Simple LED Driver.

[MarbleMad]

Hi, I've been building a few simple mechanical TVs and could use some advice on improving my LED driver circuit.
At the moment my design is generating quite a nice crisp, sharp and bright image but it has absolutely no shades of grey in it, it's total contrast. Black or white, nothing in between. Here's what i've built:



The 10k pot was supposed to adjust brightness but is actually behaving like a threshold selection control. It moves the point at which a picture element will flip from black to white.
I've not got a very good scope but the wave coming out of the LM358N does look like it's mostly jumping from zero to topping out. Very little in between.
I did at one point include a .1uf ceramic cap in this design for signal isolation... It gave the image terrible ghosting, resulting in big black bands under any very bright part of the picture.

Has anyone got any ideas about how I could improve this?


Also had a google around and found this circuit.


http://www.hawestv.com/mtv_page/mtv_page2.htm

Gave it a try.
Now I am running this at 6volts, not 12.
As is, I got this circuit running and it generated a picture not dissimilar to the one i got with my own design (I say design, lets be honest, my original bodge). That included the .1uf cap and it also included the horrible ghosting distortion. So as suggested on the page (and as i had already discovered with my own experiments) I removed the cap. All I could then get was a bunch of lit LEDs. I tried messing with the values of R1 and R2 and by replacing R1 with a 10k pot I found i could adjust it to a threshold point where it's transition from lighting the lamps so turning them off. At this point if a sent in a signal it would just barely generate the tiniest flicker of a picture but it was balanced on a knife edge and you couldn't resolve it to anything that made sense.

Does anyone know how i could adjust this to work on 6 volts? or if it's worth pursuing? will i get any gradiation from the picture or is this just going to result in a high (total) contrast image like i'm already getting with my circuit?

or can anyone suggest any other design that will give me better results?

Thanks for taking the trouble to read this.

[MarbleMad]

PS if you want to know what it looks like:
https://www.facebook.com/video.php?v=10 ... 8558810441

[Klaas Robers]

Try the simple circuit of the attached pdf. It is a chapter of the NBTV Handbook. It shows you how to make an even simpler circuit that works quite well. I think that other power FETs will work as well, but I didn't check it.

LEDs should be driven by current, not by a voltage. A resistor from the emitter to ground in your first circuit may help in this respect. Start with 47 ohm. Then gray scales may become possible. But then you lack still the needed gamma correction, which the FET circuit has. And you lack the DC restoration, which is also important in video circuits.

[semuenter]

Your circuit is a non-inverting, variable gain op-amp directly driving the base of the NPN transistor. This means that the transistor is off when the base voltage is less that ~0.6 volts, and on when the voltage is greater than ~0.7 volts. Even with a large voltage signal driving the op-amp from the audio source, the output voltage from the op-amp is clamped to ~0.7 volts by the base-emitter junction when the emitter is connected directly to ground.

Try removing the 220 ohm current limiting resistor from the LED anodes and put it between the emitter of the 2N3904 and ground. This makes a voltage follower circuit where the current through the LEDs is determined by the voltage across the 220 ohm resistor. The voltage at the emitter follows the voltage at the base minus about 0.7 volts. This should give a more linear response.

I assume that the audio signal is DC coupled where 0 volts is black and the maximum positive voltage is white video. If the audio signal is AC coupled, you'll need to make a DC restoration circuit otherwise anything in the original video image darker than grey will show as black.

[MarbleMad]

I tried a variable resistor from the emitter to ground on my original circuit. That gave some interesting results but as far as i can tell it's just moving the threshold between black and white around again just like my brightness control.

I'll give the other circuit you suggested a go although i'm not sure it's going to work with the 6 volts and 5v regulated i have available.

Thanks.

[MarbleMad]

I've got a couple of solutions now, thanks. I moved the resistors as suggested on my original circuit and also tried a strategy someone else mentioned involving an array of resistors and diodes from the emittor to ground. That's generating some contrast now.
But i also tried Klaas Robert’s suggested circuit and it works nicely even on 6 volts.
Thanks.

[MarbleMad]

some of the results:
https://www.facebook.com/video.php?v=10 ... 5779374991