I've seen this method in "vibrato" generators  and avc circuits etc. going
back well into the valve era. However, it would pay to match the wavelength
response of the LDR against the output from the LED. LDRs are, I think,
somewhat more sensitive to green light. ( = Use a green LED ?)

Richard

> -----Original Message-----
> From: Lance Allen [mailto:l.allen@AUCKLAND.AC.NZ]
> Sent: Tuesday, July 13, 1999 9:05 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: analog switch
>
>
> >         Hi to all engineers.
> >         last night I made a small circuitry to mute the analog line
> >         the pic part works ok  but analog section didn't
> work at all.
> >         on output when I add 10k pull up resister works
> fine but I can't
> >         do that on audio line.
> >         I connected pic with max4066 analog chip. the problem
> >         I found is that 4066 needs voltage on output  to operate
> >         which we do not have it my question is this is
> there any part that
> > can
> >         be used to mute the analog line "audio" beside relay ?
> >         any help will highly appreciated.
> >
> >         Andre
>
> A simple method is to use a light dependent resistor in the audio
> line. The LDR is very high resistance when dark, so a no power-mute
> would have the LDR in line with the audio followed by a lowish
> (2k-10k) resistor to ground. An LED driven by the PIC could enable/
> disable it (2 Meg dark to around 5K light).
> I have seen this system used but I have not used it personally, I
> dont think there would be any significant distortion being a resistive
> device but I don't know how linear an LDR is.
> The circuit I saw actually used 2 LDRs in a "T" attenuator . That is
> in line LDR, resistor to ground, in line LDR.
>
> Hope this helps.
>
>
> Lance Allen
> Uni of Auckland
> New Zealand
>