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Thread: PET 2001 Sound

  1. #41

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    I have built the circuit found in post #36, having replaced the potentiometer with a 10 ohm capacitor. The sound was barely audible when the volume was turned up to 100%. Perhaps the NAND gate is needed?

  2. #42
    Join Date
    Dec 2005
    Location
    Toronto ON Canada
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    7,257

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    Quote Originally Posted by tanuki View Post
    I have built the circuit found in post #36, having replaced the potentiometer with a 10 ohm capacitor.
    A 10 ohm capacitor ??? Better check that, and anyway, why replace anything?

    Perhaps the NAND gate is needed?
    No.

  3. #43
    Join Date
    Jun 2012
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    UK - Worcester
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    3,475

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    ??? ???

    I am confused also...

    Dave

  4. #44

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    1. Capacitors are not measured in OHMs. So I assume you used a 10 ohm potentiometer ?
    2. 10 Ohms is too small for the Potentiometer and will overload the output too much to hear anything. 10 ohms at 5 volts is half an amp and I'm pretty sure the chip can't output that much current.
    I would start with something like 470 ohms at least or maybe 1K Ohms for the Pot. It doesn't really need to be a log pot unless you plan on adjusting it frequently.

    A 470 ohm pot would draw about 0.010 amps from 5v (5/470) which might be fine. I'd need to check the chips specs to see what the max current is on that output.
    10 ohms (5/10) is ~0.5 amps which would overload the output and draw the voltage down to zero
    Last edited by Hutch; December 6th, 2019 at 03:46 PM.

  5. #45

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    Oops! I meant 10 ohm resistor not capacitor. Sorry for the confusion.

  6. #46

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    Quote Originally Posted by Hutch View Post
    1. Capacitors are not measured in OHMs. So I assume you used a 10 ohm potentiometer ?
    2. 10 Ohms is too small for the Potentiometer and will overload the output too much to hear anything. 10 ohms at 5 volts is half an amp and I'm pretty sure the chip can't output that much current.
    I would start with something like 470 ohms at least or maybe 1K Ohms for the Pot. It doesn't really need to be a log pot unless you plan on adjusting it frequently.
    Thank you for the advice. I'll try starting with 1KOhm and work my way down. I actually tried this in the recent past, but worth reproducing the results and reporting back here.

  7. #47

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    Where were you taking the output from a single resistor to go to the amp?

    Also worth noting that you can't use just one resistor. You need a POT or two resistors to divide the voltage.
    If you have a single resistor, you're only limiting the current, the voltage on the resistor will be the same.
    It may affect the loudness depending on the input impedance of your amplifier but it will be very touchy and difficult to adjust.

    resistors.JPG

    If you do it like this, if the amp has a high impedance input, the voltage will be the same on both sides of the resistor.

    resistors2.JPG
    Last edited by Hutch; December 6th, 2019 at 03:59 PM.

  8. #48

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    Thanks for the great explanation, Hutch. Even as an armchair "electronics engineer", I'm very rusty. I had forgotten all about split voltage via two resistors. I was using the potentiometer only as a variable resistor. I tried out the circuit you recommended. It worked, kind-of. It is much too quiet even with the potentiometer turned all the way up.

    I found an old passive piezo and have now verified that the circuit from the very first post in this thread:
    PET User Port CB2 line -----> 20 ohm resistor ------> (+) PIEZO BUZZER (-) ---------> PET User Port GROUND
    will work but only with a lot of buzz from the moment the machine turns on. I'm now surprised that people have used this circuit as I find the constant buzzing very annoying.

    When I take Dwight's advice (thanks, Dwight) to add a capacitor:
    PET User Port CB2 line --> 20 ohm resistor --> 0.47uF capacitor --> (+) PIEZO BUZZER (-) --> PET User Port GROUND
    the buzz goes away and the piezo sounds fine with plenty of volume. Does anyone see a problem with this circuit? I'm hoping not to void my CBM warranty.

    I'm waiting on a 74LS00 that I ordered to try the amplifier circuit again. At the moment I'm tempted to simply wire a capacitor, resistor and piezo directly to a port connector.

  9. #49

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    I'm using a PET2SD Future SD card solution and didn't want to cut apart the tape drive connector to pull 5V. So, I ended up building the no power piezo solution. It uses 3 components as described above:
    * 22 ohm resistor (note minor difference from 20 ohm stated previously)
    * 0.47uF capacitor
    * passive piezo buzzer
    I wired it up on some perf board & with the help of some hot glue, I have a functioning PET 2001 sound board. The circuit is as described previously:
    PET User Port CB2 line --> 20 ohm resistor --> 0.47uF capacitor --> (+) PIEZO BUZZER (-) --> PET User Port GROUND

    PET2001_sound_med.JPG

    Thanks to everyone on the thread. I learned more than I expected by piecing together this very simple circuit. Hopefully this helps someone in future to build a known working PET2001 piezo sound board.

  10. #50

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    FWIW, I just built one of these myself.
    I just used a 1K ohm resistor to a Piezo speaker.
    I added 3.5mm audio jack to optionally connect to a powered speaker, and with that 1K ohm resistor and the powered speaker connected to account for it's impedance, I measured about 1/2 volt pk-pk on the output. Low for a typical line output but good enough.
    Blog post with details and video here. https://myoldcomputers.tumblr.com/po...28310/cb2sound
    tumblr_9cd497cea95a30652043bd3a9b2648b4_163b7b98_500.jpg
    I didn't add a capacitor to the output as I didn't have one on hand. I might add one later but it's working fine as is.
    I didn't use a voltage divider since that was pulling CB2 low and causing problems with the serial port. (added feature, see blog post) I just used the impedance of the powered speaker as the bottom half of the voltage divider.
    My goal was to keep it simple and to not require +5V for the audio output.

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