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jmspaggi
11-12-2010, 12:31 PM
Hi,

For the TV Output on the PPDB, the resistors are 1100, 560 and 270.

I have thousands of resistors, but not those values :(

The closest I have are:
For 270 => 220 and 330.
For 560 => 330 and 470.
For 1100 => 1000 and 1200.

So my question is:
Can I go with 1000, 330, 220 instead of 1100, 560, 270? Or does those values need to be accurate?

Thanks,

JM

Leon
11-12-2010, 02:05 PM
Just buy the right ones, they don't cost much. Of course, you could always build up something close by putting the ones you have in series and parallel; I think there is software around to do the sums for you.

jmspaggi
11-12-2010, 02:08 PM
But if I buy some, I will not have them before Monday ;)

Do you have any link for a software able to help me with that? Else I will try to do some calculation.

Thanks,

JM

Lev
11-12-2010, 02:25 PM
@JM
(2) 2.2k resistors in parallel will give you 1100 ohms. A 220 and 330 in series is 550 ohms which is probably close enough. Add another 10 ohm resistor in series , if you have one, to get a nominal 560 ohms. If you have any 100 ohm resistors, put one in series with (2) 330 in parallel to get 265 ohms, which is close to 270.

Lev

jmspaggi
11-12-2010, 02:32 PM
I have 10 ohms, I have 100 ohms, I have 2.2k... Look like I have all what I need! Super!

I even have 1 ohms so I can put 5 in series for 265 ;) Or 2 x 10 ohms parallel.

Thanks. I will try that right away!

JM

Ariba
11-12-2010, 03:06 PM
It will work also with:
220, 470, 1000 Ohm

The values are not that critical, they just must build a 3 bit DAC, so the relation must be near to 1:2:4.
If you want it exact, use two 470 in parallel instead of the 220R.

Andy

jmspaggi
11-12-2010, 03:11 PM
Ok! I see!

so I can you an R value with 2 R in parallel, 1 R and 2 R in series?

It's already too late, but if what I just did is not working fine, I will try that way.

Thanks,

JM

Toby Seckshund
11-12-2010, 04:31 PM
As long as you are not trying to make some sort of test generator then as close as possible will be ok. I have always used 220, 470,1k2 as I usually only have E12 series resistors here.
Usually it is one of those bags of 600s, 10 of most values.

jmspaggi
11-12-2010, 04:47 PM
I just figured my mistake :(

I'm using this breakout board: http://www.sparkfun.com/commerce/product_info.php?products_id=8826 and there is 3 pins. I thought BRL and BRK are the same so just choosed one randomly.... Wrong choice :(

So I'm back with the solution Lev proposed since I already soldered it and it's now working fine.

Thanks all for your help.

JM

jmspaggi
11-12-2010, 05:45 PM
Finally, it's not working ;)

It worked for 30 seconds then the display stopped.

I just plugged back the option with the 5 x 470 and seems to we working fine (way longer than 30 seconds).

So back to the soldering iron. Just to bad I already soldered the RCA Jack to the breakout bord, else I will have make my own board with it.

JM

Cluso99
11-12-2010, 11:40 PM
Phil is the expert here, but I would thave thought 222, 470, 1000 would have been close enough. However, from Phil's comments in other threads, some monitors cannot handle a non-true 75 ohm impedance. He suggested a resistor and capacitor across the output to ground. IIRC 191R and 100pF. So I would try a 220R (which you have) across the output ans see if that helps.

The other possibility is that the TV program is not tuned correctly, or you are outputting NTSC to a PAL monitor. You may have to tell us what code you are running.

jmspaggi
11-13-2010, 01:03 AM
I used 470 resistors.

2 in //.
1 alone
2 in serie.

The screen is now on for hours and working fine. Just the colors which are a bit different than from the PPDB but I will adjust that on the software side. So so far I'm fine with the result ;)

JM

Phil Pilgrim (PhiPi)
11-13-2010, 01:14 AM
If your colors seem a little washed out (and they probably will be with those values), you can also add resistance across the final output terminals. I'd try a pair of 470-ohm resistors in parallel. See here:


http://forums.parallax.com/showthread.php?t=107596

-Phil

jmspaggi
11-13-2010, 02:26 AM
Very interesting! It's to late for the board I did, but I will keep that link.

Thanks.

JM

Cluso99
11-13-2010, 10:09 AM
Thanks Phil. I knew it was your baby but didn't have time to find the link.