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You can order the three-digit digital voltmeter module DSN-DVM-368 for less than three euros. With this module you can measure DC voltages up to 30.0 V. We investigated this module and compared the reading with that of a professional voltmeter. |
Introduction to the DSN-DVM-368
The appearance of the DSN-DVM-368
The DSN-DVM-368 is a little module of only 32 mm x 12 mm x 9 mm. The small PCB is no larger than the three-digit segment display. On both sides, a little of the PCB material protrudes, so you can screw it behind a front panel.
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| The DSN-DVM-368 is not much bigger than a finger phalanx. (© 2018 Jos Verstraten) |
When you disassembled the module you notice that there are two chips hidden in the internal. Next to the processor you can discover a 5 V stabilizer, five resistors, two capacitors and a diode.
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| The PCB, after removing the display. (© 2018 Jos Verstraten) |
It is not that difficult to reconstruct the schematic of the module. The power supply goes via the diode D1 to the voltage stabilizer IC1, which provides the +5 Vdc power supply for the microcontroller and the display. The diode D1 probably provides a better distribution of the heat power dissipated on the PCB, which is now divided between the diode and IC1.
The DSN-DVM-368 has two modes of operation:
- Mode 1, three-wire.
The jumper J1 is open. The input is then disconnected from +Ub and you can use the module to measure DC voltages from 0 V to +30 V with a fixed supply voltage +Ub of at least +5 Vdc. - Mode 2, two-wire.
The jumper J1 is closed. You can connect the module with only two wires to the voltage to be measured, which is then also the supply voltage. In this mode, the lowest measurable voltage is about +4.5 V. At a lower voltage, the circuit fails.
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| The schematic of the DSN-DVM-368-02. (© 2018 Jos Verstraten) |
An interesting mystery is how the seven segments of the display, the two decimal points and the three digits are controlled from only ten lines. Logically speaking, thirteen lines are needed! On the internet it is suggested that the segments and the decimal points are merged into a 5 x 5 matrix, as shown below.
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| A suggested solution for controlling the display from only ten lines. (© edycube.blog.fc2.com) |
The picture below shows the back of the module. A soldering iron with a fine tip and a steady hand are absolutely necessary when soldering the jumpers or replacing the wires.
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| The location of the jumpers and the connection points on the back. (© 2018 Jos Verstraten) |
- Measuring range: 0 Vdc to 30 Vdc
- Resolution < 10 V: 0.01 V
- Resolution > 10 V: 0.1 V
- Maximum input voltage: 30 Vdc
- Supply voltage: 4.5 Vdc to 30 Vdc
- Supply current: 23 mA typical
- Sample period: 300 ms typical
- Input resistance: > 100 kΩ
- Accuracy < 10 V: ±0.1 V
- Accuracy > 10 V: ±0.3 V
- Display: 3-digit 0.36 inch
- Display color: red - yellow - green - blue - white
- Dimensions: 32 mm x 12 mm x 9 mm
- Heart to heart distance fixing holes: 28 mm
- Weight: 5 g
The DSN-DVM-368 tested
Three-wire mode test
We supplied the module with a voltage of 23.3 V and compared the measurement results with those of a good universal meter.
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| Accuracy of the measurement in three-wire mode. (© 2018 Jos Verstraten) |
Two-wire mode test
Of course we repeated this test with the two-wire mode, with the results below.
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| Accuracy of the measurement in two-wire mode. (© 2018 Jos Verstraten) |
Geekcreit 0.28 Inch 30 V Mini Digital Voltmeter
