A new problem appeared... These relays don't like to oscillate !
I tried to do the circuit we discussed at How can I make a low-frequency oscillator with a relay (or more ?) and it didn't go as planned. The relay breaks pretty quickly, for an unknown reason (yet).
I wired one, it clicked once and then nothing. Maybe this is a quality problem so I changed it and made it vibrate a bit without capacitor. It made a little whine and it looked OK. Adding 330µF made it sound a bit weird but it only lasted a few seconds : the relay's contact is shut off now.
Maybe this is a problem with current limitation. This hypothesis can be tested by adding a resistor in series with the contact. I have to understand quickly because I've already broken 2 relays...
It seems that a larger capacitor, a 100nF cap across the contacts and a 47 Ohms resistor in series work. However the working voltage is too high (at the very least 5.5V) so I'm changing the design and I start a ring oscillator with 3300µF. This will allow me to tightly control the pulse lengths and sequencing.