Almost all the appliances that are produced today contain LEDs. They literally surround us from all sides, from lamps and flashlights to voltage detection in literally all household appliances. They are often used to illuminate screens, control various devices, etc.
Most often in technology, LEDs of five colors are used:
- white,
- red,
- green,
- yellow,
- blue.
They can also produce infrared and ultraviolet radiation. These are indispensable in control systems: remote controls for TVs, air conditioners and other household appliances.
We will look at how LEDs can be used to determine the voltage of devices. The main device for measuring voltage is a voltmeter. How can LEDs be useful here? They will become our visible indicators.
Usually, as a sample, an example of a voltmeter based on 12 LEDs is given. Accordingly, it can index the voltage in the range from 0 to 12 volts. Such a device can be used very effectively to measure power supplies that can be regulated. It will also be indispensable for radio amateurs, in particular for creating small devices at home.
LEDs - indicators
Using an LED as an indicator also has its own laws that you need to know if you are assembling a device with your own hands.
- It is important to observe the polarity. An LED is a semiconductor device that has two terminals: a cathode and an anode. It will only work if connected directly.
- Voltage limit. Each LED has its own. If this value is exceeded, it will break.
- As indicators, it is recommended to use LEDs that burn brightly enough at a voltage of 5 mA.
LED voltmeters
If the error of the voltmeter is no more than 4%, then it can be safely called an indicator. Such a device can be easily made with your own hands using LEDs. You can use such a voltmeter to indicate microcircuits under 5 volts. The indicators will be 6 LEDs within the range of 1.2 - 4.2 volts with an interval of 0.6 volts. The LEDs should draw 60 microamps.
The principle of operation of the indicator is based on fixing the voltage drop in the transitions: the base is the emitter of transistors and direct drops on diodes (0.6 volts).
A diagram of such a voltmeter is easy to find on the Internet.
How to assemble a voltmeter for a car battery?
This voltmeter can be used for both 12 volt battery and chargers, or on its own.
The indicator will consist of 10 LEDs with a quarter volt difference. The voltage measurement will be in the range of 10.25 - 15 volts.
Power is supplied from the voltage that you will measure.
The basis of the circuit of such a voltmeter are two polycomparator microcircuits with a linear indication law.
A microcircuit is a set of 10 comparators and resistors that form a voltage divider. The output comparators have key stages for driving the LEDs. In order for the microcircuits to work in series, resistor dividers are included in this (serial) order.
We install LEDs in one line. You can take both LED strips and 10 individual LEDs. For a voltmeter, LEDs of any type are suitable.
The good old way.
A voltmeter installed on the dashboard of a car allows you to quickly control the voltage level in its on-board network. Such a device does not require high resolution, but the ability to easily and quickly determine the readings is necessary. These conditions are best met by a discrete led indicator voltage. Similar devices have become very widespread for assessing the level of voltage and power. They are usually implemented in two ways.
First, 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. For the simplicity of such an indicator, one has to pay with a fuzzy threshold for the ignition of LEDs. 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. Due to the high gain of the comparators, most often implemented on op amps, the on and off thresholds are very clear, but the indicator requires a lot of chips. Quad op-amps are still expensive now, and one such chip can only drive four LEDs.
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 threshold switching level of inverters is about half the supply voltage of the microcircuit, so they, as it were, 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 (sink) current. When the output of the inverters high level, 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 microcircuit stabilizer DA1. 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 induced 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 \u003d Upit / 2, i.e. in the case under consideration Upr \u003d 3 V. With an input voltage of 15 V, 3 V should fall on the resistor R7, and the current through it (equal to the current through the entire divider) Id \u003d UBX / Rd \u003d 15 V / 100 kOhm \u003d 0.15 mA \u003d 150 μA, Then the resistance of the resistor R7: R \u003d Upor / Id; R7=3V/0.15mA=20kΩ.
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 \u003d 14 V / 100 kOhm \u003d 0.14 mA. Then the total resistance R6 + R7 \u003d Upop / Id \u003d 3 / 0.14-21.5 kOhm.
Hence R6 \u003d 21.5-20 \u003d 1.5 kOhm.
Similarly, the resistance of the remaining resistors of the divider is determined: R5 \u003d UporkhRd / Uin- (R6 + R7) -1.6 kOhm; R4-2 kOhm, R3-2.2 kOhm, R2-2.7 kOhm and, finally, R1 \u003d Rd- (R2 + R3 + R4 + R5 + R6 + R7) \u003d 70 kOhm-68 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 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 0.1 microns is connected to the input of the microcircuit (between pins 8 and 17). 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 the radio and sound amplifier of automotive equipment.
Literature
Considered are not complex circuits of digital voltmeter and ammeter, built without the use of microcontrollers on microcircuits SA3162, KR514ID2. Usually, good laboratory block There are built-in devices for power supply - a voltmeter and an ammeter. The voltmeter allows you to accurately set the output voltage, and the ammeter will show the current through the load.
Old lab power supplies had dial gauges, but now they should be digital. Now radio amateurs most often make such devices based on a microcontroller or ADC chips like KR572PV2, KR572PV5.
Chip CA3162E
But there are other microcircuits of a similar action. For example, there is a CA3162E microcircuit, which is designed to create an analog value meter with the result displayed on a three-digit digital indicator.
The CA3162E microcircuit is an ADC with a maximum input voltage of 999 mV (while readings are “999”) and a logic circuit that provides information about the measurement result in the form of three alternating BCD four-digit codes at a parallel output and three outputs for polling the bits of the dynamic circuit. indications.
To get a complete device, you need to add a decoder to work on a seven-segment indicator and an assembly of three seven-segment indicators included in the matrix for dynamic indication, as well as three control keys.
The type of indicators can be any - LED, luminescent, gas discharge, liquid crystal, it all depends on the circuit of the output node on the decoder and keys. It uses LED indication on a scoreboard of three seven-segment indicators with common anodes.
The indicators are connected according to the dynamic matrix scheme, that is, all their segment (cathode) outputs are connected in parallel. And for interrogation, that is, sequential switching, common anode outputs are used.
Schematic diagram of a voltmeter
Now closer to the scheme. Figure 1 shows a voltmeter circuit that measures voltage from 0 to 100V (0...99.9V). The measured voltage is supplied to pins 11-10 (input) of the D1 chip through a divider on resistors R1-R3.
Capacitor SZ eliminates the influence of interference on the measurement result. Resistor R4 sets the readings of the device to zero, in the absence of input voltage A, resistor R5 sets the measurement limit so that the measurement result corresponds to the real one, that is, one can say that they calibrate the device.
Rice. 1. circuit diagram digital voltmeter up to 100V on chips SA3162, KR514ID2.
Now about the outputs of the microcircuit. The logical part of the CA3162E is built according to the TTL logic, and the outputs are also with open collectors. At the outputs "1-2-4-8" a binary decimal code is formed, which is periodically replaced, providing serial transmission of data on three digits of the measurement result.
If a TTL decoder is used, such as KR514ID2, then its inputs are directly connected to these D1 inputs. If a CMOS or MOS logic decoder is used, then its inputs will need to be pulled up to positive with resistors. This will need to be done, for example, if the decoder K176ID2 or CD4056 is used instead of KR514ID2.
The outputs of the decoder D2 are connected to segment outputs through current-limiting resistors R7-R13 LED indicators H1-NC. Segment outputs of the same name of all three indicators are connected together. To interrogate the indicators, transistor switches VT1-VT3 are used, to the bases of which commands are sent from the outputs H1-NC of the D1 chip.
These conclusions are also made according to the open collector scheme. Active zero, therefore, transistors of the p-p-r structure are used.
Schematic diagram of the ammeter
The ammeter circuit is shown in Figure 2. The circuit is almost the same except for the input. Here, instead of a divider, there is a shunt on a five-watt resistor R2 with a resistance of 0.1 Ot. With such a shunt, the device measures current up to 10A (0 ... 9.99A). Zeroing and calibration, as in the first circuit, is carried out by resistors R4 and R5.
Rice. 2. Schematic diagram of a digital ammeter up to 10A and more on CA3162, KR514ID2 microcircuits.
By selecting other dividers and shunts, you can set other measurement limits, for example, 0...9.99V, 0...999mA, 0...999V, 0...99.9A, it depends on the output parameters of that laboratory power supply, in which will set these indicators. Also, based on these circuits, you can make an independent measuring device for measuring voltage and current (table multimeter).
In this case, it should be taken into account that even using liquid crystal indicators, the device will consume a significant current, since the logical part of the CA3162E is built according to TTL logic. Therefore, a good self-powered device is unlikely to succeed. But the car voltmeter (Fig. 4) will come out pretty good.
The devices are powered by a constant stabilized voltage of 5V. In the power source in which they will be installed, it is necessary to provide for the presence of such a voltage at a current of at least 150mA.
Connecting the device
Figure 3 shows the connection diagram of the meters in the laboratory source.
Rice. 3. Scheme of connecting meters in a laboratory source.
Fig.4. Homemade car voltmeter on microcircuits.
Details
Perhaps the most difficult to get is the CA3162E microcircuits. Of the analogues, I know only NTE2054. There may be other similar ones that I don't know about.
The rest is much easier. As already said, output circuit can be done on any decoder and corresponding indicators. For example, if the indicators are with a common cathode, then you need to replace KR514ID2 with KR514ID1 (the pinout is the same), and drag the VT1-VTZ transistors down by connecting their collector to the negative power supply, and the emitters to the common cathodes of the indicators. You can use CMOS decoders by pulling up their inputs to the power plus with resistors.
Establishment
In general, it is quite simple. Let's start with a voltmeter. First, we close the conclusions 10 and 11 of D1 to each other, and by adjusting R4 we set zero readings. Then, remove the jumper that closes terminals 11-10 and connect an exemplary device, for example, a multimeter, to the “load” terminals.
By adjusting the voltage at the output of the source, with the resistor R5 we adjust the calibration of the device so that its readings coincide with the readings of the multimeter. Next, set up an ammeter. First, without connecting the load, by adjusting the resistor R5 we set its readings to zero. Now you need a constant resistor with a resistance of 20 Ot and a power of at least 5W.
We set the voltage to 10V on the power supply and connect this resistor as a load. We adjust R5 so that the ammeter shows 0.50 A.
You can also calibrate using a standard ammeter, but it seemed to me more convenient with a resistor, although of course the error in the resistance of the resistor greatly affects the quality of the calibration.
According to the same scheme, you can make a car voltmeter. A diagram of such a device is shown in Figure 4. The circuit from that shown in Figure 1 differs only in the input and power circuit. This device is now powered by the measured voltage, that is, it measures the voltage supplied to it as a power supply.
The voltage from the vehicle's on-board network through the R1-R2-R3 divider is fed to the input of the D1 microcircuit. The parameters of this divider are the same as in the circuit in Figure 1, that is, for measuring within 0 ... 99.9V.
But in a car, the voltage is rarely more than 18V (more than 14.5V is already a malfunction). And it rarely drops below 6V, unless it drops to zero when completely turned off. Therefore, the device really works in the range of 7 ... 16V. The 5V power supply is generated from the same source, using the A1 stabilizer.
The task was to determine the state of the battery during discharge, store it and charge, I had to remember the skills and take up the soldering iron. All circuits with a bunch of comparators and other tricks inspired melancholy with their size - it was easier to tie the multimeter to the battery. Therefore, it was decided to come up with something simple and elegant, as a result, a scheme was born that can be scaled to fit your needs both in width and in depth. Only three elements are used per voltage step - a zener diode, a resistor and an LED (at this point, slap your forehead and exclaim: "How did I not think of it before!"
In general, catch the diagram and photo of the finished device based on one 12-volt lead-acid battery, as in UPSs and cars. Indication from completely discharged (voltage less than 9.5V) to fully charged (voltage greater than 14.6V). If you need other ranges or you want a wider scale, then we take the nearest voltage zener diode and consider the current-limiting resistor for the LED. (1.5V drop, 20mA current).
In general, everything is simple.
If you use SMD components, then you can meet this ten-kopeck coin, well, I didn’t have the task of miniaturization, so I assembled it on a breadboard.
The first red LED indicates that the circuit is connected and there is some voltage. the second is more than 9 Volts, the third, yellow, is more than 10V, the fourth is more than 11V, the fifth, green, is more than 12V and the sixth is more than 13V. Gradations between these points are perfectly visible by the degree of illumination of the corresponding LEDs. In this case, the battery is on charge and is about to be charged.
