86 Nicolas Collins
Breadboard the circuit shown below. This is the simple oscillator from Chapter
18 (you could also use the 4093-based design in Chapter 20,) but now we are
connecting it to an LED instead of an amplifier; it blinks the light instead of
clicking the speaker. We want it to blink at an observable rate, so we use a
reasonably large capacitor.
Vary the speed and watch the effect. Fun enough just to look at, but wait -- it
gets better!
Now take the LED and hold it against the photocell as shown below. Spread the
leads of the photocell and LED apart so they do not touch each other, then wrap
the photocell and LED in electrical tape so that they are sealed from outside light.
Be very careful not to let any of the wires touch each other or the circuit will not
work -- you can wrap each of the four wires separately if this helps.
Now run audio through the photocell while you vary the speed of the oscillator -
- you should hear the cassette/CD get chopped on and off as the LED blinks. As
you speed up the oscillator into the audio range the on/off rhythm is replaced by
a kind of a distorted, buzzing, sound. Experiment with different size capacitors
until you find a good range of speeds.
If you want to see what’s happening as well as hear it you can add a second LED
as an indicator light (and we all love blinking lights,) as shown below