I was thinking of using my Raspberry Pi to control a switch (control a lamp, for instance). I have a traditional (electromechanical) relay, but I know I will need to use an external transistor and power source to drive the relay. My question is, does the Pi have enough power to directly drive a Solid State Relay?
I used a PiFace to connect to my Solid State Relays in the end. When measured, the relays I had were drawing about 14mA, I have 5 (relays) so was over the 50mA total GPIO limit. At the end of a cable run, I was also only just getting above the necessary 3v to switch them.
That said, it did seem to work fine from the GPIO, even with all relays engaged. I guess there is margin in the 50mA limit.
I wasn't comfortable though, and had a PiFace anyway, so hooked up to that. My relays seem to draw slightly less (12mA) at 5v from the PiFace, and work perfectly. Also, when connected to the GPIO, a couple of the relays would switch at boot-up, they do not do that from the PiFace.