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Led display digital Voltmeter (ILC7107CPL)

2014-12-15 21:37  
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This article describes the Led display digital Voltmeter (ILC7107CPL). The principle is very simple, very practical. The circuit components can help you understand better grasp this principle. For example, in this circuit, you can go to find and buy these components: ILC7107CPL. 

image 2 th Led display digital Voltmeter
Front Side
General Description

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 th Led display digital Voltmeter

Schematic (fixed 22-2-04)


7seg Led display digital Voltmeter

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 tw1o
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 tw1o 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

image 3 th Led display digital Voltmeter

Back Side

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 betw1een
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 ρ 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 betw1een 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 easier.
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 every time.

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 following:

- Clean the component leads with a small piece of emery paper.

- Bend them at the correct distance from the components 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 difficult afterwards.

layout Led display digital Voltmeter

Parts placement


pcb Led display digital Voltmeter

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 the component.

- 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

- 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.

image 1 th Led display digital Voltmeter

As it is recommended start working by identifying the
components and separating them in groups. There are tw1o points
in the construction of this project that you should observe:

First of all the display ICs 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 betw1een 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 ICs 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 place.

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 voltages.

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 ρ 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?.

Parts List

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

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 wrong places.

- 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

- Check your project for faulty or damaged components.

Sample Power Supply 1
Sample Power Supply 2