Description.
The circuit given here is of a very
useful and accurate digital voltmeter with LED display using the ICL7107
from Intersil. The ICL7107 is a high performance, low power, 3.5 digit
analog to digital converter. The IC includes internal circuitry for
seven segment decoders, display drivers, reference voltage source and a
clock. The power dissipation is less than 10mW and the display stability
is very high.
The working of this electronic circuit
is very simple. The voltage to be measured is converted into a digital
equivalent by the ADC inside the IC and then this digital equivalent is
decoded to the seven segment format and then displayed. The ADC used in
ICL7107 is dual slope type ADC. The process taking place inside our ADC
can be stated as follows. For a fixed period of time the voltage to be
measured is integrated to obtain a ramp at the output of the integrator.
Then a known reference voltage of opposite polarity is applied to the
input of the integrator and allowed to ramp until the output of
integrator becomes zero. The time taken for the negative slope to reach
zero is measured in terms of the IC’s clock cycle and it will be
proportional to the voltage under measurement. In simple words, the
input voltage is compared to an internal reference voltage and the
result is converted in a digital format.
The resistor R2 and C1 are used to set
the frequency of IC’s internal clock. Capacitor C2 neutralizes the
fluctuations in the internal reference voltage and increases the
stability of the display.R4 controls the range of the voltmeter. Right
most three displays are connected so that they can display all digits.
The left most display is so connected that it can display only “1” and
“-“.The pin5(representing the dot) is connected to ground only for the
third display and its position needs to be changed when you change the
range of the volt meter by altering R4. (R4=1.2K gives 0-20V range,
R4=12K gives 0-200V range ).
Important Points :-
- Assemble the circuit on a good quality PCB.
- The circuit can be powered from a +/_5V dual supply.
- For calibration, power up the circuit and short the input terminals. Then adjust R6 so that the display reads 0V.
- The ICL7107 is a CMOS device and it is very sensitive to static electricity. So avoid touching the IC pins with your bare hands.
- The seven segment displays must by common anode type.
- I also assembled this circuit few years back and it is still working fine.
Original Source : Circuits Today
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