Page 239 - Robot Builder's Bonanza

208 ROBOT POWER SYSTEMS

Battery Voltage Monitors

Quick! What’s the condition of the battery in your robot? With a battery monitor, you’d know
in a flash. A battery monitor continually samples the output voltage of the battery during
operation of the robot (the best time to test the battery) and provides a visual or logic output.
A microcontroller- compatible battery monitor is shown in Figure 19- 13. This monitor uses a
5.1- volt half- watt zener as a voltage reference alone with an LM 339 quad comparator IC.
Only one of the comparator circuits in the IC is used; you are free to use any of the remaining
three for other applications.
The circuit is set to trip when the voltage sags below the (approximate) 5- volt threshold of
the zener (in my test circuit the comparator tripped when the supply voltage dipped to under
4.5 volts). When this happens, the output of the comparator immediately drops to 0 volts.

19-chapter-19.indd 208 4/21/11 11:49 AM