On-board LED voltmeter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Automobile. Electronic devices

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Descriptions of devices for light display of the voltage level have already been published on the pages of our magazine. It would seem, what else can be added to the published? It turns out you can! The article below confirms this: a real radio amateur is in no hurry to consider any problem solved ...

A voltmeter installed on the dashboard of a car allows you to quickly monitor the voltage level in its on-board network. Such a device does not require high resolution, but you need the ability to easily and quickly read the readings. These conditions are best met by a discrete LED voltage indicator. Similar devices have become very widespread for assessing the level of voltage and power (in sound amplifying equipment). They are usually implemented in two ways.

The first one is described in detail in [1]. Its essence is that a line of LEDs is connected to the source of the measured voltage through a multi-output resistive voltage divider. The threshold properties of LEDs, transistors and diodes are used here. The simplicity of such an indicator comes at the price of a fuzzy LED ignition threshold (as noted by the author in [2]). Such devices were once sold in the form of a radio designer.

The second way is to use a separate comparator to turn on each LED, which compares part of the input signal with the exemplary one (as, for example, in [3]). many chips are required. Quad op-amps are still expensive now, and one such chip can only drive four LEDs.

Finally, one cannot fail to note the work (4), where the principle of analog-to-digital conversion is used. This design has many advantages, but still too many details, and also uneconomical.

The voltmeter brought to your attention is optimized in the light of the above - in it clear threshold levels of LED ignition are obtained using a minimum of cheap, economical and widely available elements. The principle of operation of the device is based on the threshold properties of a digital microcircuit.

The device (see diagram in Fig. 1) is a six-level indicator. For ease of use in a car, the measurement interval was chosen to be 10...15 V in 1 V steps. Both the interval and the step can be easily changed.

The threshold devices are six inverters DD1,1-DD1.6, each of which is a non-linear voltage amplifier with a large gain. The switching threshold level of inverters is about half the voltage supplied by the chip, so they kind of compare the input voltage with half the supply voltage.

If the input voltage of the inverter exceeds the threshold level, the output voltage will go low. Therefore, the LED that serves as the load of the inverter will be turned on by the output (sinking) current. When the output of the inverters is high, the LEDs are closed and off.

From the outputs of the resistive divider R1-R7, the corresponding share of the voltage of the on-board network is supplied to the input of the inverters. When the onboard voltage changes, its shares also change proportionally. The supply voltage of the inverters and the LED line is stabilized by the DA1 microcircuit stabilizer. The values ​​of the resistors R1-R7 are calculated in such a way as to obtain a switching step of 1 V.

Capacitor C2 together with resistor R1 form a low-frequency filter that suppresses short-term voltage spikes that may occur, for example, when starting the engine. The manufacturer of microcircuit stabilizers recommends installing capacitor C1 to improve their stability at high frequency. Resistors R8-R13 limit the output current of the inverters.

How to calculate resistors R1-R7? Despite the fact that field-effect transistors are installed at the input of inverters DD1.1.-D1.6, which practically do not consume input current, there is a so-called leakage current. This makes it necessary to choose a current through the divider that is much larger than the total leakage current of all six inverters (no more than 6X10-5 μA). The minimum current through the divider will be at a minimum indicated voltage of 10 V.

Let's set this current to 100 µA, which is about a million times the leakage current. Then the total resistance of the divider RD=R1+R2+RЗ+R4+R5+R6+R7 (in kiloohms, if the voltage is in volts and the current is in milliamps) should be: Rd=Uvx min/Imin = 10V/0,1mA = 100 kOhm.

Now let's calculate the resistance of each of the resistors under the condition Upr \u2d Upit / 3, i.e. in the case under consideration Upr \u15d 7 V. With an input voltage of 3 V, 15 V should fall on the resistor R100, and the current through it (equal to the current through the entire divider) Id \u0,15d UBX / Rd \u150d 7 V / 7 kOhm \u3d 0,15 mA \u20d XNUMX μA, Then the resistance of the resistor RXNUMX: R \uXNUMXd Upor / Id; RXNUMX=XNUMXV/XNUMXmA=XNUMXkΩ.

At the input of the DD1.5 inverter, 3 V should be at an input voltage of 14 V. The current through the divider in this case is Id \u14d 100 V / 0,14 kOhm \u6d 7 mA. Then the total resistance R3 + R0,14 \u21,5d Upop / Id \uXNUMXd XNUMX / XNUMX-XNUMX kOhm.

Hence R6 \u21,5d 20-1,5 \uXNUMXd XNUMX kOhm.

Similarly, the resistance of the remaining resistors of the divider is determined: R5 \u6d UporkhRd / Uin- (R7 + R1,6) -4 kOhm; R2-2,2 kOhm, R2-2.7 kOhm, R1-2 kOhm and, finally, R4 \u5d Rd-(R6 + R7 + R70 + R68 + RXNUMX + RXNUMX) \uXNUMXd XNUMX kOhm-XNUMX kOhm.

In general, as is known, the threshold voltage of the elements of CMOS microcircuits is in the range from 1/3Upit to 2/3Upit. It is also known that the elements of the same microcircuit, manufactured in a single technological cycle on the same chip, have almost the same values ​​of the switching threshold. Therefore, to accurately set the "beginning of the scale" of the voltmeter, it is enough to replace the resistor R1 with a series circuit from a trimmer with a calculated rating and a constant one with a rating two times less than the calculated one.

The temperature stability of the device is very high. When the temperature changes from -10 to +60 °C, the response threshold changes by several hundredths of a volt. The DA1 microcircuit stabilizer also has a temperature stability of at least 30 mV within 0...100 °C.

The output voltage of the DA1 stabilizer must not be less than 6 V, otherwise the inverters will not be able to provide the necessary current through the LEDs. The inverters of the K561LN2 chip allow an output current of up to 8 mA. AL307BM LEDs can be replaced by any others by recalculating the values ​​of the current-limiting resistors R8-R13. Capacitors can also be any for a rated voltage of at least 10 V.

To establish the assembled device is connected to the output of an adjustable voltage source, which will simulate the on-board network. By setting the output voltage of the source to 10 V, and the resistance of the tuning resistor to the maximum, rotate its slider until the HL1 LED turns on. The remaining levels are set automatically.

The parts of the voltmeter are mounted on a printed circuit board made of foil fiberglass with a thickness of 1 mm. The drawing of the board is shown in fig. 2. It is designed to install a tuning resistor SPZ-33, and the rest - MLT-0,125, capacitor C1 - KM, C2 - K50-35.

The board is attached to the bottom of the plastic box with two M2,5 screws on tubular racks and another one that simultaneously presses the DA1 chip to the board. Note that this microcircuit is installed with a plastic (not metal) side to the board. A tubular stand is also installed between the microcircuit case and the board, but shortened.

The leads of the LEDs before mounting are bent by 90 degrees so that their optical axes are parallel to the plane of the board. The housings of the LEDs should protrude beyond the edge of the board and, during the final assembly of the device, go into the holes drilled in the end of the box.

The stability of the stabilizer and the entire device as a whole will be even higher if a capacitor with a capacity of 8 microns is connected to the input of the microcircuit (between pins 17 and 0,1). In order to protect the stabilizer from accidental voltage surges in the on-board network, the amplitude of which can reach 80 - 00 V. Another oxide capacitor should be connected in parallel with this capacitor. It must have a capacity of at least 1000 microfarads and a nominal voltage of 25 V. This capacitor will also have a beneficial effect on the operation of radio and sound amplifying automotive equipment.

Literature

1. Nechaev I. LED signal level indicator. - Radio, 1988, No. 12, p. 52.
2. Isaulov V., Vasilenko E. A simple recording level indicator. - RadioAmator, 1995, No. 3, p. 5.
3. Tikhomirov A. On-board network voltage indicator. - RadioAmator, 1996, No. 10, p. 2.
4. Gvozditsky G. On-board network voltage indicator. - Radio, 1992, No. 7, p. 18-20.

Author: O. Klevtsov, Dnepropetrovsk, Ukraine

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