Modding the Snes so the Tototek GBA-SNES adapter does not need separate AV cable.

Discussion in 'Modding and Hacking - Consoles and Electronics' started by MaxWar, Sep 15, 2013.

  1. MaxWar

    MaxWar <B>Site Supporter 2013</B>

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    Nice man, thanks alot for the infos! Ill try to get some of these parts next time i visit the electronic store.

    Looking at the 2N7000 datasheet I now think I understand better how MOSFETs work.
    Then I had an idea that could possibly make this whole thing work.

    If i understand the concept correctly, the gate voltage threshold of the MOSFET is the minimum voltage needed for the transistor to be turned ON and current/signal flows from drain to source. And contrary to a BPT, the mosfet is more or less fully ON or fully OFF instead of being poportional to the voltage on the Base. So better suited for logic circuits.

    The concept of my idea is that a composite signal connected to the GATE of a MOSFET would trigger the switch because the maximum amplitude of the composite analog signal would be above the Threshold voltage. ( I may need to find the right MOSFET model for this though.)

    The MOSFET would only need to be ON for a split instant for this to work, just long enough to trigger a latching OR gate and the signal switching would stay stable until console is powered off.

    Anyway, here is what I came up with. I think this might work if all the parts behave as I expect they would.
    But first I should acquire a couple of these MOSFET and test if they are proper for switching composite signals!
    If this should work, I can replace the OR gate with 3 NAND gate and so a single 4 gate IC would be perfect!

    [​IMG]
     
    Last edited: Sep 19, 2013
  2. RetroSwim

    RetroSwim <B>Site Supporter 2013</B><BR><B>Site Supporter 20

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    Yeah, that should work, as long as there's no funny business at power-on that might trip your OR-latch unintentionally.
     
  3. MaxWar

    MaxWar <B>Site Supporter 2013</B>

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    True, also, I may have to put a pull down resistor between Q1 and U1 to make it more reliable.
     
    Last edited: Sep 19, 2013
  4. RetroSwim

    RetroSwim <B>Site Supporter 2013</B><BR><B>Site Supporter 20

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    To the laboratory!!
     
  5. MaxWar

    MaxWar <B>Site Supporter 2013</B>

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    Haha damn right, If I get the chance I may be able to get some MOSFET and NAND gate this weekend, provided the local store has them in stock!
    Will keep this updated.

    Thanks for all the help!
     
    Last edited: Sep 19, 2013
  6. RetroSwim

    RetroSwim <B>Site Supporter 2013</B><BR><B>Site Supporter 20

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    My pleasure! :)
     
  7. MaxWar

    MaxWar <B>Site Supporter 2013</B>

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    Hey just got some 2N7000 and NAND gate IC this afternoon. I tried the 2N7000 to switch Composite and it totally works. I observed no visible degradation of the image quality when the signal was passing through the transistor. I had a bit of trouble to get it to close but I realized quickly that the GATE needs a 100k pull down resistor to ground, if you leave it floating its kind of semi on.

    I could not get a composite signal to trigger the GATE though, Threshold likely too high... ( I used a LED ) But if it only gets on for a Millisecond here and there I wont be able to see it on the LED. Maybe if I try to amplify the composite with a BJT... Ill do more tests..
     
  8. MottZilla

    MottZilla Champion of the Forum

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    Just curious, did you look at how the clones do it for any clues?
     
  9. MaxWar

    MaxWar <B>Site Supporter 2013</B>

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    Nope, I do not have any of the said clones :p Otherwise I would be checking this out.

    Anyway, for a quick update on my "progress".

    I am currently experiencing difficutly with connecting the 2N7000 to the Latching OR gate. For some reason the transistor triggers the latch on power up half of the times, and it is not only on console power up, it also triggers when resetting the Gate IC or when connecting the 5 v to the Drain while the Gate is grounded. I measure 0v on the source feeding the latch, I put 100k pull down resistors everywhere but some funny business still sends some kind of pulse that triggers the latch.

    I tried reducing the Pull down resistor between the SOURCE and the Gate from 100k to 12k, did not change anything. Maybe I could try an inductor...

    Well, I'll keep experimenting. If all else fail I will stick a small rubber pad on my Tototek adapter and do the whole thing Goblin style :D
     
    Last edited: Sep 23, 2013
  10. RetroSwim

    RetroSwim <B>Site Supporter 2013</B><BR><B>Site Supporter 20

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    This is where a modern digital oscilloscope is handy, you can capture one-off events that cause things like this.

    I guess there's a small pulse coming in to Q1 at power-up, as the video encoder initialises. I don't know how you'd suppress that.

    You might be able to do something with a microcontroller. Hmm.
     
  11. MaxWar

    MaxWar <B>Site Supporter 2013</B>

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    Yeah just when You are happy about getting to use your very first CRO, turns out you need the newer digital thing after all xD
    It seems that the culprit is the 2N7000, I do not think It is caused by the SNES initialization. If I use a BJT instead It works just fine. But then I cannot get the Composite to trigger anything so far, be it BJT or MOSFET. The fact that the MOSFET seems to misbehave is not helping to do testing.

    The mosfet sure works great for the Video switching part, but Its being a bitch about randomly triggering that OR gate, whereas a normal BJT seems to do the job correctly.
    Maybe I could use an OPAMP to get the composite to trigger the Latch, hmmm...
     
  12. RetroSwim

    RetroSwim <B>Site Supporter 2013</B><BR><B>Site Supporter 20

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  13. xmog123x

    xmog123x Peppy Member

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    Why not use both? One for output, the second one for triggering.
     
    Last edited: Sep 26, 2013
  14. MaxWar

    MaxWar <B>Site Supporter 2013</B>

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    Thats pretty much where i am aiming at.


    ok
    I just started messing around with Op Amps. Reading some and checking out some circuits. My initial idea was to use it in open mode ( with no feedback ). I have since learned this is often refered to being used as a comparator, which is what Retroswim mentionned :p

    There is a possibility this might work I guess, I just hope it wont spike at start up, my early tests indicates me that it does but I have not done all the proper tests so far. When using supply voltage/ground on the inputs it kinds of latches up to full output everytime (4.6v). Ive then learned this is probably normal and I will need to work out this out with the specs of my op amp.

    If this fails in the end there is the possibility of the Pic programming, Thanks again to retroswim :p Pic programming is something I have never done before, but hey Im exactly doing those mods to learn stuff .
     
    Last edited: Sep 28, 2013
  15. xmog123x

    xmog123x Peppy Member

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    if something spikes, just filter it out with a passive filter. those are easy to design even if you're a newbie.
     
  16. MaxWar

    MaxWar <B>Site Supporter 2013</B>

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    At this point my main problem Is I could not seem to properly use the Op-Amps I had on hand as a Comparator, I had 3 models. 2 just did not work at all, the output was always saturated no matter what.
    The other one "kinda" worked but in reverse to what I was expecting but before I could do more tests on it I killed it by feeding it reverse supply ( Doh!! )

    So at this point I would like to try a dedicated Comparator and I isolated the LM193 as a potential candidate. But none of the local electronic shops seem to carry anything stock as far as Comparators are concerned. Im considering my next digikey orders...
     
  17. MaxWar

    MaxWar <B>Site Supporter 2013</B>

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    Ok, well, I got a cheap 741 op amp at the part store and managed to use it as a comparator but, yes, the blasted thing spikes and triggers the Latch when I start the SNES. While I am interested in the passive filter thing, I think that wont be necessary.

    Because you see, The solution in truth was simple.
    I mean really...
    The expand line in the SNES is connected to a pull up resistor and nothing else. No SNES cart that I know of connects with it. Most SNES carts do not even have the side tabs. I checked Star Fox and Super Game boy, which are the only games I have with the tabs, and even them do not connect to this specific pin of the cart connector.

    The only thing that I know in the world that connects to this pin is this GBA adapter, and it uses it to output Composite.
    What happens, obvioulsy, is that once you connect composite out to the Expand line, the Pull up resistor is overridden by the video circuit effectively pulling it down to just under 1v, the maximum positive peek of composite.

    So there You have it. A High state when any SNES cart, or no cart at all, is inserted. Then it effectively becomes a low state when the GBA adapter is in because Video circuit connected to it drives it down.

    I made a small circuit to test this. I only used a OR gate and a LED. Totally works, does not need any latching and it works 100% reliably.
    With this I can get my MOSFET transistor to switch the video signal.

    I am happy to have found this of course but I wonder why I did not think about this before. :p

    Edit: Because Composite is bellow 2n7000 threshold voltage, it should effectively count as a 0 for all practical purpose. So I could probably make this entire switching circuit using only two 2n7000 and an Inverter, lol. Man I lost one of my transistor, crap, my place is a mess. Will post back when I find it.
     
    Last edited: Oct 4, 2013
  18. xmog123x

    xmog123x Peppy Member

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    do a proper schematic when you're done :)
     
  19. splith

    splith Resolute Member

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    If your op-amp is saturated, you're not doing feedback correctly, if it's spiking then you've got a 'spark' signal or aren't using a capacitor for voltage smoothing.
     
  20. MaxWar

    MaxWar <B>Site Supporter 2013</B>

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    As I specified, I was using it without feedback at all, in comparator mode. So you want a saturated output in this case.

    Thanks, I had kinda figured a cap would help the spiking issues, I was not sure if it would be enough by itself though. It turned out to be usefull in my latest design.


    Btw at this point I must say... : SUCCESS !!!


    So here is the super duper ultra simple circuit I came up with. I tested it and it totally works.

    The ceramic cap between Q1 and U1 was the last piece of the puzzle. Without it a small spike would always latch the circuit into adapter mode and SNES carts would not display.
    I originaly used a couple pull down resistors at key locations but once I got the circuit working properly I removed them and It did not seem to affect anything so I left them out of the design to keep it at a bare minimum. You only need 4 physical parts for this circuit:

    Part list:
    1 x SN74HCC00N ( quad NAND gate IC )
    2 x 2N7000 transistors
    1 x Ceramic cap ( minimum value not yet assessed, so far all the caps I tried worked. )
    [​IMG]
     
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