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 a 12-volt battery and chargers, or even 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.
This article describes a simple voltmeter, the indicator of which is twelve LEDs. This allows you to display the measured voltage in the range from 0 to 12 volts in steps of 1 volt, and the error in the measurement does not exceed 2 percent.
The most suitable application for this LED voltmeter indicator is in regulated power supplies. If you have all the necessary radio components at hand, then the circuit can be assembled in just an hour or two.
Description of the device of the LED voltmeter
will be a logical zero, so the LEDs are off.
When a voltage is applied to the input of the voltmeter, a low logic level will appear at certain outputs of the comparators DA1 ... DA3 (in accordance with the voltage level at the non-inverting terminals of the op-amp).
As follows from the schematic diagram, at different voltage levels at the inputs of integrated circuits DD1 ... DD3, a high logic level is set at their outputs, as a result of which the corresponding LED starts to glow. To limit the voltage at the input of the voltmeter to 12 volts, a zener diode VD2 is included in the circuit.
Details of LED Voltmeter
In the circuit, LM324 op-amps are used as comparators. Their use contributed to a decrease in the total number of microcircuits and other radio elements for interfacing the analog part of the circuit with integrated circuits. Capacitors - KM. All resistances - MLT-0.125, MLT-0.25.
LEDs HL1 - HL12 can be used AL307. The integrated voltage regulator DA5 78L12 can be replaced with KREN8B or 7812. The VD2 zener diode can be changed to KS212 with the letter E or Zh. The voltmeter circuit is powered by an unstabilized DC voltage source from 13 to 16 volts with a load current of at least 12 mA.
Source Radioamator, 8/2001
The scheme of the on-board automobile voltmeter with indication on is shown in the figure below:
The device is a six-level linear indicator, in the range from 10 to 15 volts. DA1, on K142EN5B at pin 8, outputs a voltage of 6 volts to power the digital microcircuit DD1 of the K561LN2 type. The inverters of the K561LN2 chip serve as threshold elements, representing nonlinear voltage amplifiers, and the resistors R1 - R7 set the offset at the inputs of these elements. If the input voltage of the inverter exceeds the threshold level, the output voltage will be low, the output LED of the corresponding inverter will be lit.
The printed circuit board of the on-board LED voltmeter with the layout of parts on it, 80x45 mm in size, is shown in the figures below:
When establishing an on-board LED voltmeter, instead of a battery, a laboratory stabilized source of 10 volts is connected, setting a temporary trimming resistor instead of resistor R1. By changing the resistance R1, they achieve the moment when the HL1 LED turns on. The remaining levels are set automatically. With a detailed check of the remaining levels, the resistances R2 - R6 are specified, respectively.
Greetings to all. Today I will talk about a voltmeter. Many people remember what a voltmeter is from grade 8 school physics lessons. And to be more precise, a voltmeter (volt + gr. μετρεω I measure) is a direct reading measuring device for determining voltage or EMF in electrical circuits. Connected in parallel with the load or power source. (As defined by Wikipedia)
An ideal voltmeter should have an infinitely large internal resistance. Therefore, the higher the internal resistance in a real voltmeter, the less influence the device has on the measured object and, consequently, the higher the accuracy and the more diverse the scope. Unfortunately, this does not apply to our device, since current is supplied through the wires with which we measure to power the circuit and indicators.
According to the principle of operation, our voltmeter is electronic, digital. This means that the microcircuit that is installed inside measures the signal and converts it into digital form for ease of perception.
In the last century, pointer voltmeters were common, such as: 
However, they are still widely used today.
But perhaps you are more familiar with other pictures:
level indicator / voltmeter in the tape recorder 
or even in a car of the VAZ classic family 
Pointer voltmeters have a significant drawback - the suspension of the coil with an arrow, which require careful handling and are designed to work in only one position (otherwise, the measurement error increases). This disadvantage is not present in electronic devices. Soviet industry has mastered specialized microcircuits such as 572PV2 and 572PV5, but they are also obsolete.
Delivery:
The usual package, no pimples and other protection. 
Came by regular mail without a track in about 40 days from the date of order.
Claimed characteristics and reality:
- Measurement range 3.2-30 Volts.
-Protection against incorrect inclusion
Protective diode installed.
- At voltages below 10 Volts, the accuracy is 0.01 V + - 1 digit
- At voltages above 10 volts, the accuracy is 0.1 V
- Red color LEDs
Available with other colors of seven-segment indicators
-Does not require power
In fact, it is powered by the wires on which the measurement is made
- The measurement is made on two wires
- The display consists of 3 LED seven-segment indicators with a height of 0.56 inches, which corresponds to approximately 14 mm
- Data update time 5 times per second
-Maximum changeable voltage 30 volts
Limited by on-board stabilizer
-Minimum 3.2 Volts.
In fact, about 3.6 volts.
-Declared Accuracy:
0.01V when measuring up to 10V and 0.1V from 10V and above, no more than 1% ± 1 digit
Compliant (ADC 12 bit)
- Temperature range -10℃~65℃
- Dimensions: 48mm x 29mm x 22mm (L*W*H)
Landing hole: 46*27mm
-Consumption current no more than 20mA
The current consumption depends on the numbers on the indicator - the more segments are lit, the more current is consumed, but not more than 20 mA
Appearance with small details:
The dimensions correspond to the declared, which is not surprising. Therefore, I will not dwell on them in detail.
Tabs for fixing the voltmeter in the window: 
The board dangles a little in the case, is “treated” with a drop of sealant or glue.
An empty case and a protective film, it also acts as a light filter: 
The film on the front side is matte, due to which there are relatively few glare when exposed to light: 
3 digit indicator. They didn't even shoot the film.
Photo for comparison 
The film "works in the light" With glare, it is quite acceptable: 
parameters are readable.
We finally got to the board:
Soldering is quite neat, no traces of flux were found. 
D1 protection diode prevents components from being damaged if connected incorrectly (reverse polarity). U2 stabilizer 7133H Holtek (3.3 Volts) is powered by a microcircuit. Based on the fact that a minimum of 0.1 Volts drops on the stabilizer (low drop series), and at least 0.2 Volts drops on the diode, therefore, the minimum power supply of the voltmeter, at which stable values are guaranteed, must be at least 3.6 Volts. Which does not match the declared seller. Resistors 221 (8 pieces) limit the current of the indicator segments.
The label on the controller has been removed. Initially, I thought that some kind of PIC16 was being used, but I did not find a case with 16 legs in the catalog, so I still leaned towards the thought of a Holtek series controller. In any case, a 12 bit ADC is overkill for 30 Volts and 1 decimal place accuracy. With a little stretch, an 8-bit ADC could be used.
Tests:
They are reduced to a banal comparison with existing devices.
Do not pay attention to the minus readings, this is how electricians use it in our country, but I didn’t notice right away.
Twisting wires for simultaneous connection is not a way out. I used wago spring terminal blocks. 
Work from 3.2 Volts is declared, but the internal stabilizer requires a minimum of 3.4 Volts at the input. 
forgot to switch to a higher range 
In general, the accuracy is relatively high, and it was even found that clamp meters underestimate the readings, so I will ignore them as a guide.
I did not freeze the board, but I tried to heat it with a hair dryer up to about + 50C. The results have not changed.
Occupied a child seat 
bad photo
A small optional video about glare and the refresh rate of readings for clarity:
Conclusions:
Various homemade products - the direct purpose. If the gaps are sealed, then they can be used as protected IP 67. One of the reasons that prompted me to buy these voltmeters is that the old stocks of pointer voltmeters are running out. I use them in homemade car battery chargers based on a vacuum tube transformer. Unfortunately, there are no photos of the finished device - consumers ignore my request to send photos in work. I will not post a link to an outside resource, if you wish, you can send it in a personal message.
There are also cheaper versions of voltmeters on sale - without a case.
Pros:
The design of the housing with a frame (panel version) makes it possible to close your eyes to an inaccurately made mounting hole
Big and bright numbers
There are several colors
The screen is almost non-glare
Accuracy corresponds to +-1 last digit
Minuses:
Power requires 3.6 Volts (claimed 3.2)
The board dangles slightly in the case.
This design describes a simple voltmeter with twelve LED indicators. This measuring device allows you to display the measured voltage in the range of values from 0 to 12 volts in steps of 1 volt, and the measurement error is very low.
Voltage comparators are assembled on three LM324 operational amplifiers. Their inverted inputs are connected to a resistor voltage divider, assembled on resistors R1 and R2, through which a controlled voltage is supplied to the circuit.
The reference voltage is supplied to the non-inverting inputs of operational amplifiers from a divider made on resistances R3 - R15. If there is no voltage at the input of the voltmeter, then the outputs of the op-amp will have a high signal level and the outputs of the logic elements will have a logical zero, so the LEDs do not light up.
When the LED indicator of the measured voltage arrives at the input, a low logic level will be set at certain outputs of the comparators of the op-amp, respectively, a high logic level will go to the LEDs, as a result of which the corresponding LED will light up. To prevent the supply of a voltage level at the input of the device, there is a protective zener diode for 12 volts.
This version of the scheme discussed above is perfect for any car owner and will give him visual information about the state of charge of the battery. In this case, four built-in comparators of the LM324 microassembly are involved. Inverting inputs form reference voltages of 5.6V, 5.2V, 4.8V, 4.4V, respectively. The battery voltage is directly supplied to the inverting input through a divider on the resistances R1 and R7.
The LEDs act as flashing indicators. For tuning, a voltmeter is connected to the battery, then the variable resistor R6 is adjusted so that the desired voltages are present at the inverting terminals. Fix the indicator LEDs on the front panel of the car and apply the battery voltage next to them, at which one or another indicator lights up.
So, today I want to consider another project using microcontrollers, but also very useful in the daily work of a radio amateur. This is a digital device on a modern microcontroller. Its design was taken from a radio magazine for 2010 and can be easily rebuilt for an ammeter if necessary.
This simple design of an automotive voltmeter is used to monitor the voltage of a car's on-board network and is designed for a range of 10.5V to 15 volts. Ten LEDs are used as indicators.
The heart of the circuit is the LM3914 IC. It is able to estimate the input voltage level and display the approximate result on the LEDs in dot or column mode.
The LEDs display the current value of the battery or on-board network voltage in the mode of a point (pin 9 is not connected or connected to minus) or a column (pin 9 to the power plus).
Resistance R4 controls the brightness of the LEDs. Resistors R2 and variable R1 form a voltage divider. With the help of R1, the upper voltage threshold is adjusted, and with the help of the resistor R3, the lower one.
Calibration of the circuit is done according to the following principle. We apply 15 volts to the input of the voltmeter. Then, by changing the resistance R1, we will achieve the ignition of the VD10 LED (in point mode) or all LEDs (in column mode).
Then we apply 10.5 volts to the input and R3 achieve the glow of VD1. And then increase the voltage level in increments of half a volt. The SA1 toggle switch is used to switch between dot/column indication modes. When SA1 is closed - a column, when open - a dot.
If the voltage on the battery is below 11 volts, the zener diodes VD1 and VD2 do not pass current, which is why only HL1 glows, indicating a low voltage level of the car's on-board network.
If the voltage is in the range from 12 to 14 volts, the zener diode VD1 unlocks VT1. HL2 is lit, indicating a normal battery level. If the battery voltage is higher than 15 volts, the VD2 zener diode unlocks VT2, and the HL3 LED lights up, indicating a significant excess of voltage in the car's network.
As an indicator, as in the previous design, three LEDs are used.
When the voltage level is low, HL1 lights up. If the norm is HL2. And more than 14 volts, the third LED flashes. Zener diode VD1 generates a reference voltage for the operation of the op-amp.