With each clock of the 555 (negative transition) the 4013 (flip-flop) changes state by activating the relay or turning it off, depending on its previous condition. The time constant given by the 47k ohms resistor and the 100 nF capacitor determines the minimum time between two control pulses. The relay used depends on the supply voltage and the sensitive types of 6 or 12 V are recommended. As the 4013 integrated circuit has two flip-flops available and this configuration uses only one, the other can be used for other purposes.
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