For Technology and Science Teachers:
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Denis Burchill
Index:
Circuit diagrams Investigating light sources Veroboard layout diagrams Component list
Circuit Diagrams:
These relatively simple circuits
can be used to transmit information across a small distance. The
"information" I normally use is music from a radio, cassette tape or CD
player - you could also try a microphone and amplifier.
People transmit information all the
time when they use remote controls for TVs, VCRs, stereo systems, etc.,
but they usually do not think much about the transmission as a real
event.
Here, the medium of transmission
used is a fluctuating beam of red light which can be seen and
interrupted. The fact that music or speech is being transmitted and
received adds to the impact on students.
This system gives surprisingly good
clarity of sound. If you wish to try distances greater than 50 cm
between transmitter and receiver then you will need to focus the light
from the transmitter's red LED.
Using a small lens from a
disposable camera works well. I mount it on a small wire frame. The
frame can be bent to position the lens a focal length away from the
transmitter LED. It would also help to put another
lens at the receiver end. You could
also experiment with fibre optic cable (the plastic demonstration type
- 1 mm in diameter) placed between transmitter and receiver. Another
possibility to investigate is how well the unit works if the
transmitted red light is reflected off mist or smoke or dust in the air.
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Investigating light sources:
The receiver can also be used on
its own as a detector for investigating other sources of light and
infra red. You could show visual waveforms of the light being
investigated by connecting oscilloscope inputs across the
phototransistor or across the output of the LM386 with the speaker
disconnected.
Interesting sources to investigate are:
- infra red remote controls (How is each key on the remote encoded?)
- incandescent bulbs, fluorescent tubes, new electronic fluorescent lamps
- TV and computer monitor screens
- candle flames
- displays such as VCRs and bedside clock radios
- Infra red from auto focus cameras (How accurate is the focusing optics?)
- the oscilloscope trace that monitors the phototransistor !!!
(This forms a feedback loop and shows the principle of the "light pen"
used on some computer screens.)
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Veroboard layouts:
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Component list:
Transmitter:
- 1 x 100R
- 1 x 220R 1/4 watt resistors
- 2 x 1K0
- 1 x 10K skeleton potentiometer
- 1 x 1µ0 capacitor (electrolytic or mylar)
- 1 x BC548 transistor (or equivalent)
- 1 x 5mm diameter high brightness clear red LED (Dick Smith Z-4021)
- 1 x single pole, single throw switch (SPST)
- 1 x 9 volt battery and battery clip
- 1 x 3.5 mm jack socket
- 1 x connecting lead with 3.5 mm male jack plugs at either end
- vero board, connecting wire, case, etc.
Receiver:
- 1 x 330R
- 1 x 1K0 1/4 watt resistors
- 1 x 4K7 skeleton potentiometer
- 1 x 10K potentiometer
- 1 x 1µ0 capacitor (electrolytic or mylar)
- 1 x 10µ capacitor (electrolytic)
- 2 x 100µ capacitors (electrolytic)
- 1 x LM386 audio amplifier IC
- 1 x phototransistor
- 1 x 5mm diameter red LED
- 1 x single pole, single throw switch (SPST)
- 1 x 9 volt battery and battery clip
- 1 x 8 ohm loudspeaker
- 1 x 8 pin IC socket (for LM386)
- 1 x knob (for 10K potentiometer)
- vero board, connecting wire, case, etc.
Note: All components can be
purchased from Dick Smith Electronics with the exception of the
phototransistor which is sold by Jaycar (catalogue number ZD1950)
I'm happy to correspond with anyone over modifications, improvements or further uses of these circuits.
Denis Burchill, email: d.burchill@clear.net.nz
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This page: created 27 November 2006, modified 25 September 2007.
