Would this sound relay work ?

Hi,
I'm trying to get a switch to be activated when a sound is made from my PDA, there are audio outputs on the bottom of the pda which I will be connecting up to my stereo, I need the switch to sit in between the PDA and Stereo unit.
I have found this : **broken link removed**
But the problem is that it uses its own microphone, could I simply solder one of the PDA's audio output lines onto where the Microphone connects and do away with the built in MIC ? so that it is only activated on sound from the PDA not ambient sound.
Like this :
**broken link removed**

Sorry for the crudeness of my diagram :lol:

Many Thanks
Neil

You could connect your pda direct, but not without doing some mods....

Looking at the cct diagram, delete the biassing resistor for the microphone, ac couple the output of your pda into pin 3 of the op amp, then play around with the feedback loop until there is a reasonable signal level where clipping doesnt occur prior to feeding the clipping/didode pump network formed by the diodes/caps......A good starting point I'd have thought.....Anyone else got any thoughts ?

Thanks, I thought that would be a little too simple :lol:
I'm afraid you'll have to bear with me, as I'm no electronics expert

By "delete the biassing resistor for the microphone" do you mean remove R8 ?

"ac couple the output of your pda into pin 3 of the op amp" only connect the positive line from PDA to pin 3 do not connect the ground to the curcuit at all ?

"play around with the feedback loop until there is a reasonable signal level" Adjust the part BC548 until the pickup is suitable ?

Here is what I think you mean:
**broken link removed**

Many thanks for you help
Neil

^
Anyone confirm that I understood that right, and that the above circuit would work, so I can get the bits ordered. :?:

Cheers
Neil

1,) Yes RB is a mic bias resistor, remove it.
2,) AC-coupled mean: use a serial condenser about 1uF. (GND must always common)
3,) Play around with feedback-loop: adjust the sensitivity pot (in feedback-loop)

Thanks Sebi,
Just to make sure I am on the right track, would this condenser be suitable? : **broken link removed**
(Electrolytic Cap. 7mm/63V/1uF Number: CA0246066)
And do I simply hook it up as in the ammended image above ?
Sorry for all the questions, I just want to make sure I don't blow anything up

Your help is much appreciated.
Neil

Yup, that cap will do........Aint there a Maplins near you? You could get a cap from them without having to buy 5 or more......

Cheers guys,
went down maplins this morning and got the cap, along with some other bits I was after, and I've ordered the relay kit.
Does it matter witch way round the cap is connected ?

Cheers
Neil