DC voltmeter with automatic range selection. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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DC voltmeter with automatic range selection. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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When developing this device, the task was set - to make a digital DC voltmeter as simple as possible with an automatic selection of limits, providing voltage measurement up to 999 V and consuming a small current. The scheme of the developed device is shown in fig. 1. Its basis is the DD1 microcontroller, which works according to the program, the codes of which are shown in the table.

DC voltmeter with automatic range selection
Fig. 1

The measured voltage is fed to the input of the ADC built into the microcontroller (pin 3) through resistive voltage dividers and a C1R5 low-pass filter that suppresses high-frequency noise. As a reference voltage for the ADC, a 2,56 V voltage source built into the microcontroller was used. At an input voltage of less than 10 V, the PBI and PB2 port lines (pins 6 and 7) of the DD1 microcontroller are in a high resistance state. In this case, the division ratio of the input voltage divider of the ADC is 4 (the upper arm of the divider is R3 and R6, the lower arm is R2) and the input voltage is measured with an accuracy of hundredths of a volt.

If the input voltage exceeds 10 V, using the PB1 port line, the DD1 microcontroller will connect the resistor R2 in parallel with the resistor R9, increasing the input voltage division factor to 40. In this case, the upper measurement limit will be 999 V. When the voltage at this limit becomes less than 10 V, the lines PB1 and PB2 ports (pins 6 and 7) of the DD1 microcontroller will switch to a high resistance state and the division factor of the input divider will again decrease to 4. If the input voltage reaches 100 V or more, using the PB2 port line, the DD1 microcontroller will additionally connect a resistor in parallel with the resistor R2 R8, In this case, the input voltage division ratio will increase to 400, and the upper measurement limit will be 999 V. When the input voltage exceeds 999 V (overload), "- -" characters are displayed in the first and second (rightmost) digits.

The device also provides for measuring the voltage of the G1 battery with an accuracy of hundredths of a volt. To do this, a voltage proportional to the battery voltage from the resistive divider R1R4 is fed to the PB4 input, which is configured by software as another input of the built-in ADC. All information is displayed on a ten-digit LCD indicator HG1. On the left side is the battery voltage, and on the right side is the measured voltage.

Separation of integers and tenths of a volt is carried out by an empty familiarity. Due to the limited number of input-output ports of the microcontroller, data is transmitted over one PB5 line (pin 5) with pulse-time coding (transmission time 1 is approximately ten times greater than 0, and the pause between them is equal to the duration of 1). With a short signal duration, the capacitor C3 does not have time to charge, and during the pause it is completely discharged, therefore, with a short pulse duration during its decline, there is a low level on the DAT data line (pin 4 of the HG1 indicator) and the LCD controller perceives this as 0. With a large pulse duration, by the time the pulse falls, capacitor C3 has time to charge to a high level and the LCD controller registers this as 1.

To power the device, a battery from a cell phone is suitable. At a voltage of 4,2 V, the current consumption does not exceed 5 mA. The HL1 LED is not used as a light indicator, but as a voltage regulator for the LCD supply. The voltmeter remains operational when the supply voltage drops to 3 V.

Most of the elements, except for the battery G1, the power switch SA1, the indicator HG1 and the resistor R3, are mounted on a printed circuit board made of one-sided foil fiberglass, the drawing of which is shown in fig. 2.

DC voltmeter with automatic range selection
Fig. 2

The board is installed in a plastic case of a suitable size. Resistors R1-4, MLT, C2-23 are used, the oxide capacitor is imported, capacitors C1, C3 are K10-17. LCD indicator - KO-4V2 (with controller W-1611-04) or NT-1611 manufactured by Telesystems. The LED, power switch, and cell phone battery can be any type.

To establish the device, you will need an exemplary voltmeter. First, it is connected to the battery and with a selection of resistor R4, the readings on the left side of the indicator are equalized with the readings of an exemplary voltmeter. Then, the "+" input of the device is connected to the positive terminal of the capacitor C2 and, with a selection of resistor R9, the readings on the right side of the LCD indicator are equalized with the readings of a reference voltmeter. Next, this voltmeter is connected to the input of the device, a voltage of about 30 V is applied to it from a stabilized power source, and by selecting a resistor, the readings on the right side of the LCD indicator are again equalized with the readings of the exemplary voltmeter. The input voltage is increased to 150 V, and the readings are again equalized by selecting the resistor R8.

Since the maximum divider current does not exceed 1 mA (at an input voltage of 1000 V it is about 0,6 mA), internal protective diodes quite cope with protecting the microcontroller from overloads and abnormal voltages at the input of the built-in ADC.

Text and program codes for the voltmeter microcontroller

Author: Ozolin M.

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