Bike Indicators

Function
Flashing LEDs attached to rear of the cycle indicates in which direction the cyclist intends to turn.
Circuit diagram

Components
SW1 = Toggle switch
SW2 = Switch SPDT
R1 = 1k ohm
R2 = 470 ohm
R3 = 470 ohm
VR1 = 100k ohm
C1 = 100mF
IC1 = 555 Timer
LED = 5mm Standard LED
Operation
Heart of the circuit is a 555-timer chip configured in ASTABLE mode.
This means the out put at pin 3 is constantly changing, i.e. the output goes high (9V) for a specified time, and then low (0V) for a specified time before again switching high.
The transition from high to low is called a cycle, and the number of cycles which occur in one second is called the frequency, measured in hertz (symbol Hz)
The frequency is controlled by the size of R1, VR1 and C1.
Formula to calculate the frequency:-
With VR1 at its max resistance
FREQ = 0.07Hz

The time taken to complete one complete cycle (i.e. time output pin 3 is high plus time output is low, called the period, symbol T), is calculated from the formula:-
T = 14 sec.
This means the output from pin 3 (IC1) is high (9V) for a period of 7 seconds, and then low (0V) for a period of 7seconds.
Note the period can be reduced by adjustment of VR1.
The output from the chip is then fed, via R2 and the switch, to either of the LEDS depending on switch position (SW2).
Thus when the output of the timer (pin3) goes high the particular LED will light and when output pin 3 is low LED is unlit.
Note a red LED in series with R3 is connected to the output pin of the 555-timer circuit.
Thus, when output pin 3 goes to 9V, the LED will light and when output pin goes to 0V the LED is unlit. This flashing LED serves as an indicator to the rider, circuit is working correctly.

Calculation of value resistor R2 and R3
It is most important an LED must have no more than 2 volts dropped across it. Also the current flowing through should not exceed 20mA.
When output pin 3 goes to 9V, therefore 7V must be dropped across R2.
With 7V dropped across it and max permissible current flowing through it of 20mA the size of resistor can be calculated using OHMS LAW.
R2=7/ 020 =350 ohms.
Note
In electronic circuits it is not good policy to have components operating on their top limits, so a resistor value of 390 or 470 ohms should be used for R2, and R3.