Is there a photoelectric light control such as you might use for outdoor
lighting that cleanly turns power on and off. I have one that seems to use
some sort of triac circuit like a light dimmer. It turns on and off
gradually. If you put your finger over the photocell in daylight you can
make the light come on to varying degrees of brightness by varying the
daylight on the cell. The sort of weird waveform this sort of phase
control puts out is unsuitable for most compact fluorescent bulbs. So I
need to get one that has a pure on/off output.

I was thinking of adding a relay between the control and the light socket.
That would make the switching clean but there's always the possibility that
it will chatter on and off at dusk or dawn. So another possibility is to
use a junk AC adaptor with a DC output switching a relay with a low voltage
coil. A capacitor on it would iron out any chatter.

But presumably there's already something available that is suitable.