Bicycle Rear-Light Afterglow Circuit

This article is of interest only to readers whose bicycle lights are powered by a dynamo. The laws on bicycle lights in the United Kingdom are stricter than in most other countries and a dynamo is, therefore, a rarity in the UK.

For safety’s sake, it is obligatory for cyclists in the UK to have the rear light of their bicycles on even when they are at standstill. Such a rule is, of course, also advisable in other countries, even when it is not mandatory.
The super-luminous LEDs are not powered directly by the regulator, IC1, but via a timer, IC2. The timer is driven by a pulsating voltage with a duty factor of 10%. This ensures very low current drain and also that the reservoir capacitor, C2, can energize the LEDs for a long time.

Capacitor C2 is charged when the bicycle, and thus the dynamo if this makes contact with the relevant wheel, is ridden. This happens via bridge rectifier B1, and low-drop regulator IC1. The output of the regulator is a steady +5 V.

Bicycle Rear Light Afterglow circuit diagram

When the bicycle is in use, but at standstill, for instance, at traffic lights, the potential across C2 powers astable multivibrator (AMV) IC2. With component values as specified, the switch-on time is about 0.02 s, but the LEDs are not energized until 0.25 s have elapsed.
Parts list
R1 = 330 kΩ
R2 = 33 kΩ
R3 = 47 Ω
C1 = 100 μF, 25 V, radial
C2 = 1 F, 5.5 V, Goldcap
C3 = 0.1 μF, ceramic
B1 = rectifier B40C1500, round
D2, D3 = high luminosity LED
Integrated circuits:
IC1 = LP2950CZ5.0
IC2 = TLC555CP

Bicycle rear light PCB layout

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