These Relays would be ideal for applications where many relays are needed and the load current requirements are low. Due to their small size a large number of relays could be mounted on a single printed circuit board. The relays are based on a 4N33 Optoisolator package. This device has a Darlington output transistor and as the first figure shows it can be used alone for low power applications of up to 30Ma.
http://home.cogeco.ca/~rpaisley4/SSRelays.html
The circuit breakers on this page make use of the Zetex - ZXCT1009 "High Side Current Monitor' integrated circuit. The circuits have variable trip current settings and are designed for low current applications.
http://home.cogeco.ca/~rpaisley4/CircuitBreaker05.html
There are 10 relays DPCO(dual pole change over), which are activated using a 12V supply, and around 60mA current.
http://www.geocities.com/fishziblu/relayboard.html
The door opens inserting an electronic card! Very useful also to access private parks, enable photocopiers, activate showers and night light in sport resorts shared between clubs.
http://www.nutchip.com/progetti/card/card_en.htm
The circuit uses an inexpensive C-MOS inverter package and a few small capacitors to drive two power MOS transistors from a 12v to 15v supply.
http://www.discovercircuits.com/DJ-Circuits/fetssr.htm
This circuit demands a control current that is 100 times smaller than that needed by a typical optically isolated solid state relays.
http://www.discovercircuits.com/DJ-Circuits/lowssr2.htm