LED Warning Flasher
This circuit was created to serve as a warning flasher, alerting road users to hazardous situations in low light conditions. Alternatively, it can function as a bicycle light, adhering to traffic regulations and local legislation. When utilized as a bicycle front light for road illumination, it’s recommended to use only white LEDs, whereas red LEDs are suitable for a tail light function.
In daylight, the two 1.6-V solar cells recharge the two AA batteries. Once darkness falls, the solar cell voltage diminishes, and the batteries automatically supply power to the circuit. The flash rate occurs approximately once per second, with each LED illuminating for about 330 milliseconds. The duty cycle is designed to allow the batteries to sustain the circuit’s power requirements throughout the night.
Three-Part Circuit Design
The circuit comprises three distinct sections to operate effectively. In regular daylight conditions, diode D4 facilitates the charging of the batteries. During darkness, PNP transistor T1 activates, directing battery current to the second segment, a low-frequency oscillator involving T2 and T3. The third section functions as the LED driver, centered around T4. It engages and illuminates LEDs D1, D2, and D3 when the collector voltage of T3 rises substantially.
LED Selection and Potential Enhancements
In this setup, two LEDs (D1 and D2) boast high brightness, ranging from 20,000 to 30,000 mcd, and are of the yellow variety. The third LED (D3) serves as a standard 3-mm red LED for control purposes. It’s feasible to augment brightness by adding more LEDs; however, one must consider limitations related to the maximum collector current of transistor T4. For applications demanding substantial power, it’s advisable to opt for a MOSFET transistor instead of the conventional BC547B.
Understanding the BC547B Transistor Configuration
The BC547B transistor operates as an NPN (Negative-Positive-Negative) Bipolar Transistor, which is a widely employed configuration. In this configuration, the Collector of BC547B is linked to the positive supply voltage (+ve) through the load. When dealing with inductive loads like relays, solenoids, or motors, it’s common practice to connect a diode across them to counter the Back EMF, safeguarding the transistor. The diode’s cathode is directed toward the supply voltage. The input is connected to the Base through a limiting resistor, typically around 1K ohms. The Emitter is linked to the negative supply (-ve or 0V).