Tab Electronics Guide to Understanding Electricity ... - Sciences Club

Special-Purpose Diodes and Optoelectronic Devices
221
of making the sound omnidirectional, so it will be difficult to locate. In
the meantime, everyone who is a little squeamish toward mice or large
insects will get seriously nervous.
It is important to use a very small speaker for this project to achieve
the desired effect. Virtually any type of general-purpose NPN transistors
will perform well.
If you would like to try a variation on this circuit to produce some
really weird sounds, try replacing Q1 and Q3 with a couple of three-lead
phototransistors. Connect the phototransistors into the circuit exactly
like the original transistors. This causes the changes in ambient light to
“sum” with the original base voltages. Various capacitor and resistor combinations
will produce some remarkable sounds in conjunction with
changing light intensities.
To carry this idea one step further, you can mount this circuit in
the center of a bull’s-eye target and convert a laser pointer into a “gun”
(put a dummy handle on it, and fabricate the on-off switch into a trigger).
Using various component values, the target can be made to produce
any number of strange sounds, when the laser beam hits the
bull’s-eye. Including a small power amplifier into the circuit, to boost
the output volume, will improve the effect. If you built multiple circuits,
adjusted them for individual sound effects, and mounted them
in a variety of targets, you could have a high-tech shooting gallery in
your own home!
A Sound Improvement
Figure 7-8 is a Hi-Z (high-impedance) audio amplifier circuit that will
greatly increase the volume level of a high-impedance headphone (two
of these circuits will be needed for stereo headphones). This circuit can
come in handy if you want to use your headphones to listen (loudly!) to
some of the sounds that you can create with these multivibrator circuits.
The input impedance is high enough to keep it from loading down
most circuits. You can also use this circuit with most types of speakers
that have an impedance-matching transformer connected to the speaker
frame. Don’t try to use a standard 8- or 4-ohm speaker; you’ll destroy the
transistor, or the speaker, or both!
This circuit is a modified form of the common-emitter transistor
amplifier discussed in Chapter 6. VR2 should be adjusted for the best
quality of sound, and VR1 is the volume control.