Hi MIKE.

08-Mar-04 10:37:48, MIKE ROSS wrote to Jasen Betts

 JB>> I measured 400K between the tracks on the stripboard. dunno if
 JB>> the 30C heat and 100% humidity had anything to do with that. so I
 JB>> stripped the components off the board and built the whole thing
 JB>> free-standing between two wires...  seems to work now.

 MR> Might be some invisible surface contaminant.

could be sweat given the environment :)

 MR> Try cleaning it with 99% alcohol and see if it has any effect.

 MR> I know
 MR> that some types of flux residue will conduct, obviously acid types
 MR> are one of these.

after noting voltage crossing between tracks I measured some tracks on the
unsoldered portion of the board.

 MR> So if your prototype requires megohm impedances you would do well to
 MR> always clean off all flux on a pcb.

I've got a new DVD player and the TV (a monitor really, a fault has
disabled the tuner) has only one set of AV-in sockets and the AV-in
on the VCR isn't under software control,

so I decided to make an little automatic AV switch,

4 transistors and a couple of little 12V relays.
it looks through a 1 meg resistor and a 10nf capacitor for the sync pulses
(or any high-ish frequency) from the DVD player and switchees the AV to
that source when it sees it, when it doesn't it uses the signal from the
VCR, (which I also ue as a tuner)

all the transistors are used in common-emitter configuration.

the first two NPN transsitors form a darlington pair in takes the input signal
and via a small cap pumps a larger reservoir capitor (via two diodes)

this reservoir capacitor feeds via a 100K resistor a pnp transistor
which drives the NPN output transistor
a trimpot accross the load (relay) from the wiper tag feeds back via a 1
meg resistor to the base of the PNP giving a schmitt trigger effect,

it's built from mostly junk-box parts. and so far, works really well.

the only problem I had after building the circuit was when I came to assmble
it and the relays and sockets together I discovered that the
electtromagnetic field from the relays was causing the circuit to oscilate.
(at about 5-10Khz I think) I tweaked the level control of the schmitt
trigger a bit and that was fixed. biasing the darlington pair was a
little tricky too,

1M input impedance is probably over-kill but anyway it was fun to build and
proves useful.

 -=> Bye <=-

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