Go to cocoforcannabis.com and see the DIY section on self-draining saucers and the whole fertigation system. It even has links to purchase components from Amazon US and Amazon UK. I just followed the links and it couldn't have been easier. I ended up with a Nearpow digital timer, which seems very popular. It is a bit quirky to program (big hint: use the Confirm button every time you change something), but it works fine. You can set up at least 10 separate on times and 10 separate off times, down to the second. (Mine are set to 35 seconds, 4x/day).
One thing I did NOT see at the above site was how to deal with a siphon effect, from reservoir out to the plants. I have a 14 gallon reservoir that is pretty tall, and at first it was just draining out onto the pots via siphon after a watering. I solved that by making a tiny hole in the tube coming up out of the reservoir, but still on the inside. Now a few drops of water get pumped out there when the pump runs, and it just falls back into the tank. But as soon as the pump shuts off, the siphon breaks, a bit of water drains out of the tubes and all is done.
