Make: More Electronics

Chapter 6. Experiment 6: Easy On, Easy Off

You saw in the last two experiments that a phototransistor changes its output gradually in proportion with the amount of light falling on it. This is a useful capability—although not as useful as it could be. For practical purposes, we often want a light-sensitive gadget that has two precisely defined states: “on” and “off.” An intrusion alarm, for instance, triggered by someone interrupting a light beam, has to give a clear signal. It cannot function gradually or intermittently.

Is there a way to convert a gradual output from a phototransistor into a clearly defined signal? Most definitely. A comparator is the tool for the job.

Making Comparisons

Put together the schematic in Figure 6-1. The 500K potentiometer ...