CHAPTER 11 ■ POWER ON!
With this setup, all of the electricity that leaves the battery is going into the meter’s red probe, through
the meter for counting, out the meter’s black probe, and into the red alligator clip. With this arrangement,
the meter is guaranteed to count every drop of current pushed out by the battery and used by the circuit.
If all goes well, the red LED should be lit and the meter should be displaying between 5 mA and 10 mA.
My circuit measured 7.2 mA (see Figure 11-10).
If the LED doesn’t light, check all of the connections and the dial settings. If the value displayed on the
meter is negative (like -7.2 mA) then you’ve got the black and red test probes reversed.
Calculating Battery Life
Assuming you have a fresh 9 V battery in your circuit, you can now determine how long it will last. Usually
the fresh battery capacity is published on the package or on the manufacturer’s web site. If you don’t
know the official capacity rating of your 9 V battery, you can assume 595 mAh for alkaline and 150 mAh for
rechargeable.
Take the mAh (milliamp hour) battery capacity and divide it by the mA (milliamp) current usage of your
circuit. This tells you how many hours the battery can power your circuit.
For example: I have a 150 mAh rechargeable battery and my power indicator circuit is using 7.2 mA.
150 mAh / 7.2 mA = 20.8 h
That’s almost 21 hours.Extending Battery Life
If you can reduce the current flow, you can extend the battery life. Recall that resistors not only protect the
other parts from the force of the battery, but that resistors also prevent wasted power.
Instead of the 1 kW resistor, try substituting a 2.2 kW resistor (red, red, red, gold color bands). That’s
slightly more than double the resistance. It’s twice as good as resisting flow. Go ahead and try that now.
The multimeter should reflect the decrease in current flow due to the 2.2 kW resistor. My circuit went
from 7.2 mA down to 3.4 mA. That means the battery life is now 44 hours:
150 mAh / 3.4 mA = 44 h
However, the LED isn’t as bright as it was.
Try some other resistor values (see Figure 11-11). You can pick values as large as you want, but don’t go
below 470 W. Watch the meter and observe the brightness of the LED.
Figure 11-10. Multimeter displaying 7.2 mA of current flowing through circuit