> A cap C-B in that circuit is stupid,
> a cap B-E will fix his problem.

Maybe.  It depends on where the feedback is coming from that causes the
oscillations.  He said the relay was "vibrating" or something like that
during the transition.  The circuit is just a DC amplifier driving the
relay, and as you said, relays do have some inherent hysterisis.  We also
know his input signal varies very slowly (outdoor ambient light at dusk).

So, why does it oscillate at the threshold input?  One possibility could
be AC noise coupled back into the amplifier from the switching transients.
Your cap from B to E should help with that.

However, I suspect at least part of his problem is DC feedback via the
power supply when voltages change due to the relay switching on
significant power.  A B-E cap won't fix that, only decrease the
oscillation frequency.  I think the best way to address both problems
together is to spend a few more cents and add another transistor stage.
This will give him more gain and provide an easy opportunity for some
deliberate positive feedback in the amplifier to get some hysterisis.
This will always drive the relay solidly on or off, and should provide the
necessary noise immunity.  It will also make insensitive to small
variations in light level at the thresholds, like a cloud momentarily
passing over the sun shortly after it has set.


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