This circuit was designed as a warning flasher to alert road users to dangerous situations in the dark. Alternatively, it can act as a bicycle light (subject to traffic regulations and legislation). White LEDs only are recommended if the circuit is used as a bicycle front light (i.e. for road illumination) and red LEDs only when used as a tail light. During the day, the two 1.6-V solar cells charge the two AA batteries. In the darkness, the solar cell voltage disappears and the batteries automatically power the circuit. The flash frequency is about one per second and the LED on-time is about 330 ms. The duty cycle should enable the batteries to power the circuit overnight.
The circuit is composed of three parts. Under normal daylight conditions, the batteries are charged through diode D4. In the darkness, PNP transistor T1 is switched on, supplying battery current to the second part, a low-frequency oscillator comprising T2 and T3. The third part is the LED driver around T4. It conducts and switches on the LEDs D1- D2-D3 when the collector voltage of T3 swings high. Two LEDs (D1, and D2) are 20,000- 30,000 mcd high-brightness yellow types and one (D3) is a normal 3-mm red LED for control purposes. Of course, it is possible to increase the number of LEDs to obtain higher brightness. However, you will run into limitations regarding the maximum collector current of transistor T4. For really high-power applications a MOSFET transistor is suggested instead of the common or garden BC547B.
A BC547B Transistor is an NPN (Negative-Positive-Negative) Bipolar Transistor: the most commonly used transistor configuration BC547B Collector is connected to the positive (+ve) supply via the load. NB If the load is inductive i.e. has coil windings such as a relay, solenoid, or motor, then it is usual to connect a diode across it to prevent the Back EMF from damaging the transistor. (Cathode to supply). The input is connected to the Base via a limiting resistor (typically 1K). The Emitter is connected to the negative (-ve) supply (0V).