Regarding Audio Signal Detection...

From: "jos" (soj at sympatico.ca)
Tue, 27 Nov 2001 17:03:09 -0500

Greetings!  Use a peak detector made of an OP-AMP and a diode followed by
a comparator! Jos

You can use a comparator or op-amp to compare the audio voltage from the
jack to a DC level adjustable with a pot. It would be adjustable over a
range from zero volts to over 2 volts to trigger on a peak of the audio
signal. Next you need to determine how long the relay is to stay on, once
triggered. Without a time delay circuit or latch to hold the relay on, it
will just chatter with the audio peaks, mostly in the bass. If you want a
momentary on time, a few seconds to an hour or so, use a 555 or equivalent
timer to develop this time. If you want the relay to stay on till reset,
use a flip-flop, made with gates, op-amps, comparators, or the relay
contacts directly to hold the relay on until a push button unlatches it.
The comparator, op-amp, 555 or other circuit must handle enough current to
drive the relay coil, so you have to start with the relay you intend to
use.

Bob

Suppose you use a cmos fet as your power switch.  Suppose you choose
one that will be "off" when there is no signal applied to the gate.
(I think you can find those, the other choice is a switch that is "on"
until the signal is applied).  Now from your audio input you use a
series resistor (to provide high enough impedandce), a diode to rectify
the audio, a cap to charge up, and a bleed resistor to discharge the
cap when the audio goes away.  Now connect the high end of the cap to
the gate of the cmos fet.  When audio is present the cap charges, the
cmos fet turns on, and the power flows to your circuit.  When the audio
goes away the cap discharges, the cmos fet turns off and your circuit
turns off.

Sound reasonable?

>On 21 Jun 1999 14:25:21 GMT, "Alex C"  wrote:
>
>>Hi,
>>Can any one teach me how to use a transistor to drive a relay, where the
>>base of the transistor will be control by an audio input. when after an
>>interval there is no audio signal, the transistor will be switch off, once
>>again the audio signal is presence, the transistor switch on again.
>
>I'm not exactly sure what to do for what you're asking, but I can tell
>you a transistor won't work in that configuration. If you give the
>base of the transistor an audio signal, the transistor may or may not
>turn on and that will be at irregular times. You need 0.7v (approx.)
>to allow current flow from the collector to the emitter, and I don't
>think audio is normally that high (without amplification). What you
>could do is use an OpAmp to amplify the audio signal to saturation
>(say maybe +5v) and then send the output of the OpAmp to a digital
>timing circuit. Sorry if this seems confusing, if anyone has any
>better ideas, shoot...

If the audio signal is high enough (2 volts or more) you can rectify it
and charge a capacitor that will keep the transistor and relay on.
Hook the anode of a diode to the audio source and the cathode
to the + side of the capacitor. Connect the (-) side of capacitor to
ground.  Install  the relay between the collector of a NPN transistor
and (+) power supply and hook the emitter to ground.  Connect
a resistor between the transistor base and  (+) side of capacitor.
Relay will open after a delay time when the audio stops. Delay
will depend on RC values.

-Bill