Digital battery voltage meter. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Digital battery voltage meter. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells

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Almost all old domestic cars have dial gauges for battery voltage. The indicators are simple and indistinct, but they, working in a narrow voltage range, allowed the driver to notice in time the generator overload, contact failure or changes in the operation of the relay-regulator.

In modern domestic cars and in almost all modern "foreign cars" there is no voltmeter. There is only a signal lamp that should light up when the battery voltage drops sharply. But, firstly, not only a decrease but also a strong increase in voltage is harmful to the battery, which can lead to its overcharging and boiling. Secondly, as the experiment shows, such an indicator does not even react in any way to disconnecting the battery while the engine is running. That is, if, for example, the terminal has fallen off, you will only know about it when you try to start the engine, but it's already too late.

In the radio engineering literature, there are many descriptions of automotive indicators - voltmeters that show the voltage on the battery in the form of a column of luminous LEDs. But this is inconvenient, in my opinion, digital indication is better perceived.

Figure 1 shows a diagram of an automotive voltmeter operating on the analog principle, but giving an indication on a two-digit digital indicator.

(click to enlarge)

The measurement range is from 10 to 17 V. This is more than enough.

The circuit consists of a meter on the LM3914 polycomparator chip and an indication circuit on a diode decimal-binary converter, a binary-seven-segment decoder and two seven-segment indicators!.

Chip. A2, using trimmers R4 and R5, is set to measure the input voltage supplied to the divider R1-R3 in the range from 10 to 17 V. In this case, A2 actually indicates from 0 to 7, that is, the voltage of 10 V is taken as zero. The indication at output A2 is set to the type of moving dot.

That is, only one of its output keys is open at any given time.

Instead of indicator LEDs, the inputs of the decoder D2 are connected to the outputs of A1, but through a diode circuit VD2-VD12, which, together with R7-R8, is a mounting decimal-binary converter that converts decimal numbers from 0 to 7 into a three-digit binary code. This code is fed to the inputs of the decoder D1, designed to work with a seven-segment LED indicator.

In practice, the function of indicating the input voltage lies on the indicator H2, showing the numbers of the least significant digit, that is, in the range from 10 to 17 V, it shows from "0" to "7". The H1 indicator serves only for visual convenience of reading the readings - a unity is always lit on it.

The voltmeter is powered by the measured voltage.

In order for the stability of operation not to depend on fluctuations in this voltage, the supply voltage of the circuit is chosen to be 5V (this corresponds to the power supply standard of the 74LS247 digital microcircuit). Voltage 5V produces an integral stabilizer. A1. Diode VD1 and capacitor C1 make the power circuit little dependent on dips and surges that may be in the car's on-board network.

Capacitor C3 serves to ensure that the measurement takes place smoothly, with some barely noticeable delay. This avoids the occurrence of chaotic unreadable readings due to impulse noise in the vehicle circuits and too rapid voltage changes.

Now for the details. Chip. A2 should be exactly LM3914, since a linear indication is needed, not a logarithmic one. The 74LS247 chip can be replaced by any available decoder chip to work with a seven-segment common anode LED indicator.

The decoder will need to be turned on according to the typical scheme, turning off all its additional functions - memory cells, strobing, etc. (if any).

Seven-segment LED indicators can also be almost anything with a common anode. You can use an assembly of two indicators with separate segment outputs. from the indicator. H1 can be abandoned, but then you need to mentally imagine a unit in the highest digit.

Stabilizer 7805 can be replaced by our type KR142EN5A. Diode 1N4007 - any rectifier diode of low or medium power, for example, KD105. Diodes 1N4148 can be replaced with KD522, KD521.

Capacitor C1 must be at least 20 V.

It is most convenient to set up a voltmeter when it is powered from an adjustable laboratory DC voltage source. Apply a voltage of 17 V and adjust R4 to achieve the reading "17". Then, apply 10 V and adjust R5 until the reading is "10". Then check the compliance of the readings with the real voltage within the entire range (10-17 V). If necessary, repeat the setting of R4 and R5 a few more times.

Author: Lyzhin R.

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