To prevent the dynamo-driven lights of your bicycle going out when you stop in the dark for, say, traffic lights, the simple circuit here may offer help.
The circuit uses four NiCd batteries with a capacity of between 0.25 Ah and 1.25 Ah, which are constantly charged when the dynamo is driven via R1 and D1. Since the battery voltage is rather less than the dynamo output, the lights are dimmed to a small extent when the cycle is stopped, but in practice that is hardly noticeable.
Monostable ICia, which has a mono time of 1 s (R5-C2), is used to detect whether the dynamo generates a voltage with the aid of D3, R3, and R4. As long as there is a voltage, the monostable holds 1Cib, also a mono-stable, in the reset state. The relay is not energized and the lights are powered by the dynamo.
When the dynamo voltage drops, ICia is no longer triggered so that its outputs change level. This causes the reset state of ICib to be removed, whereupon its T input is actuated and remains so for two minutes, during which time the relay is energized and the -cycle lights are powered by the batteries.
Strictly speaking, ICib is not essential, but it does ensure that the lights are switched and that the battery cannot be discharged completely. The relay should be a type that operates faultlessly when its supply voltage reaches 4.8 V.
It is advisable to build the circuit in a watertight, or at least waterproof, enclosure.