Schematic Diagram

This is LED Flasher circuit that is built using 2 transistors. Many other NPN transistor have a small signal or switching transistors can be used, such as 2N4401, PN2222 or 2N2222 using the circuit on the left figure. The circuit can also be inverted using PNP transistors such as 2N3906, 2N4403, PN2907, or 2N2907 as shown to the right. This is the figure of the simple LED Flasher circuit;


The 470 ohm resistors determine the LED brightness. Lower resistance means higher current, and more light. LEDs that require more current or have a higher operating voltage (such as green and yellow) may work better with 300 ohms. The RC time constant of the 39K ohms resistor and the 10uF capacitor determines the on time for each side. (The two sides do not need to match - vary the RC time constant for one side to get a lower or a higher duty cycle). With the values shown, the flash rate is about 1 cycle per second at 50% duty cycle.

1. This makes it possible for a charge of 3 mAh to be available after a 10 minute ride, 1.e., enough for the E light to flash for about4 minutes after the bicycle is halted. A ! relay is used to switch between operation while riding and while standing still.
2. The circuit is essentially composed of a battery charger and a logic switching section. The NiCd battery is charged from a I voltage doubler C1-Cz-Di—Dz—C to ensure a charge current of about 20 mA when riding at a reasonable speed.
3. Whén the dlI9CI Y voltage drops below approximately 2XL N1 switches on multivibrator Nz—Na-N4 which causes the relay to toggle at a rate determined by R4-Ca
4. Of course, due care must be taken to avoid the battery con- tacts touching the metal inside of the light.
5. Since Ca is only slowly discharged via R1, N4 remains enabled for about 4 minutes after halting.
6. When the bicycle is in motion, the voltage from the dynamo, G, ensures that N4 is enabled, so that T1 actuates Re, and the small 6 V bulb is illuminated.
7. Push- button S1 enables immediately switching off the rear light, because R2 then discharges Ca in a few seconds. Gate N1 monitors the dynamo voltage, which is rectified by Da-C4-Rs.
8. The 5V DIL relay requires only llmA, while the current consumption of the 4093 is virtually negligible at about l nA. 

Bicycle Turn Signal Flasher Circuit Diagram



 

Flashers are useful as visual metronomes, and for decoration and advertising purposes, here we learn how to build one at home. Conventional. thermal flashers make use of a bimetallic strip which alternately heats and cools, thereby breaking and making an electrical contact. Such flashers are uureliabledue to various mechanical problems. glimther, due to sparking at the contacts, their life is limited, and also their flashing rate is not constant. The solidstate electronic flasher described here suffers frdm none of the above disadvantages. A 555 timer IC operated as an astable multivibrator is used to control the on and off times of triac, which in tum controls power to the lamps. The flashing rate is determined by settings on the potentiometer, and is approximately given by  

f = 43/(VR1+R2)C flashes/minute

Hence by varying the resistance included through 250K potentiometer (VR1), it is possible to adjust the flashing rate   from about 6 flashes/ minute (slow) to about 60 flashes/ · minute (fast). The on and off times are almost equal due to symmetry of the astable circuit.  The ST44A triac used here has a 400 PlV, 4-amp rating; With it the load must be restricted to a maximum of 500W. The triac should be clipped to at suitable electrically isolated heatsink. The unit should be assemble inside a plastic or wooden box, and on switching it on, no part of the circuitry, except the rate adjust potentiometer (which must preferably have a plastic knob) should be touched as all parts will be alive. An approximate cost of the electronic ponents used is Rs 100/-