Analog-Resolution Digital Multimeter Features Audible Accessory
What is wrong with every digital multimeter in the world? Basically, the fact that it’s digital!
Mar 9, 2015
Dr. Sam Green
Digital multimeters are precise and accurate instruments, with some
models being very expensive. No matter the cost, one problem pervades
every one of these devices: They do a poor job of conveying the analog
behavior of an input signal.
To perform an operation where you want to maximize or minimize some
parameter when making an adjustment, you must visually monitor the
reading closely and really study the numerical display to determine
whether the rapidly changing digits are increasing or decreasing, or
varying randomly over a small or large range of values. This process
requires both your eyes and your concentration, while still attending to
the process of performing the adjustment and perhaps holding a test
probe steady in the correct position.
high-end digital multimeters (DMMs) come with a crude bar graph on the
liquid-crystal display to convey analog information. However, these are
generally useless and make one yearn for an old analog moving-needle
Instead of a bar graph or a moving needle, my solution produces a
sound whose audible frequency (pitch) changes along with the magnitude
of the digital display. As the displayed magnitude increases, the
audible frequency increases; and as the displayed magnitude decreases,
the audible frequency decreases. This turns out to be an incredibly
effective way to convey precise analog behavior, as good as or better
than watching an analog-meter needle, and you don’t have to watch the
Audible-Tone Frequency Represents Magnitude
human ear is able to resolve small changes in pitch (frequency). This
arrangement provides better resolution than an analog-meter needle, and
you can look at what you’re probing and adjusting while listening for
the resultant parameter changes. This method yields eyes-off
measurements. You can maximize, minimize, or make relative changes
without looking at the instrument!
the prototype that I built in 2008, I used an LM331 precision
voltage-to-frequency converter (VFC) to produce an audible tone
proportional to the voltage across the digital meter within the DMM. The
DMM conveniently serves to convert all inputs, whether voltage or
current or resistance or any other parameter, to a voltage in the range
of ±199.9 mV or sometimes ±399.9 mV, depending on the particular unit.
1. Shown is an illustration from the patent for the digital-multimeter prototype
the signal from the input to the internal digital meter, amplify it,
take the absolute value, apply it to the VFC, convert the low duty-cycle
pulse output to a high duty-cycle signal, and apply it to a loudspeaker
to generate a suitable audible tone. Figure 1 illustrates the concept,
while Figure 2 shows the schematic diagram of one successful early
The audible frequency will vary with the magnitude of the voltage to the meter within the DMM (Fig. 3).
So far, this circuit performs like an audible bar graph with very high
resolution. With an absolute-value converter between the input buffer
amplifier and the VFC, it ignores signal polarity. Just like a DMM with
an analog bar graph, it gives the same indication for positive and
3. The plot indicates audio frequency versus voltage applied to the meter within the DMM.
the magnitude of a measured parameter as audible pitch isn’t a new
idea. I used this method of presenting signal values as varying audible
frequencies in previous inventions1 and an article in QEX.2
I still have some hardware I used around 1988. It actually has the
earphone in a rubber cup from a 300-baud modem to couple the audible
output signal into a plain-old-telephone handset to convey measurements
over a telephone connection to me at a remote location, where I made
adjustments to a free-space optical data link that affected the
Audible Polarity-Indicating Feature
make this device more usable, I set out to make positive polarity and
negative polarity signals sound different with the polarity sensor and
the waveform changer. I initially went between a sine wave and a square
wave, but the difference was more pronounced when I used tremolo, a
low-frequency amplitude variation, to denote negative signals. The
amplitude envelope of the audio pitch varies at low frequency when the
input polarity is negative to give negative inputs a very noticeable and
Bosenbecker, my friend and colleague, pointed out how annoying the
audible tone became, especially when it wasn’t changing between
adjustments. Ray suggested the addition of squelch when the input signal
was invariant for a while. At this point, he became a co-inventor.
the squelch was the most difficult feature to implement. Four op amps
serve to buffer and differentiate the signal at the input of the VFC and
sense sudden changes.
reading remains invariant for a while, the audible tone ceases. When the
reading begins to vary again, the tone resumes. This was touchy and
difficult to adjust in the prototype circuit with squelch (Fig. 4). It did function well enough to demonstrate the concept.
wanted to reduce this circuit to fit within a DMM, but suitable analog
equivalents of complex programmable logic devices (CPLDs) and
field-programmable gate arrays (FPGAs) never happened. Consequently, I
had only this complex and unsuitable analog circuit that was too large
to fit and required inconvenient power-supply voltage levels.
friend Lee Johnson encouraged me to attempt a digital version. I
resisted for a long time, though, since it seemed silly to digitize the
signal both in the DMM and my audio accessory.
I finally sat down to try a digital implementation, it proved to be
very simple and worked on the first try. The digital prototype
breadboard was an Arduino Uno R3 running an example program called
“tonePitchFollower,” which is equivalent to a VFC. The circuit still
needs op amps to buffer and amplify the input to a range that’s
acceptable for the unipolar analog-to-digital converter (ADC) on the
Arduino. Matching voltage ranges to frequency ranges, adding tremolo to
distinguish negative polarity, and implementing squelch are just
The first fully
functional digital prototype used an Arduino ProMini and readily fit
into the available space within a low-end Cen-Tech DMM from Harbor
Freight. The next used a full-size 28-pin Atmel ATmega328P chip from an
Arduino Uno R3, and it fit just as well. I recently got the 8-pin Atmel
ATtiny85 chip to work as well as the higher performing chips, so there’s
now plenty of room, and it requires less current.
5. Progressive implementation versions from the analog prototype through various Atmel processors.
5 shows three generations of prototypes with various Atmel processors.
The first is an Arduino Pro Mini, the second is an ATmega328P chip from
an Arduino Uno R3, and the third is an ATtiny85. An early version of the
simple program for the Arduino Uno demonstrates the method without
squelch (Fig. 6).
6. This simple program for Arduino Uno demonstrates the method without squelch.
Harbor Freight DMM takes power from a 9-V battery and delivers a
regulated reference potential 3 V below the positive battery potential.
The input signal to the internal digital meter varies ±200 mV with
respect to this reference potential. I sampled the reference and digital
meter input signal on two sides of a chip capacitor using blue and red
wires (Fig. 7, lower left). I connected to the switched +9-V
power with the red wire near the center and to the –9-V power with the
blue wire to the upper right. This power arrangement holds the secret to
hacking the Harbor Freight DMM to provide power to the bipolar op amps
and the Atmel processor.
7. Signal and power sample points.
8 shows the DMM voltage levels and the circuit diagram of this audible
accessory. The 79L05 negative voltage regulator derives a potential 5 V
below the +9-V battery (after the power switch). The op amps and the CPU
chip use this as the negative supply and the +9-V battery potential as
the positive supply. In this way, the DMM reference and DMM
output-signal potentials lie just about midway between, and in a fixed
relationship with, the op amp and CPU supplies.
ATtiny85 version draws less than 14 mA from the battery, including the
DMM when operating with a 150-Ω loudspeaker. The loudspeakers come from
old cordless telephones. I see them on the Web, but the supplier hasn’t
responded to an enquiry.
A Fluke 23
DMM I examined uses the same power-supply configuration as the Cen-Tech
DMM from Harbor Freight. I conclude that this hack applies generally,
and not just to Cen-Tech models. I recommend starting with Cen-Tech
models, because Harbor Freight offers coupon deals that give them away
for free from time to time.
Johnson suggested that it would be useful to have the audible DMM
indicate the approach to particular input-signal levels other than zero
for purposes of repetitive calibration in an industrial setting. I added
this feature to the software and solved my own calibration problem.
each unit required a slightly different constant in the software to
force the minimum audible frequency to occur exactly when the DMM
display indicates zero. Now, if pin 2 of the CPU is low at power-up, the
unit will self-calibrate to whatever input level is present and store
the calibration constant in the non-volatile EEPROM memory of the CPU.
Short the input, pull pin 2 low, and apply power to calibrate the unit
normally. I use an internal reed switch and an external magnet to pull
pin 2 low without drilling a hole in the case for a switch, but that
would work as well.
any positive or negative input level to the unit while measuring any
parameter on any range. Turn power off, pull pin 2 low, and reapply
power to calibrate the unit to indicate minimum frequency and zero
crossings at that specific input level. Depending on your meter, you
could do this with capacitance, resistance, and frequency as well as
voltage, current, and resistance.
you can pay attention to probe placement or the specifics of performing
your adjustment while simply listening and adjusting for minimum
audible frequency. This is how the instrument indicates whether the
input level approaches or departs from the parameter value you desire.
believe that this offset calibration feature is a new and possibly
patentable idea, but Lee and I instead chose to disclose it in this
article. You’re welcome. Name it after us.
whether a potentiometer is dirty (noisy) or makes good contact
throughout its range by setting up to measure resistance and listening
for dropouts while you rotate the shaft. They will be very noticeable.
You may also gain insight into whether the resistance variation is
linear or has a logarithmic taper.
using the accessory board with any ±199.9-mV or ±399.9-mV digital panel
meter that you build into your own applications. You will have to
change the power arrangement to provide a symmetrical or nearly
symmetrical power-supply range of about 5 V that extends both above and
below the reference input to your meter. Different meters will require
different solutions. Someone should eventually offer digital panels with
this functionality built in. This accessory provides a very versatile
audible continuity check to digital meters that lack this feature.
On a unit with the squelch feature, leave the instrument to monitor a
critical supply or bias voltage that shouldn’t vary. When it does, the
unit breaks squelch to provide an alarm.
do not plan to make and sell digital multimeters. However, to
popularize my method, I’m considering ways to provide this accessory as a
small assembled and operating circuit board that users can build into
their own DMMs. These units have sockets for the ATtiny85 CPU, so users
can tailor software to their own applications. Figure 9 shows a new
printed-circuit board that accommodates a reed switch for self
calibration when enabled with an external magnet.
9. The printed-circuit board accepts a reed switch for self-calibration
This accessory should become available from Citrus Electronics.3 Check the website for status. If there’s demand for the remaining boards, we will fabricate more of them.
can provide these accessory boards with software that generates an
audible tone whose frequency increases and decreases with the magnitude
of the DMM’s numerical display. The Boeing Company owns U.S. patent
8,803,560,4 which covers the audible polarity-indicating feature and the squelch feature, so I cannot provide those (see “Intellectual-Property Patent Restrictions” below).
what else you can do with this setup. I can think of several
improvements, and have already incorporated a couple into the available
version. My goal is to see DMMs with these features available to users
who have long complained about the lack of decent analog indication. I
hope this happens. I even hope Boeing makes some money.
Sam Green is a retired aerospace engineer who specialized in free space
optical and fiber-optic data communications and photonics. He holds
degrees in electronic engineering from Northwestern University and the
University of Illinois at Urbana. Listed as inventor on 18 patents, he
is a Registered Professional Engineer in Missouri and a life senior
member of IEEE. Contact Sam at email@example.com.
1. See patents 5,729,335 and 7,898,395.
2. “Fun with Voltage-to-Frequency Converters,” QEX, March/April 2013, pp. 7-10.
use of a voltage-to-frequency converter (VFC) to indicate magnitude
changes as proportional changes in audible frequency is not a new
concept. The voltage-to-audible-frequency principal is public property
by now, because that’s what VFCs do and because I did it so long ago.
addition of tremolo to distinguish between positive and negative
polarity is new, though. The use of squelch to silence the tone when it
ceases to change and to turn the tone on again when change resumes is
another new twist.
Company holds patent number 8,803,560 that covers these two features. I
describe the implementation of this invention here, because I would like
to see this method in wide use. Anyone interested in licensing the
features of this method in the U.S. must deal with my former employer.
owns the patent on the squelch and tremolo features, so I cannot offer
to sell units with tremolo or squelch. I can instead provide the
ATtiny85 CPU with alternative software that audibly indicates zero
crossings in place of indicating polarity, which proves remarkably
effective. You will have to interrupt power to make it quiet or program a
fixed operating time until shutoff, rather than sensing when the input
ceases to vary or again begins to vary.
that there is only a U.S. patent. Therefore, anyone can build and sell
units with the tremolo and squelch features outside the U.S. I do hope
one or two manufacturers will license this method so that they become
available here in the United States. Call it the Green Whistler or
something cool so I get some credit.