A transistor can either be used to amplify a signal (we used this very same transistor—the utilitarian 2N3904—as an amplifier in the Two-Transistor Fuzztone, Chapter 16) or to switch a signal. Here, we use it as a switch—although it might make more sense to think of it as a valve (like a bathroom faucet), rather than a bedroom light switch. If the transistor is a valve, then its base is the handle on that valve, and the battery’s current is the hand waiting to turn that handle. In darkness, the photoresistor puts up a lot of resistance, and little to no current can reach the base. When the photoresistor is illuminated, its resistance decreases, and when the resistance drops beneath a certain threshold, enough voltage passes to the transistor’s base to crack that valve open. The transistor permits some current to trickle from the collector into the emitter, which feeds the motor. The Jitterbug then quakes and palpitates. As more light shines on the photoresistor, more current reaches the base, which cranks the transistor open further, letting more current pour into the motor, which rattles even more furiously. This is all proportional, so if you like, you can double the fun by cramming two 3-volt CR2032 batteries into the battery clip (just be sure to stack them so their positive sides are up). The Jitterbug will shimmy and buzz until it stumbles into soothing darkness again, or until you cut the light.