Re: RCX sound circuit

In lugnet.robotics, Philippe Hurbain wrote:
> In lugnet.robotics, John Reynolds <john-reynolds@xxxxxxxxxxxxxx> wrote:
>>  I was getting to the point where I could generate a
>> number of similar friendly sounds and then the RCX
>> stopped generating sounds completely (except for beeps
>> from the On-Off button when pressed).  This happened
>> within an hour!
>>
>> I can only assume that the repeated intensive use of
>> the oscillator circuit has burnt out some component.
>> All other functions of the RCX seem to be intact.
>>
>> Does anyone know the location of a circuit diagram for
>> the RCX?
>
> Here is the sound driver section:
> http://www.brickshelf.com/gallery/Philo/RCX-comp/sound_driver.gif
> (Mark Bellis, if you read this you can add this schematics to your RCX circuit
> collection to get everything together)
>
> It seems impossible that you have fried anything if you still get on/off 
> sound:
> there is only one buzzer driven by a simple circuit, so the hardware can't be
> wrong here (and I don't think it could ever get "tired" with the 330 ohm 
> series
> resistance...)
>
> So I suggest you follow Claude's suggestions who posted while I wrote this ;o)
>
> Philo

Your circuit diagram checks out Philo, though a 47k resistor to ground should be
added to the processor connection.  As soon as I get access to Visio again, I'll
write it up and post it with a credit.

I was wondering what the other NAND gate is used for.  Input pin 1 traces to
somewhere under the ROM.  Input pins 2 and 13 and output pin 12 disappear
elsewhere.

I suggest that a 3-input NAND gate was needed by the main computer circuit and
that the speaker driver circuit just mopped up the spare gates.  The speaker
circuit is effectively an H-bridge design, using the CMOS output stages of the
two gates, which allows 10 volts differential across the speaker and series
resistor.

Mark