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R1=169K, R2=86.6K C=1000uf gives the timing you
want. The timing may be a few seconds off either way mainly due to the tempco and tolerance of the capacitor.
The time the 555 output is at +V has to be longer than the off period so you have to make the relay
turn on when the 555 output is low. Either a) check the relay current is within the capability of your 555 (200mA for the national semi LM555 but you it typically onlys pulss down to 2.5V when sinking 200mA) and connect your relay and back-EMF diode between +V and the 555 output or b) Use a PNP transistor +base resistors to drive the relay.
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