MEET
BLINKER – Our alternating dual blinking LEDs kit is based on
“a-stable-multi-vibrator.” Using 2 transistors as a power
switch to control (turn ‘on’ and ‘off’) the
corresponding LED. This basic kit can work with many
conventional NPN transistors i.e. 2N3904, 2N4401, 2N2222, etc.
The
150 Ohms resistor determines the LED's brightness and limits the
current flow to about 15mA when running with 9V battery.
Standard LED will operate on 10-20mA and the voltage developed
on the LED is about 1.6VDC, therefore:
R_Led
= (V_Bat – V_CE – V_LED) / 20mA =
(9VDC - 0.2VDC – 1.6VDC) / 20mA = 360 Ohms
The
duty cycle (time on divided by the full cycle) of each
transistor is less then 30%. Therefore, 0.3 * R_LED = 108 Ohms.
Since the BLINKER is designed to be powered from 3VDC and up to
18VDC, we chose 150 Ohms resistors to limit the current through
the LED’s for a larger range of operation. To get more
lumination (light) from the LEDs, use smaller resistors. Please
be advised you should never exceed the LED maximum current
limitations. Doing so will result in burning the LED.
The
BLINKER flash rate is determined by the 51K resistors, and the
10µF capacitors combination determines the ‘on’ time
of each alternate transistor. Using matching components R1 = R4
and C1 = C2 will result in matching duty-cycles of the
transistor, therefore the LED light time. The BLINKER flash rate
is about 1 cycle per second. Additionally the 51K resistors are
used to limit the current flow to the transistor base. Remember,
the Ohm’s law... the larger the resistor, the smaller the current.
V
= I * R
Finally,
decreasing the blinking rate can be done easily by increasing
capacitors size i.e. C1 = C2 = 22uF. Or on the contrary, using
smaller capacitors value i.e. 4.7uF to increase the rate. Please
be advised any capacitor use with this kit should carry 25V
rating and above. Any modification you do at your on own risk.
Note:
The 9V battery is for demonstration only
and is not included in this kit.
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