From: firstname.lastname@example.org (N. Thornton)
Subject: Re: Help needed in designing a press-release button to switch on a logic+analog circuit and "explosion" in headphones!
Date: 22 Oct 2002 08:08:43 -0700
NNTP-Posting-Date: 22 Oct 2002 15:08:43 GMT
> Help needed in designing a press-release button
So you need to be able to release the members of the press without
getting too close to them? :)
Old Lady wrote in message news:...
> I have a problem I can't solve without your help.
> I need to power on a circuit in which there is a logic section powered
> +5V and an analogic section powered +12-15V.
> The circuit is battery powered, so it should be disconnected when not
> in use.
> I need to press a button (I don't know the exactly English name, but
> is a button that you can press closing the circuit and when you
> release it, it immediately open the circuit again) to power on the
> circuit, which starts a digital counter and other few things. When the
> counter reach the status 111, one cmos-AND port should switch off the
> circuit and NO POWER should be drained from the battery, at least
> until one press the button again switching on the circuit.
> My English isn't so good, but I hope I'm understandable!
This should be doable with a FET and some logic.
Eg a series FET is the switching element.
The press switch connects B+ or 0v to FET to power it on.
When the circuit receives power the counter sets off, when it reaches
111 a connected AND gate changes state and shorts out the FET's drive
V, so everything shuts down again.
> Another problem is that when I switch on the circuit manually (with a
> classic on-off switch), I ear like an explosion in the headphones
> connected to an LM386 in the analog part of the circuit. Any idea how
> to avoid this?
Yes, this is inevitable. The LM386 is a class B amp, and you're
running it off a single sided supply. So you have an output capacitor
between 386 and speaker, with speaker going to ground or +5v, so the
switch on surge is inevitable. Its a result of the circuit design.
Some solutions are:
use a +/- supply to run the amp
use a series FET between amp and speaker, ramping down its resistance
slowly at switch on.
use a series FET / tr as the on/off power control element, and switch
it on gradually rather than suddenly.
> Should I separate the digital and analog grounds? How
> to do this? I also tried to put a ceramic cap between +12V and GND but
> none happened.
No, thats not the problem here.