Hardware Hacking 89when the LED is on -- if so, you’ve got a simple solution to automating some of
the toy’s functions; if not, try another switch or another toy. Don’t use the
photocell as a substitute for the on/off switch of a circuit or between the circuit
and its speaker, since it can’t pass enough current.
Channel Surfing Music
In Chapter 3 we used coils to pick up the electromagnetic signals given off by
various appliances and electronic devices. We can also eavesdrop on light
signals of various kinds by using a specialized type of photocell. The
“phototransistor” is the heart of any infra-red remote control receiver circuit,
such as that in your TV. It detects the pulses of infra-red light sent by your
remote control and converts them into a stream of on/offs, that are in turn
translated into digital data by the microprocessor in the TV.
Aim a remote control at the simple circuit below (keep it close) and you should
hear pulse trains as you press the buttons (remember to connect the shield of
your jack to the circuit ground.) If not, reverse which leg of the phototransistor
connects to +9 volts and which connects to the load resistor. The differences
between one button and another may sound pretty subtle, even though the
encoded data is different. Try different remotes -- the fundamental frequency
and basic timbre may differ from one to another, but it’s a minor difference, since
they’re all sending similar pulse trains.
Since the lights on many electronic circuits look steady but are in fact “scanned”
by the central processor unit, you can use this circuit to extract sound from
almost any device with LEDs. Try it on bicycle flashers, toys with blinking lights,
the front panels of studio gear, TV screens, computer monitors.
You can substitute an ordinary photocell for the phototransistor; you may need
to increase the size of the load resistor from 2.2kOhm to 10kOhm or larger.
Because photocells are sensitive to light across the spectrum (not just infra-red)
you will get much more interference from the power grid AC frequency present
in incandescent and fluorescent lighting (60hz in America, 50hz in Europe,) but