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On/Off switch using MOSFET idea needed

E

elfa

Jan 1, 1970
0
I'm building a standard LM386 low power amplifier for use with small portable
radios to amplify the sound via the earphone connection. What I need is a way
to turn on the low power amplifier JUST by using the voltage it receives from
the input (earphone jack). The idea is that the speakers circuitry will
automatically turn ON when the radio is turned on and turned OFF when the radio
is turned off. Portable, amplified, speakers like this used to be made but
apparently not anymore (I have an old one from Radio Shack...my prized
possession).

I'm thinking that a MOSFET would be the likely transistor for a job like this.
Anyone with any idea of the best way of doing this?

Any ideas or webpage link will be appreciated.

thanks

elfa
 
L

Lord Garth

Jan 1, 1970
0
elfa said:
I'm building a standard LM386 low power amplifier for use with small portable
radios to amplify the sound via the earphone connection. What I need is a way
to turn on the low power amplifier JUST by using the voltage it receives from
the input (earphone jack). The idea is that the speakers circuitry will
automatically turn ON when the radio is turned on and turned OFF when the radio
is turned off. Portable, amplified, speakers like this used to be made but
apparently not anymore (I have an old one from Radio Shack...my prized
possession).

I'm thinking that a MOSFET would be the likely transistor for a job like this.
Anyone with any idea of the best way of doing this?

For decades, phono jacks have been made with a switch built in. These
normally
break the speaker connection when the earphone plug is inserted. You can
use
that switch to activate your battery powered amp.
 
D

Dimitrij Klingbeil

Jan 1, 1970
0
Lord Garth said:
a receives the

For decades, phono jacks have been made with a switch built in. These
normally
break the speaker connection when the earphone plug is inserted. You can
use
that switch to activate your battery powered amp.

And when no switching jack is available a one-transistor-amp with a reed
relay may help.
 
E

elfa

Jan 1, 1970
0
For decades, phono jacks have been made with a switch built in. These
normally
break the speaker connection when the earphone plug is inserted. You can
use
that switch to activate your battery powered amp.

That's not what I'm looking for. I want it turned on 'electronically' when the
radio is turned on.

elfa
 
L

Luhan Monat

Jan 1, 1970
0
elfa said:
I'm building a standard LM386 low power amplifier for use with small portable
radios to amplify the sound via the earphone connection. What I need is a way
to turn on the low power amplifier JUST by using the voltage it receives from
the input (earphone jack). The idea is that the speakers circuitry will
automatically turn ON when the radio is turned on and turned OFF when the radio
is turned off. Portable, amplified, speakers like this used to be made but
apparently not anymore (I have an old one from Radio Shack...my prized
possession).

I'm thinking that a MOSFET would be the likely transistor for a job like this.
Anyone with any idea of the best way of doing this?

Any ideas or webpage link will be appreciated.

thanks

elfa
Hi,

Technically, what you are describing is a 'retriggerable, one-shot,
multivibrator' (if memory serves). You want audio to trigger a timer
and the timer to only turn back off after a specified time with no audio.

This envolves amplifying the audio to a sufficient level to convert it
to a set of digital pulses that keep resetting some timer circuit. Lots
of ways to do all this with a bit of prototyping and experimentation.
 
L

Lord Garth

Jan 1, 1970
0
elfa said:
That's not what I'm looking for. I want it turned on 'electronically' when the
radio is turned on.

elfa

So your sensor for this battery powered amp is always on ... seems wasteful.
 
W

Watson A.Name - Watt Sun, Dark Remover

Jan 1, 1970
0
Hi,

Technically, what you are describing is a 'retriggerable, one-shot,
multivibrator' (if memory serves). You want audio to trigger a timer
and the timer to only turn back off after a specified time with no audio.

This envolves amplifying the audio to a sufficient level to convert it
to a set of digital pulses that keep resetting some timer circuit. Lots
of ways to do all this with a bit of prototyping and experimentation.

You may be able to find a schematic of such a circuit by googling for
clap switch. Here's one.
http://www.discovercircuits.com/S/sou-operate.htm

This one needs some work. B1 should be a wall wart. The relay should
have a 1N4002 across the coil, cathode to positive. The R10 should be
much lower, 2.2k or 1k. As for performance, don't expect much out of
it. It doesn't have any bandpass filters at all. So noise at any
freq can trigger it. http://home.maine.rr.com/randylinscott/dec97.htm


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E

elfa

Jan 1, 1970
0
Watson A.Name - said:
You may be able to find a schematic of such a circuit by googling for
clap switch. Here's one.
http://www.discovercircuits.com/S/sou-operate.htm

This one needs some work. B1 should be a wall wart. The relay should
have a 1N4002 across the coil, cathode to positive. The R10 should be
much lower, 2.2k or 1k. As for performance, don't expect much out of
it. It doesn't have any bandpass filters at all. So noise at any
freq can trigger it. http://home.maine.rr.com/randylinscott/dec97.htm

Actually...I've already got an idea that works! In my simple test, I used 2
transistors in an arrangement similiar to a Darlington Array. Except that Q2 is
a MOSFET (IRF510), not a second NPN (904). The test was to input just the
signal from the earphone jack of a radio into the base of Q1 which had a
separate power supply and make its way from Q1's emitter into the MOSFET's gate
and light up an LED which was connected to Drain; then, when the radio is turned
off, the LED goes out, which would tell me the power is turned off. If it
worked with an LED, then it could do the same with LM386.

It works...but for one radio only (a real junker). It won't work with any other
radio I've tried.

Now...to figure out why.

elfa
 
L

Luhan Monat

Jan 1, 1970
0
elfa wrote:


Actually...I've already got an idea that works! In my simple test, I used 2
transistors in an arrangement similiar to a Darlington Array. Except that Q2 is
a MOSFET (IRF510), not a second NPN (904). The test was to input just the
signal from the earphone jack of a radio into the base of Q1 which had a
separate power supply and make its way from Q1's emitter into the MOSFET's gate
and light up an LED which was connected to Drain; then, when the radio is turned
off, the LED goes out, which would tell me the power is turned off. If it
worked with an LED, then it could do the same with LM386.

It works...but for one radio only (a real junker). It won't work with any other
radio I've tried.

Now...to figure out why.
elfa,

If you get steady light from the LED, you are probably getting some DC
offset from your radio. Serendipitous solution at best.
 
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