Hi,
The NTE3036 photo-transistor will handle 250 mA
that should be enough for most small relays.
Dont forget the 'quenching' diode across the
relay coil, cos that can have quite a kick
and may damage the sensor.
bogdanfirst,
Putting the coil in series with the supply to the
lamp would mean using an AC relay (or rectifying)
there is no problem with the relay contacts
switching AC or DC,
its the relay coil.
Most of these smaller relays are DC only,
they buzz badly on AC,
you could of course rectify and smooth,
but getting an AC working relay in the first
place would be easier.
However, i think leon mean a light sensitive unit,
and since he hasn't been back,
i shant worry any more.
John