Hardware Hacking - Nicolas Collins

Hardware Hacking 83

Chapter 22: On/Off (more fun with photocells)

You will need:

• Two things to turn on & off, such as a radio, cassette or CD player with
  an audio output jack.


• Some photocells.


• A flashlight (torch.)


• A basic oscillator circuit from one of the previous chapters, on a
  breadboard.


• Some LEDs (Light Emitting Diodes.)


• A phototransistor.


• Some heat shrink tubing (optional.)


• Assorted resistors, capacitors.


• Some solid hookup wire.


• Some plugs and jacks.


• Clip leads and Y-cords.


• A 9 volt battery and connector.


• An amplifier or two.


• Hand tools.


• An infra-red remote control from a TV or other appliance.

As we have seen in our earlier experiments with toy clocks and simple
oscillators, the photocell changes resistance in response to changes in light level;
this change is resistance in turn affects the speed of a clock or pitch of an
oscillator. A photocell can also be used as a kind of gate to pass or block any
audio source.

Flashlights

Breadboard or clip together the simple circuit shown below:

photocell

Connect the “hot” of any audio signal (such as the output of a portable cassette
or CD player) to the input of the circuit; connect the output of the circuit to the
“hot” of your amplifier input. Connect the shield/ground of the audio source to
the shield/ground of the amplifier input (Rule 10.) Turn on the amp, play the