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Leds and how to use them
Leds feature a specific voltage
drop, depending on type and
colour. Check the datasheet for
exact voltage drop and rated
current !
How to Calculate the series resistor:
Example: operate a red led (1.7V) on a 9Vdc source.
Required led current for full brightness: 5mA (this can be found in the datasheet of the led)
Supply voltage (V) - led voltage (V)
= series resistance (ohms)
required current (A)
9V - 1.7V
= 1460 ohm
0.005A
Required resistor power handling=
voltage over resistor x current passed trough resistor
(9V - 1.7V) x 0.005A = 0.036W
LEDs in series:
Example: 3 x red led (1.7V) on 9V battery
Required led current for full brightness: 5mA
(this can be found in the datasheet of the led)
Supply voltage (V) - (number of leds x led voltage (V))
required current (A)
9V - (3 x1.7V)
= 780 ohm
0.005A
closest value :
use a 1k5 resistor
a standard 1/4W resistor
will do the job
= series resistance (ohms)
use an
820 ohm resistor
Never connect leds in parallel
open collector outputs
An open collector output can be compared to a switch which switches to
ground when operated
Example: How to switch an LED by means of an open collector output
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