Download lm3909

[CIRCUIT IDEAS]
MUDIT AGARWAL
Bicycle
Turning Indicator
Sanjay Kumar
Turning vehicle without indicators on is always
dangerous. Two wheelers and four wheelers are
always equipped with indicators, whereas Bicycle
doesn’t have indicators. This may prove dangerous
to children. A simple solution to it is to build a circuit
for it.
The circuit consists of LM3909 IC. The LM3909 is a
monolithic oscillator specifically designed to flash
Light Emitting Diodes. Most ICs are designed to
operate on voltages of 4.5 volts to 40 volts.
However, the LM3909 IC is a low-voltage chip that
operates from a 1.5-volt battery. Four LEDs are
suitable for any LED flasher using a supply up to 6V.
LM3909 IC performs functions similar to circuits
containing many transistors, resistors, and
capacitors. To get a diode to flash on and off at a
constant rate, the flasher part of the IC is used. A
flasher uses an electronic timer to control the flash
rate of the LED. Electronic timers contain a resistor
to restrict current flow so that a capacitor charges
up slowly. Once the capacitor is charged to a
predetermined voltage level, a transistor starts to
conduct current to the LED. This discharges the
capacitor, shuts off the transistor, and causes the
S1
L
R
1
6
5
Off
4
L
R
S2
FASTRC
SLOWRC
RL
V+
OUT
LM3909
8
Off
2
+
10nF
V-
+
D1
Front Left
D2
Front right
D3
Back left
U
D4
Back right
1.5V
used indicating the front and back indicators.
To turn left move the switch to left. This will turn on
the front left LED (D1) and back left LED (D3). To
turn right move the switch to right .This will turn on
the front right LED (D2) and back right LED (D4).
By using the timing capacitor for voltage boost, it
delivers pulses of 2 or more volts to the LED while
operating on a supply of 1.5V or less. The circuit is
inherently self-starting, and requires addition of
only a battery and capacitor to function as an LED
flasher. Timing capacitors will generally be of the
electrolytic type, and a small 3V rated part will be
LED to stop glowing. The capacitor now starts to
charge again and the process is repeated. In the
LM3909 IC, electrons from the battery pass
through a resistor and charge up the capacitor. The
size of the capacitor determines how long it will take
for the capacitor to charge. Once the voltage of the
capacitor reaches the level that was designed into
the IC, a transistor will start conducting current from
the capacitor to the LED. This results in the LED
lighting up and discharging the capacitor. Once
the capacitor is discharged, the LED will go out and
the transistor will stop conducting current.