Still, 1 kilo-ohm resistors have kept it simple for me with LEDs on 12 Volts DC power (in that particular case, my modified Halogen spotlight which I chucked in metal halide HID lamp, and I opted to use LED-illuminated rocker switches - it worked fine with voltage up to almost 15 Volts DC from the Lithium-ion battery pack). Some LEDs are quite efficient nowadays, so it doesn't matter if it consume 1 or 20 mA, the outcome is still the same.

I have a box of 1.1k just for that reason :))))

If it works, who cares if you guessed? 🤔

@@aviandragon1390 In a professional environment, appearing to 'guess' might not be the best career move. Being able to guess correctly is great, but it perhaps needs to be backed-up with page of 'Design File Memo' supporting data. I've seen six pages of math to select a rivet, so EEs should be seen to take as much care. Up to you !!

In a professional product, of course you want to calculate for whatever parameter is most important to the design. This guy is just spitballing a circuit for demonstration. I'm not sure the same level of care is required here.

Sure, but from an educational point of view the first, naive approach is good to show you how it does NOT work and why. And leaves space for improvement and success stories 😉

Yep, to a point. Nothing is literally linear, unfortunately. Not to mention the subtle differences between a few different transistors in an OP-AMP chip causing rare instance of weird events to happen in a specific application (a few people, including me, have been bitten by it). Analog wizardry is rarely simple.

It was my idea at the very first glance. Thanks you wrote it. Voltage to current converter, just one resistor in negative feedback of first op-amp, thus voltage produces directly current of LED, LED light is ~propotional to the current and light intensity on the Base of transistor works like Base current, at the end Emitter current is proportional to Base current (light intensity). Almost perfect isolated follower. Then later we could add some extra circuits, current mirrors, thermal compensation and even optocoupled negative feedback.

With one huge price tag too, I'm sure.

​@@tonyfremont Yup. Ceramic and metal-capped chips are usually insanely expensive, even before the pointless economic inflation.

Those are called vactrols in the audio electronics world. You can also diy it, quite a simple and powerful part. An alternative to it is the FET output opto H11F1. Have you seen that part youtube.com/@IMSAIGuy ?

This was what I was thinking they should do. You could even have a 555 on one side for simple pwm. Half retro junk.

A professional designer should also consider the cost of the product. Do you know the price difference between these two optocouplers ?

regulation circuits of high voltage power supplies or transmitters

or even use XY mode and look for a straight line. Great things for you to try

yes, you can try and find a more linear portion of the LED and only use that. not as good as this, but might be fine in some applications

the two sides have different supplies, total isolation. see part 2

@@IMSAIGuy thank you for the quick reply, i didn't see part 2 yet untill now. Grtz

Experience and rule of thumb!

@@JacquesMartini Then you try it out in circuit and see what value works best. Capacitor substitution boxes are very handy for lower frequency circuits.

All this stuff is available fully integrated in a single, cost effective IC nowadays.

??? there is an input side and output side, just turn the paper 180 degrees

@@IMSAIGuy now my turn! ('???') What I meant was placing the second LED on the right and dioide on the left to complete the feedback loop. I edited my post to clarify it. This design is liken a transmiter/receiver set which only measures the power needed at the transmitter side. Today, the radio (specifically TV) stations measure the transmitted power some distance away in multiple points and send the power readings back to the transmitter to adjust the power accordingly. For example, for rainy/snowy or sunny days or reducing power at night . Then power would not be wasted. In PSUs, I think there are opto couplers to adjust the PWM. But the transformer's output voltage is used to compare and adjustments.

@@bayareapianist compete the feedback? remember this is high voltage isolated.

this would also if you just DC offset the signal to 1/2 Vcc

@@IMSAIGuy Then it wouldn't be linear.

@@williamogilvie6909 I just showed it is

@@IMSAIGuy You don't know the definition of linear. The function f(x) = x + A is not linear.

Y = Ax + B seems to be a straight line, no? that IS the definition of linear.

if in parallel, you have two LEDs in parallel. since the Vf curve will be not the same there will be current hogging of one LED before the other and they will not match well

​@@IMSAIGuysomething that could be taken care of with a current mirror? 😊

your are missing. see part 2

Yes, non linearity then becomes an advantage. The trade off is bandwidth though, as optocouplers are typically slow.

nF is for the youngsters 😎

@JacquesMartini I'm overseas (NZ) and we call it by both.

If you go back farther, pF is for youngsters. uuF for the old timers!

I used a PC817 but any optocoupler should work fine

@@IMSAIGuy Thnks, some people use 6N137 for MIDI, it's almost 2€ piece, got some PC900 (obsolete, used by Roland), they all have some logic inside. a PC817 + 74LVC1G175 (Single Schmitt-Trigger Buffer) would be cheaper, although it might cost a litle more more pcb real estate

Doing analog linear isolation. Logic is not linear. Using A/D, then D/A would introduce delays and other artifacts to the signal

One popular application was isolated 4-20mA current loops. Good use there.