Started experimenting on clock generator. I designed a scheme which consists of 2 relays and RC circuits:

Then I bought two 100 Ohm resistors and two 1500 uF capacitors.

Soldered clock works with 2.66 seconds period:

Time for energizing the capacitor and switch on the relay is about 0.2 sec (as I planned). And discharging the capacitor and switching off the relay takes 0.93-1.2 sec. Thus the left relay performs the charging-discharging cycle. It is switched on within ~1.2 sec and off within ~1.4 sec. This is almost perfect square wave.

I’ll probably reduce the capacitors and make two speeds of execution (switched by “turbo” button).