If you like running your car sound system hard and long when you’re parked,
this project is for you. It’ll save you the embarrassment of having to beg some
jumper leads when you find out that you ran your system just a little too long... Or if you’re running a small
race-style battery – either for space or weight reasons – you might’ve found
that the capacity isn’t as great as you’re used to. Again, this project’s for
you. And hey, if you just like trick-looking digital displays in your car, we’re
sure you can find a space for this one...
So what is it then? It’s a four-digit LED battery voltage meter that looks
great. Plus it’s easy to put together and won’t break the bank. There are a few
electronic components to assemble but 95 per cent of the hard work’s already
been done for you in the shape of the off-the-shelf LED digital panel meter.
The meter is Jaycar cat no QP-5580. It costs just AUD$26.95 and boasts those
four digits, a super-bright and very accurate 14.2mm high LED display, and a
configurable decimal point position. Unlike some meters, this one is also happy
measuring the voltage being used to run the meter – when asked to do this it
won’t have kittens, as many meters in this situation are prone to do.
The meter needs a regulated 5-volt power supply and reads to a max of 200mV
(ie 0.2 volts). Now these two specs might look kind of daunting – since you want
to power it from the running car voltage of 13.8V and you want to be able to
measure up to that voltage as well – but they’re both easily fixed.
First up, the power supply.
We’ve deliberately over-rated the power supply so that you can run multiple
meters from it. That means you can not only monitor battery voltage, but also
use extra LED displays to do other things (which we’ll cover another time). In
addition to supplying a regulated 5V, the power supply is designed to protect
the meter from the voltage spikes that can occur in car electrical systems. It
is largely the same as the one we used in our ‘TempScreen Part 4’
TempScreen: Part 4 - Building a Custom Temperature Display
article of Dec 7, 1999, which was also designed to run a digital
The 5 volts is provided by a 7805 voltage regulator (12 volts in, 5 volts
out) and this works with 10 and 100uF electrolytic capacitors, a 1W 18 volt
zener diode and a 10 ohm 1 watt resistor. This circuit shows how these
components go together. Note the pin-outs on the voltage regulator and the fact
that the capacitors and zener diode have a polarity (ie go into the circuit only
one way around). The capacitors have the negative lead marked on their bodies,
and the orientation of the diode is indicated by the band on its body which
corresponds with the band on the diagram. The voltage regulator will grow quite
warm with the load – you may want to place a small heatsink on it, especially if
you’re running multiple displays.
Build the circuit on a piece of punched board, physically laying it out just
as the circuit is drawn. This will reduce the chance of making mistakes. When
you’ve finished, apply 12V and earth and measure the output, which should be
very close to 5V.
You may notice an extra component on the board – a 100 kilo-ohm multi-turn
trimpot, which we’ll come to in a minute.
The display is easy to hook up. As the instruction sheet shows, there are ten
pins on the back of the LED meter. Pin 1 is marked.
Pin 1 connects to the +5V provided by your power supply and Pin 2 to earth.
To trigger the decimal point in the correct position, connect Pin 3 to Pin
A 100 kilo-ohm multi-turn trimpot is used to divide down the voltage to suit
the input range of the meter. One side of the pot is connected to car battery
voltage, and the other to earth. The centre pin (the wiper) connects to the
positive signal input of the meter, Pin 7. Pin 8, the signal ground, connects to
We mounted the display and power supply in a plastic box.
When you apply power and earth (both to the power supply and the meter’s
input via the pot), the LEDs should come alive but not yet show anything
meaningful. However, by turning the pot you should be able to bring up numbers
on the display. Use a digital multimeter to measure the input signal voltage and
then very carefully adjust the pot
until the two meters agree. Let the system run for 10-15 minutes and then again
check the relationship between the two meters. The pot may need a slight tweak
after the system has warmed up.
(Note that when building this project we initially had a LED meter that
proved faulty – it constantly drifted in measured value. However, a second meter
behaved perfectly – if the value of the meter keeps changing, return it for a
Easiest is to feed the same source of power to both the 5V power supply and
also the meter’s input. That is, the meter is powered from the voltage it is
However, in some cases you might want to do it differently. For example, if
you want to measure the real-time voltage at a car sound amp, power the 5V power
supply from an ignition-switched 12V supply and connect the meter’s signal
inputs to the amplifier’s constant 12V and ground. That way, you can measure the
actual voltage at the amplifier – but when the car is turned off, the LED
display won’t needlessly stay on.
LED Panel Meter – cat no QP-5580 - $26.95
100 kilo-ohm multi-turn trim-pot – cat no RT-4620 - $1.50
7805 1A 5V regulator – cat no ZV-1505 - $1.20
Heatsink – cat no HH-8502 - $1
16V 1W zener diode – ZR-1416 – 40c
100uF 16V electrolytic capacitor – cat no RE-6130 – 32c
10uF 16V electrolytic capacitor – cat no RE-6066 – 25c
10 ohm 1W resistor – cat no RR-2526 – 32c for two
Punched board – cat no HP-9562 - $4.10 for about ten times as much as you’ll
These parts and prices are from Jaycar Electronics
in Australian dollars.