The post illustrates a simple bicycle light circuit which can be used as a normal front/back indicator lights, and also as an emergency alarm signal such as an SOS signal.
Rear and front lights are a must in bicycles or tricycles, but this light system serves a second purpose. It serves as a visual alarm system for impaired travelers at night, since, unlike during the day, handicapped people in need of assistance are not easily noticed.
A rechargeable lead-acid battery of 6 V is used here instead of the conventional battery or dynamo so that it operates even when the bicycle is stationary. If the rider ends up in an unwanted situation, switching on the alarm will start flashing a HELP or SOS sign on a display, besides the normal lights.
The bicycle light and alarm circuit, constructed from an astable multivibrator, remains inactive as long as the alarm switch S2 stays open. But if S1 is closed, the front and rear lights will be lit.
Closing S2 activates the multivibrator and causes the alternate flashing of the normal lights and the alarm display lights. The 6 V, 1.8 Ah battery, if fully charged, can power the circuit for a good 3 hours.
The circuit is best packed into a waterproof case, with the HELP letters cut out on its lid to be lighted by the lamps La4 to La6. The BC141 must be fitted into a small heat sink.
The case should be easily removable and attachable since the battery will need regular charging. The following figure shows the circuit for an appropriate charger for the battery.
P1 is used to adjust the charging voltage at an exact level of 6.9 V while the charging current is kept at 650 mA. The voltage must be set right or the battery will not charge properly. The given values will get the battery charged in 3 hours.