Led display digital Voltmeter
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Copyright of this circuit belongs to smart
kit electronics. In this page we will use this circuit to
discuss for improvements and we will introduce some changes
based on original schematic.
This is an easy to build, but nevertheless very accurate and
useful digital voltmeter. It has been designed as a panel meter
and can be used in DC power supplies or anywhere else it is
necessary to have an accurate indication of the voltage present.
The circuit employs the ADC (Analogue to Digital Converter) I.C.
CL7107 made by INTERSIL. This IC incorporates in a 40 pin case all
the circuitry necessary to convert an analogue signal to digital
and can drive a series of four seven segment LED displays
directly. The circuits built into the IC are an analogue to
digital converter, a comparator, a clock, a decoder and a seven
segment LED display driver. The circuit as it is described here
can display any DC voltage in the range of 0-1999 Volts.
Technical Specifications - Characteristics
Supply Voltage: ............. +/- 5 V (Symmetrical)
Power requirements: ..... 200 mA (maximum)
Measuring range: .......... +/- 0-1,999 VDC in four ranges
Accuracy: ....................... 0.1 %
- Small size
- Easy construction
- Low cost.
- Simple adjustment.
- Easy to read from a distance.
- Few external components.
How it Works
In order to understand the principle of operation of the circuit
it is necessary to explain how the ADC IC works. This IC has the
following very important features:
- Great accuracy.
- It is not affected by noise.
- No need for a sample and hold circuit.
- It has a built-in clock.
- It has no need for high accuracy external components.
Schematic (fixed 16-11-09)
7-segment display pinout MAN6960
An Analogue to Digital Converter, (ADC from now on) is better
known as a dual slope converter or integrating converter. This
type of converter is generally preferred over other types as it
offers accuracy, simplicity in design and a relative indifference
to noise which makes it very reliable. The operation of the
circuit is better understood if it is described in two stages.
During the first stage and for a given period the input voltage is
integrated, and in the output of the integrator at the end of this
period, there is a voltage which is directly proportional to the
input voltage. At the end of the preset period the integrator is
fed with an internal reference voltage and the output of the
circuit is gradually reduced until it reaches the level of the
zero reference voltage. This second phase is known as the negative
slope period and its duration depends on the output of the
integrator in the first period. As the duration of the first
operation is fixed and the length of the second is variable it is
possible to compare the two and this way the input voltage is in
fact compared to the internal reference voltage and the result is
coded and is send to the display.
All this sounds quite easy but it is in fact a series of very
complex operations which are all made by the ADC IC with the help
of a few external components which are used to configure the
circuit for the job. In detail the circuit works as follows. The
voltage to be measured is applied across points 1 and 2 of the
circuit and through the circuit R3, R4 and C4 is finally applied
to pins 30 and 31 of the IC. These are the input of the IC as you
can see from its diagram. (IN HIGH & IN LOW respectively). The
resistor R1 together with C1 are used to set the frequency of the
internal oscillator (clock) which is set at about 48 Hz. At this
clock rate there are about three different readings per second.
The capacitor C2 which is connected between pins 33 and 34 of the
IC has been selected to compensate for the error caused by the
internal reference voltage and also keeps the display steady.
The capacitor C3 and the resistor R5 are together the circuit that
does the integration of the input voltage and at the same time
prevent any division of the input voltage making the circuit
faster and more reliable as the possibility of error is greatly
reduced. The capacitor C5 forces the instrument to display zero
when there is no voltage at its input. The resistor R2 together
with P1 are used to adjust the instrument during set-up so that it
displays zero when the input is zero. The resistor R6 controls the
current that is allowed to flow through the displays so that there
is sufficient brightness with out damaging them. The IC as we have
already mentioned above is capable to drive four common anode LED
displays. The three rightmost displays are connected so that
they can display all the numbers from 0 to 9 while the first from
the left can only display the number 1 and when the voltage is
negative the «-« sign. The whole circuit operates from a
symmetrical ń 5 VDC supply which is applied at pins 1 (+5 V), 21
(0 V) and 26 (-5 V) of the IC.
First of all let us consider a few basics in building electronic
circuits on a printed circuit board. The board is made of a thin
insulating material clad with a thin layer of conductive copper
that is shaped in such a way as to form the necessary conductors
between the various components of the circuit. The use of a
properly designed printed circuit board is very desirable as it
speeds construction up considerably and reduces the possibility of
making errors. To protect the board during storage from
oxidation and assure it gets to you in perfect condition the
copper is tinned during manufacturing and covered with a special
varnish that protects it from getting oxidised and also makes
Soldering the components to the board is the only way to build
your circuit and from the way you do it depends greatly your
success or failure. This work is not very difficult and if you
stick to a few rules you should have no problems. The soldering
iron that you use must be light and its power should not exceed
the 25 Watts. The tip should be fine and must be kept clean at all
times. For this purpose come very handy specially made sponges
that are kept wet and from time to time you can wipe the hot tip
on them to remove all the residues that tend to accumulate on it.
DO NOT file or sandpaper a dirty or worn out tip. If the tip
cannot be cleaned, replace it. There are many different types of
solder in the market and you should choose a good quality one that
contains the necessary flux in its core, to assure a perfect joint
DO NOT use soldering flux apart from that which is already
included in your solder. Too much flux can cause many problems and
is one of the main causes of circuit malfunction. If nevertheless
you have to use extra flux, as it is the case when you have to tin
copper wires, clean it very thoroughly after you finish your work.
In order to solder a component correctly you should do the
- Clean the component leads with a small piece of emery paper.
- Bend them at the correct distance from the component’s body and
insert the component in its place on the board.
- You may find sometimes a component with heavier gauge leads than
usual, that are too thick to enter in the holes of the p.c. board.
In this case use a mini drill to enlarge the holes slightly. Do
not make the holes too large as this is going to make soldering
PCB dimensions: 77,6mm x 44,18mm
or scale it at 35%
- Take the hot iron and place its tip on the component lead while
holding the end of the solder wire at the point where the lead
emerges from the board. The iron tip must touch the lead slightly
above the p.c. board.
- When the solder starts to melt and flow wait till it covers
evenly the area around the hole and the flux boils and gets out
from underneath the solder. The whole operation should not take
more than 5 seconds. Remove the iron and allow the solder to cool
naturally without blowing on it or moving the component. If
everything was done properly the surface of the joint must have a
bright metallic finish and its edges should be smoothly ended on
the component lead and the board track. If the solder looks dull,
cracked, or has the shape of a blob then you have made a dry joint
and you should remove the solder (with a pump, or a solder wick)
and redo it.
- Take care not to overheat the tracks as it is very easy to lift
them from the board and break them.
- When you are soldering a sensitive component it is good practice
to hold the lead from the component side of the board with a pair
of long-nose pliers to divert any heat that could possibly damage
- Make sure that you do not use more solder than it is necessary
as you are running the risk of short-circuiting adjacent tracks on
the board, especially if they are very close together.
- When you finish your work, cut off the excess of the component
leads and clean the board thoroughly with a suitable solvent to
remove all flux residues that may still remain on it.
As it is recommended start working by identifying the components
and separating them in groups. There are two points in the
construction of this project that you should observe:
First of all the display IC’s are placed from the copper side of
the board and second the jumper connection which is marked by a
dashed line on the component side at the same place where the
displays are located is not a single jumper but it should be
changed according to the use of the instrument. This jumper is
used to control the decimal point of the display.
If you are going to use the instrument for only one range you can
make the jumper connection between the rightmost hole on the board
and the one corresponding to the desired position for the decimal
point for your particular application. If you are planning to use
the voltmeter in different ranges you should use a single pole
three position switch to shift the decimal point to the correct
place for the range of measurement selected. (This switch could
preferably be combined with the switch that is used to actually
change the sensitivity of the instrument).
Apart from this consideration, and the fact that the small size of
the board and the great number of joints on it which calls for a
very fine tipped soldering iron, the construction of the project
is very straightforward.
Insert the IC socket and solder it in place, solder the pins,
continue with the resistors the capacitors and the multi-turn
trimmer P1. Turn the board over and very carefully solder the
display IC’s from the copper side of the board. Remember to
inspect the joints of the base of the IC as one row will be
covered by the displays and will be impossible to see any mistake
that you may have made after you have soldered the displays into
The value of R3 controls in fact the range of measurement of the
voltmeter and if you provide for some means to switch different
resistors in its place you can use the instrument over a range of
For the replacement resistors follow the table below:
0 - 2 V ............ R3 = 0 ohm 1%
0 - 20 V ........... R3 = 1.2 Kohm 1%
0 - 200 V .......... R3 = 12 Kohm 1%
0 - 2000 V ......... R3 = 120 Kohm 1%
When you have finished all the soldering on the board and you are
sure that everything is OK you can insert the IC in its place. The
IC is CMOS and is very sensitive to static electricity. It comes
wrapped in aluminium foil to protect it from static discharges and
it should be handled with great care to avoid damaging it. Try to
avoid touching its pins with your hands and keep the circuit and
your body at ground potential when you insert it in its place.
Connect the circuit to a suitable power supply ń 5 VDC and turn
the supply on. The displays should light immediately and should
form a number. Short circuit the input (0 V) and adjust the
trimmer P1 until the display indicates exactly «0».
|R1 = 180k
||P1 = 20k trimmer multi turn
|R2 = 22k
||U1 = ICL 7107
|R3 = 12k
||LD1,2,3,4 = MAN 6960 common
anode led displays
|R4 = 1M
|R5 = 470k
|R6 = 560 Ohm
|C1 = 100pF
|C2, C6, C7 = 100nF
|C3 = 47nF
|C4 = 10nF
|C5 = 220nF
If it does not work
Check your work for possible dry joints, bridges across adjacent
tracks or soldering flux residues that usually cause problems.
Check again all the external connections to and from the circuit
to see if there is a mistake there.
- See that there are no components missing or inserted in the
- Make sure that all the polarised components have been soldered
the right way round. - Make sure the supply has the correct
voltage and is connected the right way round to your circuit.
- Check your project for faulty or damaged components.
Sample Power supply 1
Check construction details on Dimitris site