ICL7107 PANEL METER

ICL7107 PANEL VOLTMETER CIRCUIT

This is a 3½ digit panel meter circuit using ICL7107 IC. Its voltage  range is 20V. But you can change it to 200mV, 2V, 20V and 200V.

PARTS LIST
R1 100K
R2 470K
R3 1K
R4 470
R5 10K
R6 10K
R7 1M  metal film 1%
R8 100K metal film 1%
VR1 10kΩ multi turn
VR2 20kΩ multi turn
C1 100pF ceramic
C2 0.47µF polystyrene or Mylar
C3 0.22µF polypropylene
C4 0.1µF polystyrene or Mylar
C5 10µF
C6 10µF
C7 100µF
C8 0.01µF
C9 0.01µF
C10 10µF
C11 0.1µF
IC1 ICL7107
IC2 ICL7660
IC3 TL431
IC4 7805
DS1, DS2, DS3, DS4 Common Anode seven segment

R1 and C1 use for oscillator, R2 integrating resistor, C3 integrating capacitor, C4 reference capacitor and C2 auto zero capacitor.

The oscillator capacitor should be a dipped mica or ceramic type, and the reference and auto-zero capacitors should be either polystyrene or Mylar types. The integrating capacitor should be polypropylene, with
polystyrene and polycarbonate as second and third choices, respectively. The integrating capacitor must have good dielectric absorption characteristics for the A/D converters to have optimum linearity.

ICL7107 is a high performance , low power , 3.5 digit A/D converter. This IC includes seven segment decoders, display drivers, a reference and a clock. Also This circuit can indicate polarity information. This IC is an electrostatic sensitive device (ESD). When you are handling this IC, you need to care about it. My advice is to use an IC base. If you don’t have ESD safe soldering iron, do not solder IC base with IC.

This IC has internal reference. But this circuit uses an external reference for to get great accuracy. I build this circuit with using internal reference, but it gives many accuracy problems. I don’t now extract the problem. But I think it is depends on the IC and component quality. Because TL431 reference uses for that. In this circuit configuration we need 1V as a reference voltage.

We need a voltage divider for adjusting the voltage range. For that we use R7 and R8. VR2 for fine adjustment.  Our circuit use 1V as the reference voltage. Because we can only measure up to 2V without voltage divider. If you need to measure 20 volts you need to divide the input voltage by 10. Because we use a resistor voltage divider for that. 

In this circuit

Vin=20, Vout=2V, Rx=R7=1MΩ, R8+VR2=Ry=?

Vout=(Vin X Ry)/(Rx+Ry)

Ry=(Vout X R1)/(Vin-Vout)

=(2 X 1MΩ) / (20-8)

=2MΩ/18

=0.111111MΩ

=111.11KΩ

R8+VR2 need to equal to 111.11kΩ. Because we use R8=100KΩ and VR2=20KΩ. 

Use below chart if you need to change voltage range.

Voltage Range R8 VR2
0-20 100KΩ 20KΩ
0-200 10KΩ 2KΩ

 

Now we need to calibrate our circuit. First, you need to adjust VR1 for 1.000 volts in the ICL7107 PIN 36 (REF_HI). Then you need to input know voltage to the circuit (Like 5V), after you need to adjust VR2 to read the same voltage in our display.

Also, you need to change the decimal point accordingly voltage range. 0-20V range you need to connect R4 to DS3 pin5 and 0-200V range you need to connect R4 to DS4 Pin5.

D1 and D2 use to adjust the brightness of the seven segment display. If you add another diode in series you can further reduce brightness.

If your power supply is 5V, you can remove IC4 and installed jumper between pin 1 and 2.

IF you need more information about this IC, please refer these documents,

ICL7107 maxim datasheet Tips for Using ICl7107 ICL7107 Meter Designs

This circuit needs negative 5V supply. Because we use ICL7660 voltage converter. It is very simple to use. If you difficult to find this IC, you can use NE555 negative 5V generator circuit or any other -5V generator circuit.


This circuit use common anode seven segment display. 

This is our PCB design, please use good quality PCB board. Otherwise, it gives some accuracy problems.

ICL7107 PCB traces ICL7107 PCB components

TP1 for test seven segment display, TP2 to adjust the reference voltage.





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