"Wiebe Cazemier" wrote in message
...
On Wednesday 12 April 2006 19:56, I.F. wrote:
An easy way to make a time delay is with a long tailed pair of
transistors
like a differential amplifier stage and add a high value cross coupling
resistor so it acts as a Schmidt trigger and avoids relay chatter.
Connect
one base to a zener about half the voltage of the secondary rail used and
the other base to a potential divider shunted by a time delay capacitor -
don't forget a discharge diode from the capacitor to Vcc, this ensures
that
as Vcc falls below 1/2 value at switch off it discharges the cap. A relay
driver transistor tapped off the collector resistor of the Schmidt output
transistor completes the line up of only 3 transistors, be sure to
remember
also the back emf diode on the relay coil, the relay of course must
provide
adequate isolation between primary & secondary supplies!

I have a circuit in use which is sensitive to relay chatter, so anything
better
is welcome.

My circuit design skills are somewhat limited (I really need to work on
it), so
I can't completely follow what you mean. Especially the LTP, I never
really
understood how they work. Could you draw up a schematic? You can post
images
to www.imageshack.us, should you be unable to send attachments. Thanks in
advance.

A better alternative is to re-post on alt.binaries.schematics.electronic as
I have sketched a basic circuit, but as I find math highly tedious I usually
do just enough to get by and determine most of the values by experiment.
There are some great engineers on a.b.s.e so you might get some even better
circuits to try.