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audio signal activated switch circuit needed
audio signal activated switch circuit needed
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Old 29th September 2007, 06:00 PM   #1
setmenu is offline setmenu
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Default audio signal activated switch circuit needed

I have not done any diy electronics for ages!
And it seems any little knowledge I had has flown out of my head,

I wonder if any of the kindly folks here could help

I am building a motor controller for a friend and it needs to be activated by a signal from a portable cd player.
It is for a display model and will be powered by a 9v battery.

Being driven mad today, no patience wasted lots of time

Many thanks
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Old 29th September 2007, 08:40 PM   #2
ocool_15 is offline ocool_15  Canada
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Couldn't you just place a cap from the signal to the base of a regular transistor? You would be able to increase the delay to turn off without a signal by putting a R in series to the base or increasing the value of C.
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Old 30th September 2007, 08:24 PM   #3
setmenu is offline setmenu
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first used an opamp comparator , this can give a relatively steady signal but it still pulses.[used a cap for some smoothing too]
Probably not doing things right...
the signal is only a few millivolts.
What I need is something that turns on hard at the presence of the signal and off when it ends.

No doubt there are many ways to achieve my ends!
Wish I knew em!

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Old 30th September 2007, 10:05 PM   #4
I_Forgot is offline I_Forgot  United States
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You need something that will detect an input signal, even a small one, then switch amp power on, and hold it on for a while (maybe a few minutes) after the input signal goes away. You don't want to cycle amp power between tracks on a CD. This would be trivial programming with a uC that has an A/D converter. It could also be done with analog circuits. I am sure someone sells a kit for such a device somewhere.

Of course, you'll be sneered at by people who will think the switch circuit must be wrecking the audio signal...

Here's a circuit that might work. You'll have to drive a relay or SSR with the output.


The CMOS comparator is essentially a rectifier that slams to the rail when the input signal exceeds 50 mV. It charges the capacitor via the diode. The diode keeps the cap from discharging into the output of the comparator when the audio signal, and therefore comparator output, is low. The 4.7 M resistor in parallel with the cap keeps the circuit switched on for a while after the audio stops. You could change the R and C values to get whatever time delay to shut-off that you want. If you don't include the resistor, the gate, diode, and capacitor leakage will determine the shut-off delay time and it will vary with temperature and humidity and phase of the moon.

This circuit was designed for long battery life, but 3V out is a little low for driving relays and SSRs, so you could easily redesign using a 5-15V supply that will make driving a common relay a lot easier.

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Old 3rd October 2007, 11:46 AM   #5
setmenu is offline setmenu
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Don't worry about it's audiophile credentials, the circuit is for controlling an electric motor!
It's for a Friend who wants a static scale model to simulate an
engine run up.
Prop has to do 1 turn in about 1-2 seconds then kick in to simulate the engine firing.
The cd payer has a soundtrack to accompany the model.

After a bit of messing I am using a duel opamp one half to boost the signal the other as a comparator to kick it in.
Using a LED as visual guide it looks pretty good,cant remember the fig on the meter for ac but not much, and that is using a music signal.
The engine one will be more constant.
Cant realy use huge smoothing caps as i want the thing to turn of quickly.

The circuit switched by the audio signal is the power for555 timers and a 4017 decade counter.

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Old 3rd October 2007, 05:46 PM   #6
AndrewT is offline AndrewT  Scotland
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the ESP site has an auto switch on circuit and also an auto switch off after a time delay.
regards Andrew T.
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Old 3rd October 2007, 06:17 PM   #7
setmenu is offline setmenu
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That pretty much what mine is, but I don't have the caps large to leave the thing switched on in the absence of a signal.

When I have got a spare moment i will test to see whether the noise upsets the other circuit, I feel pretty confident it wont.

Been pondering how to get my motor to turn even more slowly today.
It goes pretty slow at the moment but it often stalls at startup at the slowest speed i can drive it.
Poor thing was never intended to run at 1 - .5 HZ...
I only need 1-1.5 revolutions at that speed.
Tried closed loop but it goes slower open loop.
reduction drive would be a great help, but not possible here.
What i need is a little pulse to kick it over that magnetic hump......
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