kit 61. general purpose 3 1/2 digit led panel meter

kit 61. general purpose 3 1/2 digit led panel meter
In Kit 34 we presented a general purpose 3 1/2 digit panel
meter (DPM) using the 7106 IC and an LCD. The sister
chip to the 7106 is the 7107 which uses an LED display
for its output. There are two advantages to using the 7107
over the 7106. The first is that because it is powered by
+5V and -5V it can accept inputs in the range +4V to 4V. A second advantage is when a digital reading of zero
is desired for a VIN not equal to zero as, for example,
temperature and weighing systems with a variable tare.
Both the offset voltage and the transducer voltage
can be conveniently input directly to the 7107. In 7106
systems null calibration requires additional electronics
Kit 61 has been designed as a versatile 3 1/2 digit panel
meter with these considerations in mind. Also in this Kit
we have used small, monoblock capacitors rather than the
much bigger mylar or metallized types. This saves space
as well as making the meter look better. Some
components are under the IC to save even more space.
Voltage is the most frequently measured electrical
quantity. In DPM’s to measure temperature, current, wind
speed, resistance, lux etc., what is actually being
measured is voltage. After calibrating the meter for its
particular purpose the voltage measured will give an
accurate digital reading of the analog quantity being
Digital displays have many advantages over analog
meters which use a pointer and moving coil. Firstly they
are easier to read especially by unskilled labour. In the
majority of applications it is better that the value
displayed is exactly the value being measured, for
example, 13.6V. To use an analog display with its many
graduated scales (some going up and others going down)
and switches requires considerable practice. But a simple
LED display which reads '13.6' can be understood by
Second, DPM built using the 7107 are physically stronger
and more robust than analog meters because they have no
moving parts. Thirdly, for the manufacturer the assembly
of the complete DPM unit can be done by relatively
unskilled labour in third world countries. Fourthly, the
7107 by its very nature can be adapted to so many uses at
such a low cost that it has actually created markets for
itself. All of these factors add up to a better, cheaper
product which everyone can afford.
Two circuit boards have been used. The 4 LED displays
mount on one board which is soldered to the mother
board at right angles. This allows low profile end
The kit is constructed on single-sided printed circuit
boards. Protel Autotrax & Schematic were used.
Check the components in the Kit against the Component
listing. Make sure you identify every component.
Display PCB. Fit the 4 LED displays to the display PCB
by following the overlay and solder. Do not attach to the
mother board yet. That will be the last thing to do.
Main PCB. It is easiest to solder the lowest height
components first - the diodes, resistors monoblock
capacitors and the single link. Use some of the leads cut
off from the resistors to attach four wires coming out of
the TEST, REF HI & REF LO pads. These will be used
later. Note that R5 R6 R7 stand up on the board. The
link and 3 of the components fit under the 7107 IC. Then
attach the IC sockets, switches and remaining
components. Watch the orientation of the electrolytic
capacitors and the two IC's. The two terminal blocks
which are next to each other slide together in a tongue &
groove arrangement. Put them together before soldering
them. We have supplied resistor RY1 of 10M resistance.
Because the input impedance of the 7107 is so high this
will make sure that the display does not pick up stray
charge and will calibrate correctly when you first turn it
on. Leave this resistor in place except when you use the
input voltage divider as explained below.
The last step in construction is to attach the two PCB's
together using the 27 pin post header connector. The
short, 90o pins fit into the top of the mother board. Solder
it into place first. Then fit on the display board to the long
straight pins. Make sure you get the display board the
correct way around. Look for the V+and the G pins.
These are marked on both PCB's. On the display board it
is marked on the bottom side under the solder mask.)
Make sure you match them up: V+ to V+, G to G.
Connect +5V to the board and turn it on. Put the switch
in the Normal position. When the TEST pin 37 of the
7107 is connected to VCC then the display should read 1888. You can easily do this by touching together the two
wires you previously soldered into the two pads at TEST.
This will tell you that the IC and the display are working
Poor soldering is the most likely reason. Check all solder
joints carefully under a good light. Next check that all
components are in their correct position on the PCB.
Thirdly, follow the track with a voltmeter to check the
potential differences at various parts of the circuit. Check
that -5V is going to pin 26 of the 7107.
As you can see from the schematic on the previous page
most of the connections are between the 7107 and the
LED's. The 7107 contains a number of inputs which can
be varied to do different things. Of prime importance is
the reference voltage, VREF, which is set by the 10K
trimpot and R3. These will be discussed below.
-5V Generation. With the addition of only two
capacitors the 7660 IC performs the complete supply
voltage conversion from positive to negative for an input
range of 1.5 to 10V, resulting in complementary output
voltages of -1.5V to -10V.
VIN = 2 x VREF
7107. The heart of the meter is the a/d converter built into
the 7107. It uses a dual slope conversion technique. It
relies on the charging and discharging of an integrating
capacitor and having a counter count when the capacitor
voltage is above a set value. Since the capacitor discharge
is linear the counter reading is proportional to the input
voltage. There are three phases to the process:
Phase 1. Auto Zero. The autozero capacitor is charged to
the integrators offset voltage. This voltage is subtracted
from the input signal during phase 2. The integrator thus
appears to have zero offset voltage.
And VREF must be in the range 100mV to 1.0V.
Components supplied in this Kit are for VREF of 100mV.
For a VREF of 1V two components should be changed (as
mentioned above) to maintain sensitivity and recovery
from over-voltage. The 10K trim pot and resistor R3 will
allow adjustment for either value, and for intermediate
values when required (discussed below.) In the following
discussion we will assume that we are using a VREF of
Phase 2. Signal Integrate. The signal input is averaged
1000 clock pulses.
Calibration is done by attaching a (preferably digital)
multimeter to REF HI and REF LO and adjusting the
trimpot to read 100 mV. This is why wires were put into
these pads during construction. Now the meter is
calibrated to read 0 - 199.9mV.
Phase 3. Reference Integrate. Input low is internally
connected to Common (which may be an offset voltage.)
VREF is averaged back to either zero volts or the offset
voltage over another 1000 clock pulses. The number of
clock pulses counted to return to this value is a digital
measure of VIN.
Voltage Divider. To measure voltage greater than 0.2V
an input voltage divider is required. See the figure below.
This is the purpose of the space for 4 resistors on the
main circuit board. The general relation for full scale
sensitivity is now:
System Timing. This is determined by the components
connected to pins 38, 39 & 40. Values are unchanged for
all ranges measured. The internal oscillator runs at
48kHz, or 3 readings per second.
Decimal Point. A jumper selects the decimal point
position in the displays.
VIN (full scale) = 2VREF x RY / ( RX + RY)
For example, a 0 - 20V range (when VREF is 0.1V) can be
obtained using a 100:1 voltage divider. This can be done
by making RX = 1M and RY = 10K. The decimal point
jumper is placed at position '2' so a full scale display of
19.99V is indicated. Similarly, a 0 - 200V range can be
obtained with RX = 1M and RY = 1K.
Analog Section. C1 is the reference capacitor and is
unchanged for all ranges measured. IN LO is tied to the
analog COMMON pin 32 by the Normal position of the
switch except when an Offset voltage is input. See below.
The integration capacitor C5 is suitable for all ranges
measured but the value of the integration resistor R1
should be increased to 470K for a VREF of 1V.
Auto-Zero Capacitor. This is C4 connected to pin 29. It
has some influence on the noise of the system and
recovery from overload input. On the 2V scale a 0.047uF
capacitor may give better results.
Voltage Measurement. Since the maximum value which
can be displayed is 1999, voltmeters with full scale
readings of 199.9mV, 1.999V, 19.99V etc. can be made.
The user must decide their own need. Then a reference
voltage and maybe an input attenuator must be selected.
To use the meter to measure 0 - 199.9mV the trimpot is
adjusted so that the reference voltage between pins 35 &
36 is 100mV. And to set the meter for 0 - 1.999V, VREF
must be set to 1.0V. Measuring higher voltages and nonstandard voltages will be discussed below.
The relationship between full scale input voltage and the
reference voltage is:
Figure 1
If VREF is 1V a similar pattern of voltage divider resistors
can be determined. Remember that if no input divider is
used to put the 10M resistor back in RY1 across the input
Input an Offset Voltage. A major advantage of a 7107based meter over a 7106-based meter is that an offset
voltage may be read directly by the 7107. The same
function with a 7106 meter requires additional electronics
to be built. To use the Offset input first move the switch
the 'Use Offset' position. The offset voltage is input to pin
32, Offset, and pin 30, In Lo, while the transducer is
connected as normal using Input 2 and Input 3. Refer to
the schematic.
Non-standard Voltage Input. In many applications it is
required that the output of a transducer is converted by a
scale factor into some meaningful result. For example, a
load cell of a weighing system may have an output
voltage of 0.682V when it has 2.0 Kg weight on it. You
want the meter to read the range 0 - 1.99 Kg directly.
It is an easy matter to adjust VREF to 0.341V (half the
output voltage), put the decimal point in the correct
position by moving the jumper and the panel meter now
reads off 0 - 1.99 Kg directly from the display.
Current Measurement. Currents up to 2A can be easily
measured using the space on the board for a 5W shunt
resistor, R. The current is converted into a voltage by the
shunt resistor. The voltage divider resistors RX and RY
(including RY1) are not used. The principal is shown in
Figure 2 below.
If R = 0.1 ohms then 200mV will be developed when the
current through it is 2A. This voltage is applied to the
meter which is set up for the 200mV range. (That is, VREF
is set to 100mV.) Power disspiation at the maximum
reading is I2R which is 0.4W, well within the 5W rating
of the resistor. See Table below.
To measure a full scale of 200mA then R should be 1.0
ohms in order to generate 200mV input to the meter. For
a 20 mA meter then R = 10 ohms. Note that because of
wide tolerances in the shunt resistors it may be necessary
to adjust the reference voltage in order to get the correct
reading. So further adjustment of VREF using a known
current may be required.
Resistors 5%, 1/4W:
R1 yellow violet orange
100K R2 brown black yellow
R3 brown green orange
R4 brown black green
470R R5 R6 R7 yellow violet brown
RY1 brown black blue
10K trimpot 103
1N4148 diode
0.1uF 104 monoblok C1
0.01uF 103 monoblok C2
100pF 101 monoblok C3
0.47uF 474 monoblok C4
0.22uF 224 monoblok C5
100uF 16V miniecap C6
10uF mini ecap C7 C8
LED CA display KW1-561ASA
7107 IC1
7660 IC2
40 pin IC socket
8 pin IC socket
K61 main PCB
K61 display PCB
2 pole terminal block
SPDT pcb-mounted switch
3 post dual row header
27 pin single-in-line 90o header
See our kit range at
For more technical information about using the 7106 and
7107 IC’s go to
The Kit shows how much of electronics today can be
contained in a single chip. Commercial low to medium
cost digital multimeters are nothing more than this kit,
some switches and passive components and a plastic case.
The main reason today for the failure of meters is more
likely due to switch contact and mechanical failure rather
than failure of the electronics itself.
Figure 2
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