Testing point A:

This figure shows the output signal at point A at 20 kHz, which produces a square waveform. We place the probe at the output of the logic gate and measure it with the 100 nF capacitor.

Testing point B

This figure shows the output at point B. The probe is placed after the 100 nF capacitor. The capacitor is charging and discharging in the circuit as shown on the figure.

Testing point C:

This figure shows a burst square wave at 40 kHz at point C. This illustrates the of LED is controlled by the the transistor.

Testing point D:

This figure shows the output of an IR sensor at point D. It is the envelope of the burst waveform in testing point C.

Testing point E:

This figure shows the threshold voltage for detection at point E.