Hey guys i\'m having some trouble with this tut question. The LED torch circuit
ID: 1813353 • Letter: H
Question
Hey guys i'm having some trouble with this tut question.
The LED torch circuit design is to conform to the following specications:
1. Needs to illuminate 5 LEDs.
2. All LEDs are to have equal current
owing through them (equal brightness).
3. Current through each LED is to be no greater than 20 mA.
Current through each LED is to be no less than 10 mA.
4. The torch is to be powered using AA alkaline batteries.
The Alkaline AA batteries are described as 2500 mAH, 1.5 V when fully
charged, and 1.1 V at 85 % discharge. Ensure that specication 3 is met for
these battery states.
You may:
use 1 to 6 alkaline AA batteries.
use any resistor.
Consider issues of battery life, cost, number of components and any other
factors in making a good LED torch design.
Design the Circuit
Explanation / Answer
Current through each LED is to be no greater than 20 mA.
V = IR
1.5 = 0.02 * R
R(min) = 75 Ohms
Current through each LED is to be no less than 10 mA.
1.1 = 0.01 * R
R(max) = 110 Ohms
Let R = 110+75/2 = 185/2 =~ 93 Ohms
I = 1.5/93 = 0.01622A = 16.22mA
NO. of Batteries Life Cost
1 2500mAh/16.22mA = 154.13hrs 10
2 308.26hrs 20
..........
.........
........
Circuit diagram:
+------+-------------+-----+------+-------+------+
| | | | R R R R R
AA AA | | | | |
| | | | (LED) (LED) (LED) (LED) (LED)
----------------------+-----+------+-------+-------+
R = 93 Ohms
It is economics to have 2 batteries regarding the cost and the lifetime...