Voting resources, early voting, and poll worker information - VOTE. ... Adafruit is open and shipping.
0

Transistor for video generation
Moderators: adafruit_support_bill, adafruit

Please be positive and constructive with your questions and comments.

Transistor for video generation

by mojo on Tue Sep 28, 2010 7:37 pm

Hi. I am trying to generate a video signal using a couple of transistors and an AVR microcontroller.

The basic idea is to use the SPI interface to generate the luminance part and two transistors connected to I/O pins to generate foreground and background colour. That way I get high resolution on individual pixels and lower resolution colour.

The schematic looks like this:

Image



The problem is that the transistors do not switch on and off fast enough:

Image

As you can see the on/off slopes are far too slow when the base of the transistor is switching. The luminance slopes, which come directly from the AVR, are fine.

I am having a lot of trouble finding a suitable high speed transistor available in the UK. Farnell is a few but I was hoping to find a better source. My research indicates that 1GHz is about the minimum transition speed for video.


BTW, here is code for simulating the circuit in Circuit:

Code: Select all | TOGGLE FULL SIZE
$ 1 5.0E-6 10.20027730826997 50 5.0 50
r 512 224 512 288 0 380.0
r 512 320 512 384 0 75.0
L 272 160 224 160 0 1 false 5.0 0.0
L 224 272 176 272 0 1 false 5.0 0.0
t 400 160 448 160 0 -1 4.999999999408999 -5.000009295486052E-10 100.0
r 368 160 304 160 0 10000.0
r 304 272 240 272 0 100000.0
w 304 160 272 160 0
w 400 160 368 160 0
g 512 400 512 432 0
R 368 240 368 192 0 0 40.0 5.0 0.0 0.0 0.5
w 448 96 448 144 0
O 576 304 624 304 1
w 512 384 512 400 0
w 576 304 512 304 0
w 512 304 512 288 0
w 512 304 512 320 0
x 472 171 544 177 0 24 BC327
x 426 45 473 51 0 24 Lum
x 99 283 152 289 0 24 Back
x 145 173 194 179 0 24 Fore
L 448 96 448 64 0 1 false 5.0 0.0
t 320 272 368 272 0 -1 4.999999994909 -4.999999525523435E-9 100.0
w 368 240 368 256 0
w 320 272 304 272 0
w 240 272 224 272 0
w 368 288 368 320 0
w 368 320 448 320 0
w 448 320 448 224 0
w 448 224 448 176 0
w 448 224 512 224 0
x 272 235 344 241 0 24 BC327


Unfortunately it does not simulate transistor on/off times...

mojo
 
Posts: 136
Joined: Mon Jan 21, 2008 5:04 pm

Re: Transistor for video generation

by oPossum on Tue Sep 28, 2010 7:56 pm

I think a 74HC257 would work for what you are trying to do.

Bipolar transistors are rather slow when saturated. That is a big part of your problem. Adding a resistor between base and emitter would probably help a bit, but not enough.
I am the Possum, and I approve of this message. Sent from MacBook Wheel Sorry for my bad German.
oPossum
 
Posts: 636
Joined: Fri Oct 26, 2007 12:42 am
Location: Michigan, USA

Re: Transistor for video generation

by mojo on Wed Sep 29, 2010 5:10 am

Unfortunately a digital IC won't work. If you look at the base resistors you can see that they are different. The 10k saturates the transistor for foreground colour. The 100k only turns it on enough to supply about 300mV to provide a darker background colour.

There are some RF transistors which might be suitable in the GHz range. I might have to re-design using NPN though because there are even fewer PNP models.

mojo
 
Posts: 136
Joined: Mon Jan 21, 2008 5:04 pm

Re: Transistor for video generation

by richms on Wed Sep 29, 2010 7:53 am

Couldnt you just put different resistors on the output side of the transistors rather than trying to guess with different base currents?

richms
 
Posts: 558
Joined: Tue Jan 20, 2009 3:05 am
Location: New Zealand

Please be positive and constructive with your questions and comments.