David, try the circuit below.
The supply should be about 12V DC.
The on time is adjusted by RV1 and the off by RV2. For the values quoted the theoretical max for each is approx 175 seconds and the minimum 23 seconds with 100 seconds about the mid point of the adjustment. The tolerance of the electrolytic C2 tends to be quite wide so the actual times can vary a bit. Also you may find that the first cycle is a little longer than subsequent ones.
Edit, I see Greg had the same idea, it took me a while to do the drawing.