Success! I can now trigger the pump with the remote control (and with Nethome Server). I went for the simple robust solution with separate batteries for the pump and for the electronics. This also means that I don’t have to compromise with the voltage. I use four 1.2 V cells for the electronics which gives 4.8 V which is ok both for the receiver and the NE555 and I use six cells for the pump which gives 7.2 V. This means I get a bit more pressure on the water. The downside is that I cannot fit all in the box I intended to use, so I will have to use a separate box for the extra batteries.