CHAPTER 23 ■ THE MOTHERBOARD
The other pin of the power switch’s Molex connector should be wired to the positive voltage bus.
Double-check all wires connected to the positive voltage bus. Most of them should be going to resistors. In
Sandwich’s circuit, the exceptions are the collector pins on two transistors and pin 8 on the comparator IC.
Everything else runs through resistors.
Measuring the Resistance of the Entire Circuit
After completing the visual inspection, here are a few quick multimeter tests that can spot some problems.
Note: Toward the end of the book, I recommend the addition of a diode between the power switch and
the rest of the circuit that protects against damage from a battery installed backwards. The resistance tests
described in this section won't work through the diode. So, if you choose to include the diode, you'll need to
temporarily install a wire (alligator or hook jumper) to both ends of the diode so that the multimeter power
can skip around it during resistance testing.
Measuring the Power Off Resistance
- Attach all of the parts that have Molex connectors to the finished circuit board
(see Figure 23-16). That means the motors, power switch, battery snap (without
battery), line-following switch, and tube LEDs (if desired) should now be
connected to the circuit board.
Figure 23-16. Testing resistance of circuit through 9 V battery snap connector
- Connect the multimeter’s test probes to the positive and negative snaps of the
battery connector (it helps to use alligator clips or IC hooks). Note that the
battery is not installed! The multimeter is pretending to be the battery. - Dial the multimeter to measure resistance (W).