switches on the relay when input voltage reaches 2.5V
http://electronics123.com/amazon/datasheet/cps44.pdf
a simple circuit which can switch a relay off/on by pressing any key on any IR remote controller.
http://www.u-net.com/epr/electron/issue2/feat0303.htm
http://www.citycom.gr/electronics/downcircuits/78.zip
simple transistorised relay into which can be plugged personality modules to turn the base unit into a timer (pulse, delay on energise or delay on de-energise) or a latching relay or whatever is required
http://www.4qdtec.com/trd.html
Also describes a Domestic heating boiler control schematic.
http://www.4qdtec.com/csrd.html
circuit acts like a latching solid-state relay
http://www.e-insite.net/ednmag/archives/1997/091297/19di_03.htm
http://www.hut.fi/Misc/Electronics/circuits/semiconductor_relays.html
output a control signal suitable for relay depending on the light level.
http://www.spelektroniikka.fi/kuvat/valokytk.pdf