Re: Some simple design help?


"ben norton" <nospam.com> wrote in message
news:lgj7s1hh9j0b8n183761fa9qsamujscaqv@xxxxxxxxxx
> "Arfa Daily" <arfa.daily@xxxxxxxxxxxx> wrote:
>
>>If the drive to the sounder is being driven with a pulsed square wave -
>>which it almost certainly is - I would suggest that if you got a scope on
>>it, you would see that it was actually going from 3v to virtually ground.
>
> Yep - I did something like this last night. I suppose I
> should have known the meter couldn't respond.
>
>>You could glue a little AC coupled peak rectifier circuit on the end.
>>Connect the + lead of a capacitor C1 of say 10uF, to the ' bottom ' end
>>of the
>>buzzer, and then connect the - lead to the cathode of a small signal diode
>>D1
>>ie 1N4148. Connect the anode of the diode to watch ground. Now connect the
>>anode of another similar diode D2 to the cathode / cap junction, and
>>finally,
>>connect the + lead of another capacitor C2 of say 47uF to the cathode of
>>the
>>second diode. The - lead of this cap goes again to watch ground.
>
> I drew this circuit out, but I'm not quite sure of the
> function of D1 and C1 (see labels above) My simple design
> just used the equivalent of D2 and C2, connecting the anode
> of D2 to the positive end of the buzzer (B+) and the
> negative end of C2 to the negative end of the buzzer (being
> switched low). The watch is battery operated and floats
> relative to the transistor in the device being switched, so
> D2 just charges C2 when the buzzer is being sounded - which
> switches the transistor. That's the way the timer was set
> up in my first attempt.
>
> Thanks for your comments.

It's a slightly unconventional way of doing it, but if it works for your
application, then that's fine. My way of doing it kept all the grounds
together - the watch and the external circuit, but ensured that the two
circuits were isolated from each other, and the correct polarity of drive
signal was always produced at the highest level, irrespective of the duty
cycle of the drive waveform. C1 AC couples the high frequency drive, which
is then floating with respect to ground. D1 then behaves as a basic shunt
rectifier, reclamping the voltage pulses about 0.6v above ground. D2 and C2
then form a charge pump to largely remove the low frequency pulsing of the
high frequency drive ie the reason that the buzzer goes BEEP BEEP BEEP.

Buzz '-' ------| |-------|>|----------^^^^--------> Drive to transistor
C1 | D2 |
-- --
D1 ^ -- C2
| |
------------------------------------------------- Gnd

Don't know if that will come out ok. Gives you the idea if it does.

Arfa


.

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