Sensitive audio detector
PARTS AND MATERIALS
- High-quality "closed-cup" audio headphones
- Headphone jack: female receptacle for headphone plug (Radio Shack catalog # 274-312)
- Small step-down power transformer (Radio Shack catalog # 273-1365 or equivalent, using the 6-volt secondary winding tap)
- Two 1N4001 rectifying diodes (Radio Shack catalog # 276-1101)
- 1 kΩ resistor
- 100 kΩ potentiometer (Radio Shack catalog # 271-092)
- Two "banana" jack style binding posts, or other terminal hardware,
for connection to potentiometer circuit (Radio Shack catalog # 274-662
or equivalent)
- Plastic or metal mounting box
Regarding the headphones, the higher the "sensitivity" rating in
decibels (dB), the better, but listening is believing: if you're serious
about building a detector with maximum sensitivity for small electrical
signals, you should try a few different headphone models at a
high-quality audio store and "listen" for which ones produce an audible
sound for the
lowest volume setting on a radio or CD player.
Beware, as you could spend hundreds of dollars on a pair of headphones
to get the absolute best sensitivity! Take heart, though: I've used an
old pair of Radio Shack "Realistic" brand headphones with perfectly adequate results, so you don't need to buy the best.
Normally, the transformer used in this type of application (audio
speaker impedance matching) is called an "audio transformer," with its
primary and secondary windings represented by impedance values (1000 Ω :
8 Ω) instead of voltages. An audio transformer will work, but I've
found small step-down power transformers of 120/6 volt ratio to be
perfectly adequate for the task, cheaper (especially when taken from an
old thrift-store alarm clock radio), and far more rugged.
The tolerance (precision) rating for the 1 kΩ resistor is irrelevant.
The 100 kΩ potentiometer is a recommended option for incorporation into
this project, as it gives the user control over the loudness for any
given signal. Even though an
audio-taper potentiometer would be appropriate for this application, it is not necessary. A
linear-taper potentiometer works quite well.
CROSS-REFERENCES
Lessons In Electric Circuits, Volume 1, chapter 8: "DC Metering Circuits"
Lessons In Electric Circuits, Volume 2, chapter 9: "Transformers"
Lessons In Electric Circuits, Volume 2, chapter 12: "AC Metering Circuits"
LEARNING OBJECTIVES
- Soldering practice
- Use of a transformer for impedance matching
- Detection of extremely small electrical signals
- Using diodes to "clip" voltage at some maximum level
SCHEMATIC DIAGRAM
ILLUSTRATION
INSTRUCTIONS
This experiment is identical in construction to the "Sensitive Voltage
Detector" described in the DC experiments chapter. If you've already
built this detector, you may skip this experiment.
The headphones, most likely being stereo units (separate left and right
speakers) will have a three-contact plug. You will be connecting to
only two of those three contact points. If you only have a "mono"
headphone set with a two-contact plug, just connect to those two contact
points. You may either connect the two stereo speakers in series or in
parallel. I've found the series connection to work best, that is, to
produce the most sound from a small signal:
Solder all wire connections well. This detector system is extremely
sensitive, and any loose wire connections in the circuit will add
unwanted noise to the sounds produced by the measured voltage signal.
The two diodes connected in parallel with the transformer's primary
winding, along with the series-connected 1 kΩ resistor, work together to
"clip" the input voltage to a maximum of about 0.7 volts. This does
one thing and one thing only: limit the amount of sound the headphones
can produce. The system will work without the diodes and resistor in
place, but there will be no limit to sound volume in the circuit, and
the resulting sound caused by accidentally connecting the test leads
across a substantial voltage source (like a battery) can be deafening!
Binding posts provide points of connection for a pair of test probes
with banana-style plugs, once the detector components are mounted inside
a box. You may use ordinary multimeter probes, or make your own probes
with alligator clips at the ends for secure connection to a circuit.
Detectors are intended to be used for balancing bridge measurement
circuits, potentiometric (null-balance) voltmeter circuits, and detect
extremely low-amplitude AC ("alternating current") signals in the audio
frequency range. It is a valuable piece of test equipment, especially
for the low-budget experimenter without an oscilloscope. It is also
valuable in that it allows you to use a different bodily sense in
interpreting the behavior of a circuit.
For connection across any non-trivial source of voltage (1 volt and
greater), the detector's extremely high sensitivity should be
attenuated. This may be accomplished by connecting a voltage divider to
the "front" of the circuit:
SCHEMATIC DIAGRAM
ILLUSTRATION
Adjust the 100 kΩ voltage divider potentiometer to about mid-range when
initially sensing a voltage signal of unknown magnitude. If the sound
is too loud, turn the potentiometer down and try again. If too soft,
turn it up and try again. This detector even senses DC and
radio-frequency signals (frequencies below and above the audio range,
respectively), a "click" being heard whenever the test leads make or
break contact with the source under test. With my cheap headphones,
I've been able to detect currents of less than 1/10 of a microamp (<
0.1 µA) DC, and similarly low-magnitude RF signals up to 2 MHz.
A good demonstration of the detector's sensitivity is to touch both test
leads to the end of your tongue, with the sensitivity adjustment set to
maximum. The voltage produced by metal-to-electrolyte contact (called
galvanic voltage)
is very small, but enough to produce soft "clicking" sounds every time
the leads make and break contact on the wet skin of your tongue.
Try unplugging the headphone plug from the jack (receptacle) and
similarly touching it to the end of your tongue. You should still hear
soft clicking sounds, but they will be much smaller in amplitude.
Headphone speakers are "low impedance" devices: they require low voltage
and "high" current to deliver substantial sound power. Impedance is a
measure of opposition to any and all forms of electric current,
including alternating current (AC). Resistance, by comparison, is a
strictly measure of opposition to
direct current (DC). Like
resistance, impedance is measured in the unit of the Ohm (Ω), but it is
symbolized in equations by the capital letter "Z" rather than the
capital letter "R". We use the term "impedance" to describe the
headphone's opposition to current because it is primarily AC signals
that headphones are normally subjected to, not DC.
Most small signal sources have high internal impedances, some much
higher than the nominal 8 Ω of the headphone speakers. This is a
technical way of saying that they are incapable of supplying substantial
amounts of current. As the Maximum Power Transfer Theorem predicts,
maximum sound power will be delivered by the headphone speakers when
their impedance is "matched" to the impedance of the voltage source.
The transformer does this. The transformer also helps aid the detection
of small DC signals by producing inductive "kickback" every time the
test lead circuit is broken, thus "amplifying" the signal by
magnetically storing up electrical energy and suddenly releasing it to
the headphone speakers.
As with the low-voltage AC power supply experiment, I recommend building
this detector in a permanent fashion (mounting all components inside of
a box, and providing nice test lead wires) so it can be easily used in
the future. Constructed as such, it might look something like this:
