Sainsmart_Relay.txt

I seem to have bought these:
http://wiki.netduino.com/SainSmart-5V-Relay-Module.ashx
(From: http://www.ebay.com.au/itm/181026509345 )

It appears that the four-pin header with GND, IN1, IN2, VCC
is for receiving 3V3 or 5V signals from the arduino. Note
that logic is reversed -- high signal = relay off, low signal
= relay on.

The JD-VCC, VCC and GND pins on the other side are for
driving the actual relays. These want a moderately high current
5V line. Remove the jumper entirely, and apply power to the GND
and JD-VCC pins. (It sounds like the jumper is there for if you
have a 5V controller and don't care about optoisolation)

If driving from a 3V3 controller (eg. Teensy, Beaglebone) then I think
you need to drive it like this, according to the site:

All GND pins --> to GND

One of JD-VCC and VCC (next to it) will need 5V power, ignore other one.
Site linked above is wrong, IMO. Check which pin is connected thru to the
other VCC and leave that one unconnected here.

VCC (input header) -> 3.3V
Input1, Input2 -> 3v3 signals