Hi All,

I'm sending this because the list seems to contain a bunch of
friendly, knowledgable people who might be able to help. I'm a
hobbyist in electronics, my background is in computers and thus whilst
i'm doing OK with the PIC side of things, other bits of electronics
i'm struggling with.

I need to design a circuit such that a signal from a PIC will cause a
beep signal. My definition of a beep signal is aproximately 1KHz,
close to a sine wave, and in the region of 75ms long. I need the
output signal to be audio "line level" - which although poorly defined
seems generally to be regarded at 1V peak to peak, AC, driven by a
lowish impedance. The audio signal I generate needs to connect to a
line in jack on a tape recorder. The whole circuit runs in a car - so
it's a noisy environment. I'm not looking for Hi-Fi quality audio, but
it does need to be reasonable.

Furthermore, I need six of these outputs, which switch on together.

I've tried to make this myself, with mixed success. My first attempt
was a basic astable 555, the output of which went through a multiple
stage integrator, so i ended up with a (rather low voltage) sine-ish
signal. This signal then fed the inputs of 6 LM386's. To "trigger" the
beep, I hooked the power supply rail of the 555 though a BC548, and
used my PIC output line to switch the BC548 when ever I wanted a beep.

That circuit sort of worked, but not particularly well. The beeps
didn't sound very clean at all, and when capturing a beep on my DSO,
you can see that (presumably due to capacitor charging in the
integrator stages) the waveform takes about 70ms to stabilise - hence
there is a lot of distortion.

So, my second design was a slight modification of the first. In this
design, the 555 was running continually, and instead I used the BC548
to switch the LM386's on and off.

But this didn't work too well either. Although the waveform stabilises
very quickly (within 1 or 2 cycles), the size of the smoothing caps i
had to put on the LM386's meant that when I shut off the beep, they
carried on beeping, with diminishing amplitude, for about another 70
ms. So the audio sounds like a cleanish beep, which fades out, rather
than stops abrubtly. Additionally, listening to the audio tapes
recorded from this design, there is a LOT of background noise - engine
frequency whining, etc. I'm not sure why this one made more of that
than the last.


So, the question is, can anyone give me any pointers on how I can
implement my requirements? I've not managed to find any chips that do
much of what I want. I'm a bit stuck on how to proceed.

Any pointers GREATLY appreciated.

Thanks

Jon
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist