CHAPTER 15 ■ COMPARATORS
Check that positive power is being supplied to the top wire of R1, R7, R8, and IC1 V+ (pin 8). You can
confirm this by connecting the black test probe to any negative bus and then (one at a time) touching the
tip of the red test probe to the top wire lead of each of the resistors and to pin 8 of IC1 (see Figure 15-8).
By touching the multimeter test-probe tip directly to the metal portion of the component that is supposed
to be getting power, you can eliminate the possibility that the component wire is loose and not actually
receiving power from the board.
Figure 15-8. Testing the comparator’s positive voltage pin with the multimeter test probe tip
Check that negative connections are being supplied to the bottom wire of R3, R6, and IC1 GND (pin 4).
You can confirm this by connecting the red test probe to any positive bus and then (one at a time) touching
the tip of the black test probe to the bottom wire lead of each of the two bottom photoresistors and to pin 4 of
IC1. This is a slightly backwards test, since the red test probe stays in place and the black test probe moves to
the points being tested. So, note that the meter displays 9 V (or whatever) if a proper connection exists, not 0 V.
Faking Input to the Comparator
For diagnostic purposes, the reinforced jumper wires connecting the comparator’s inputs to the sensor test
points can be redirected to ideal, unwavering test points. Connect one jumper wire to the positive bus and
the other jumper wire to the negative bus to provide the highest-quality test signals (see Figure 15-9). One of
the LEDs should light. You can then swap the jumper wires and the other LED should light.