can just add a resistor in the emitter of the BJT as a current feedback. The current through the LED will be roughly: \$ I = \frac{U_{out} - U_{BE}}{R_E} \$

This leads to a circuit like this:

Answer 2

If this is a fairly low-frequency wave you want to represent, you could sample your waveform with a microcontroller ADC, then convert the waveform voltage to separate green LED and red LED PWM outputs. Then you have control over the visual effect, including doing a non-linear (gamma) mapping to improve the visualization of voltage.

Why non-linear mapping? The LED brightness (luminance) will be linear with its current (or average current if PWM is used). But the perceived brightness to the human eye will have a logarithmic relationship to current.

In broad terms, the eye is more sensitive to changes in intensity at low light levels than it is at high light levels, so gamma mapping applies an inverse curve to compensate for that.

More about gamma here: https://www.cambridgeincolour.com/tutorials/gamma-correction.htm

You will also need an input circuit to level-shift and normalize the input to the ADC range.

I know this sounds complicated, but note that you could do this with a single 8-pin uC like ATTiny and a dual op-amp package. This could be made small enough to fit in a pen and cost less than a dollar.

Answer 3

Thanks for all the help. I will definitely check out the solution @ringk89 came up with. I had idea of my own, trying to solve this using PWM. This is the solution I came up with:

Simple sim: falstad sim

It is somewhat exponential, and I have tested (without the inverter so far) it on a breadboard and it seems to work great. Values are a bit different as this simulator is not accurate.