Arduino Forum

Using Arduino => General Electronics => Topic started by: PolyGrafik on Mar 27, 2015, 09:16 am

Title: How to LED clip with low input voltage?
Post by: PolyGrafik on Mar 27, 2015, 09:16 am
Hi.

I am making a VGA generator/hack module. I am using a Arduino Uno for the VGA signal generation. But in addition to that I also want to make a distortion/clipper module.

The thing is that I want to use LEDs for the clipping (Because it looks cool on the front panel). But the problem is that the forward voltage is quite high and the VGA output is only 0.7v. Do I have to amplify the input? Or can I use the LEDs in a configuration that makes it possible without the amplification?

Also - Since the VGA color max rating is 0.7v per channel, is it enough with a voltage divider resistor configuration in the end if the output voltage is too high on the distortion/clipping module?

I hope there is enough information.

Thanks.  :)
Title: Re: How to LED clip with low input voltage?
Post by: aarg on Mar 27, 2015, 09:56 am
You could apply a negative voltage to the LED cathode, so that it would conduct somewhere less than 0.7V. But how will you limit the current? It's a passive circuit, so the driver impedance will determine the clamp (clip device) current. Otherwise you have to amplify the signal before clipping (probably a better way anyway). In that case, you also have to consider how to limit LED current.
Title: Re: How to LED clip with low input voltage?
Post by: Zapro on Mar 29, 2015, 05:46 pm
If you are generating VGA signal as in VGA (http://en.wikipedia.org/wiki/Video_Graphics_Array) the pins on your Arduino is at 5V-level, so no need to do anything extra.

If guess you are generating the signal with some ports pins and resistors on the output. Just put the LED's directly at the port pins and use corresponding resistors for those.

// Per.
Title: Re: How to LED clip with low input voltage?
Post by: DVDdoug on Mar 29, 2015, 06:16 pm
If you are using an Arduino you can use regular diodes to clip the signal at 0.7V, and program the Arduino to turn-on an LED when the level is high-enough to clip.
Title: Re: How to LED clip with low input voltage?
Post by: raschemmel on Mar 30, 2015, 03:46 am
I'm not sure what you mean by "clipping" as that is an electronic term that means this. (http://en.wikipedia.org/wiki/Clipping_%28audio%29)

How does that relate to a VGA signals (https://www.google.com/search?q=vga+signal+waveform&biw=994&bih=503&tbm=isch&imgil=prffEC9NU0EEqM%253A%253BCS6Lp7lfnWunvM%253Bhttp%25253A%25252F%25252Fpeople.ece.cornell.edu%25252Fland%25252Fcourses%25252Fece4760%25252FFinalProjects%25252Fs2012%25252Fraf225_dah322%25252Fraf225_dah322%25252F&source=iu&pf=m&fir=prffEC9NU0EEqM%253A%252CCS6Lp7lfnWunvM%252C_&usg=__BveWeHqWXpeWYh2jFcADhL2m_RM%3D&ved=0CCcQyjc&ei=CqwYVfiyJ5DdoASr9YGgAQ#imgdii=prffEC9NU0EEqM%3A%3BTXHGKVANeX-jKM%3BprffEC9NU0EEqM%3A&imgrc=prffEC9NU0EEqM%253A%3BCS6Lp7lfnWunvM%3Bhttp%253A%252F%252Fpeople.ece.cornell.edu%252Fland%252Fcourses%252Fece4760%252FFinalProjects%252Fs2012%252Fraf225_dah322%252Fraf225_dah322%252Fimages%252Fvga_waveform.gif%3Bhttp%253A%252F%252Fpeople.ece.cornell.edu%252Fland%252Fcourses%252Fece4760%252FFinalProjects%252Fs2012%252Fraf225_dah322%252Fraf225_dah322%252F%3B533%3B321) ?

(SEE ATTACHED IMAGE)

Quote
The thing is that I want to use LEDs for the clipping (Because it looks cool on the front panel). But the problem is that the forward voltage is quite high and the VGA output is only 0.7v. Do I have to amplify the input? Or can I use the LEDs in a configuration that makes it possible without the amplification?  
The only problem with your idea is that you need diodes with a forward voltage of 0.7V (if I understood you correctly that is the target voltage you want) and leds typically have forward voltages starting just above 2V and could go over 4.7V for some colors. How many leds are you talking about and where would you put them ? (what front panel ? front panel of what ? a vga monitor ?) If you want to create a led special effects circuit that's driven from a VGA signal then that would probably involve some op amps. (although not necessarily)