This article was first published in 2008.
Recently I needed to monitor the operation of an
Exhaust Gas Recirculation Valve. I could have used a digital multimeter (and did
in fact do this for the initial testing) but seeing what the valve was doing
proved to be much easier with an analog meter.
Even better, for someone who is prepared to
salvage old discards, such a meter is available at zero cost.
Furthermore, the meter can be easily set up to
monitor not just EGR valves, but anything that’s changing in voltage. It can be
used temporarily during testing or even built into a panel for permanent
Finally, it’s also easy to illuminate for night
Monitoring the Pulsed EGR
In the case of the car I was working on, the EGR
system valve is electronically controlled by the ECU. It uses what is called a
variable pulse width signal. That is, it is switched on and off very fast (108
times a second in this case) but the proportion of ‘on’ to ‘off’ time is varied.
If it is switched on all the time, the duty cycle is said to be 100 per cent. If
it is switched on for half the time, the duty cycle is 50 per cent. The lower
the duty cycle, the less the valve is open.
The very fast pulsing frequency of 108Hz means
that in fact the valve doesn’t fully open and shut each cycle – it hovers. In
fact, if you put a digital multimeter across the valve, you can see that at 50
per cent duty cycle, the meter is seeing about half battery voltage. At 25 per
cent duty cycle, the meter reads about one-quarter battery voltage. Because the
valve is being pulsed so quickly, the meter reads the average voltage
that the valve is seeing.
During on-road testing I was using a good quality
multimeter to directly read duty cycle and voltage. However, once I got a feel
for the readings, on the road this measuring system was rather awkward. The
meter couldn’t be placed in my line of sight, so I had to keep looking down at
the passenger seat or centre console. The digital numbers also had to be
actually read, a process that again took my eyes away from the road for longer
However, by using a much more compact analog
meter, the readout could be placed on the dash and the opening of the EGR valve
seen at the merest glance. That was especially useful when in most of the
testing, just a rough idea of the EGR valve opening was needed – ie, was the
valve fully open, half open, a quarter open or closed?
Monitoring Other Voltages
As you’ll see in a moment, these meters can also
be used to monitor other voltages, like battery voltage. For example, if you
have a big sound system, the meter can be built into a panel near the amplifier,
so monitoring the voltage actually reaching it (and therefore taking into
account real-time voltage drop in the cables).
However, it’s important that you realise that
these meters have low input impedance, that is, they will load down circuits
that don’t have the capacity to supply a reasonable amount of current. This
means they’re OK with battery voltage, or monitoring voltages running solenoids,
but you could not use these meters to monitor oxygen sensor voltage (instead, if
you want an analog meter to do this, see
AutoSpeed Needle Mixture Meter - Part 1).
OK so what are these meters we keep talking about?
They’re available at zero cost if you pick up an old discarded cassette deck
that has analog VU meters.
In addition, you’ll need a 100 kilo-ohm multi-turn
trimpot, used as a variable resistor to calibrate the meter for its new use.
These can be salvaged from old equipment, or bought new – eg Jaycar Electronics
RT-4620 at $1.50.
Old cassette decks with large illuminated VU
meters turn up all the time in garage sales, at the tip, and at hard rubbish
kerbside collections. It is extremely rare for the VU meters to be dead, so you
can be fairly safe in collecting any old cassette deck for this purpose. About
the only thing to look out for is that it’s preferable to use a deck that has
two separate meters, rather than one that has them built into a single
VU meters are moving coil voltmeters that have a
very high sensitivity. This means that it takes little voltage to move the
needle across the full scale – typically, just 0.3 – 0.4V.
If you need to measure a voltage that rises only
to about 0.4V, all you need do is connect the meter straight across the supply.
However, it’s much more likely that you’ll want to measure a peak voltage of 5
or 12V. However, it’s very easy to decrease the sensitivity of the meter – just
wire a variable resistor in series with the meter, as shown here.
At its simplest, that’s all you need to do – just
wire the meter to the pot, and then wire the circuit to whatever you want to
measure. The circuit you’re measuring provides the power to run the meter, so
it’s very simple to make it all happen. Just adjust the pot until the highest
signal that you are measuring moves the needle across to full scale.
But what if you want to calibrate the system?
That’s easy too.
Most VU meters use a non-linear movement – that
is, the needle moves less for a given voltage input at the top end of the scale.
So as you go around the scale, you can either make your markings closer
together, or keep the increments spaced the same distance apart and jump further
between numbers. We chose to do the latter.
You can use your PC and printer to make the new
scale. While at first it seems as if it would make most sense to scan in the
original scale and modify it, in practice it’s best to start with the scale from
another instrument. The meters shown here use a scale that was originally
scanned from an old speedo.
Once you have the arc and the increments, you can
use the software to delete the numbers and other markings you don’t want.
Measure the width of the scale on the meter and then size the scale to match
this, at this stage printing it out without any numbers on it.
Carefully remove the original scale (most meters
are held together with just sticky tape with the scale glued into place) and
then temporarily place your un-numbered scale behind the pointer.
To provide a variable calibration voltage, take
the approach shown here, where a 10 kilo-ohm pot is wired across a battery.
(This diagram shows how you’d wire a conventional, cheap pot.) By turning the
pot you can gain an output that varies from 0 to the maximum voltage provided by
the battery. (If you have an adjustable output power supply, use that instead.)
Use a multimeter to measure the actual voltage
being fed to the VU meter.
So how do you perform the calibration? Firstly,
adjust the meter’s pot so that the most commonly read maximum voltage is towards
the end of the meter’s scale. After you’ve set this, don’t touch this pot again.
Next, alter the input voltage, measure it with the
multimeter, and work out what each of the other graduations on the modified VU
meter scale show. Round off the voltages to the nearest whole volt. For example,
on one of the meters shown here, the scale goes: 4, 7, 11, 14, 18, 25, 34
When the scale markings are worked out, use the
image manipulation software to put the correct numbers on the scale, along with
any other writing you want. Then print it out on gloss paper, cut it to shape
and stick it in place.
You can of course put whatever markings you want
on your scale. For example, if you are monitoring the car battery, the markings
could be simply ‘normal’ ‘high’ and ‘low’.
the meter needs to be used at night, it’s easy to illuminate the scale – the
cassette deck probably had a suitable bulb in it, although check it matches 12V.
Otherwise, use a LED (with dropping resistor) or any small 12V light bulb.
Next time you see a dumped cassette deck with
mechanical VU meters, grab it! Storing the salvaged meters take up little space
and they can be very useful, either as permanent instruments or for short-term