Instructions
12
Using a suitable meter, you could measure how much cur-
rent flows through the LED. But you can also work out how
strong the voltage, at the moment, the battery is and what the
voltage is at the LED. If the battery is still new, it will have a
voltage of 9 V. The LED needs about 3 V, which leaves another
6 V for the resistor. You can calculate the low brightness like
this:
Current = voltage/resistance
Current = 6 V/4700 Ω
Current = 0.0013 A = 1.3 mA
That’s not very much, only 1.3 mA are flowing, although the
LED can tolerate a current of 20 mA. But the battery will last
long! It usually has a capacity of 500 milliamps (500 mAh),
so could deliver 500 mA for one hour or 1 mA for 500 hours.
Or the lamp will glow for about 400 hours at 1.3 mA; that is,
for more than two weeks.
For the higher brightness you get about ten times the cur-
rent (13 mA) and then you are closer to the allowed limit of
20mA. But the battery only manages to do this for around
40hours, which is just under two days.