Add a *1K* pullup to 5 volts
Turn the circuit on by driving Q1.
This turns the 5V on and activates the pullup
Measure the voltage at the top of the pullup (= 5 volts)
Measure the voltage at the bottom of the pullup.
It MUST be 5 volts.
It it's say 1 volt then the resistance to ground equivalent must be 
200 ohms.

The most obvious solution is that you have D1 (the upper one) 
reversed.
Its job is to block the short that Q2 applies to ground.
If D1 is reversed then A1 sees a two diode drop + Vsat of Q2 to 
ground.
The two diode cathodes (line on diode) should face each other.

Reverse D1 and away you go.
Hopefully :-)


        Russell


>I have seen this exact same circuit on the net several times, just 
>redrawn.
> In some versions it shows it connected to PortB.  I suspect someone 
> redrew
> it with PortA but didn't add a pullup.  I have looked at this 
> circuit over
> and over and just don't see a wiring flaw.  Sometimes it is staring 
> you in
> the face though and you just can't see it.  I'll have another look 
> at it.
> Thanks to all,
> FJ

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