QKits Electronic Kits: QK127, 31/2 Digit Panel Meter Kit

The 7106 chip is one of the long term survivors in the IC world. It was launched in 1977 yet it has remained as popular as ever. The reason is that it contains all the active circuitry for a 3 1/2 digit panel meter (DPM) in a single chip. It was designed to interface diredtly to a liquid crystal display (LCD). (It has a sister chip, the 7107, intended for connection to light emitting diode displays.) So the chip contains BCD to seven segment decoders, display drivers,clock and reference voltage as well as the necessary analog to digital (a/d) circuitry to convert the input voltage to a digital form. The a/d system also indicates the polarity of the input voltage.

Voltage is the most frequently measured electrical quantity. In temperature meters, current meters, wind speed meters and resistance meters what is actually being measured is voltage, or more correctly, the potential difference between two points. After calibrating the meter for its particular purpose then the potential difference measured will give an accurate digital reading of the analog quantity being measured.

With a few additional external passive components the 7106 chip may be made into an easy to use meter for any of these purposes especially multiple range digital volt meters.

Digital meters have many advantages over analog meters that use a pointer and moving coil.
1. They are easier to read. In the majority of applications it is better that the value displayed is exactly the valued being measured, for example, 13.6V. To use an analog display with its many graduated scales and switches requires considerable practice. But a simple LCD which reads 13.6 can be understood by everyone.
2. DPMs built using the 7106 car physically stronger and more robust than analog meters because they have no moving parts.
3. The 7106 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 better, cheaper product which everyone can afford.

In this kit we have supplied the 7106, the LCD and the essential components and information necessary for you to custom build it into a panel meter of you choice. The pCB has a printed overlay on it so that the position of all the components is clearly indicated and construction only takes a few minutes.

CIRCUIT DESCRIPTION
The heart of the meter is the A/D converter built into the 7106. 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 auto zero capacitor is charged to the integrator's offset voltage. This voltage is subtracted from the input signal during phase 2. The integrator thus appears to have zero offset voltage.

Phase 2 - SIGNAL INTEGRATE
The signal input is averaged for 1000 clock pulses.

Phase 3 - REFERENCE INTERGRATE
VREF is averaged back to zero volts. the number of clock pulses counted to return to zero is a digital measure of VIN.

The reference voltage supplied to the 7106 at pins 35 and 36 should be between 100mV and 1V for most purposes. This corresponds to a full scale reading of 199.9 mV and 1.999V respectively. In this kit VREF is set to 100mV but is adjustable fro 89 - 107mV.

A jumper is used to select the decimal point position in the LCD. The following table shows the resistor values to use for different voltage ranges and the position of the decimal point jumper.

Range	RA	RB	DP jumper
200mV	10M	Wire Link	2
2V	300K	2.7M	4
20V	100K	10M	3
200V	10K	10M	2

	~ Celebrating 16 Years of QualityKits in 2012 ~		17-May-2012