CHAPTER 23 ■ THE MOTHERBOARD- Turn on the multimeter.
- Toggle the robot’s power switch back and forth. In one direction, the resistance
should be infinite. That’s power off.
If you don’t get an infinite resistance in either switch position, something is wrong with the power
switch or its connections. Recall that in an off state, the positive entrance to the circuit is completely
disconnected, thus the infinite resistance to electricity trying to enter the circuit.
Measuring the Power On Resistance
- Toggle the robot’s power switch to the power on position.
- The resistance should be between 5 kW and 50 kW.
A value of infinity, with the power switch on, suggests a wire is missing or broken between the battery,
power switch, and either power bus.
A value below 1 kW suggests a partial short circuit or bad component. Then again, a trimpot could be
dialed to a minimum value.
Anything below 10 W indicates a serious short circuit! If you find that’s the case, absolutely do not
connect the robot to a battery. Trace each circuit and wire to find the cause. Switching the multimeter to
continuity mode and touching the test probes between stretches of components can help find the problem.
Measuring the Sensor Resistance
- Cover the photoresistors with your hand. The resistance of the circuit should be
quite high. Sandwich measures 45 kW. - Expose the photoresistors to light. The resistance of the circuit should be lower.
Sandwich measures about 5 kW when held against a light bulb.
If the resistance doesn’t vary with the amount of light the photoresistors are receiving, then the sensor
circuit isn’t correctly wired to the positive and negative bus. Or, maybe there’s a bad solder joint somewhere
in the sensor circuit.
Measuring the Voltage Drop
There are some semiconductors in the circuit that won’t pass significant current until the voltage from the
meter exceeds 0.5 volts or better.
- With the meter test probes still connected to the battery snap, switch the
multimeter dial to diode mode. - When the robot’s power switch is in the off position, the meter should display
“open” or “infinity.”
If the meter indicates that any voltage is being dropped through the circuit when the robot’s power
switch is in the off position, that should warn you that the power switch isn’t completely disconnecting the
robot’s circuit from positive voltage. It is not safe to connect the circuit to a battery until you’re sure that the
power switch works.
- When the robot’s power switch is in the on position, the voltage coming from
the multimeter should be high enough to cause diodes, chips, and transistors to
conduct. For Sandwich, the multimeter displays “good” or 0.742 V.