This design describes a simple voltmeter with an indicator on twelve LEDs. 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 inverse inputs are connected to a resistor voltage divider, assembled across resistors R1 and R2, through which a controlled voltage is supplied to the circuit.
The non-inverting inputs of the operational amplifiers receive a reference voltage from a divider made across 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 will not light up.
When the measured voltage is received at the input of the LED indicator, a low logical level will be established at certain outputs of the op-amp comparators, and accordingly the LEDs will receive a high logical level, as a result of which the corresponding LED will light up. To prevent the supply of voltage level at the input of the device there is a protective zener diode of 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 used. The inverting inputs generate 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 across resistances R1 and R7.
LEDs act as flashing indicators. To configure, a voltmeter is connected to the battery, then the variable resistor R6 is adjusted so that the required voltages are present at the inverting terminals. Fix the indicator LEDs on the front panel of the car and plot next to them the battery voltage at which one or another indicator lights up.
So, today I want to look at another project using microcontrollers, but also very useful in the daily work of a radio amateur. This is a digital device based on a modern microcontroller. Its design was taken from a radio magazine for 2010 and can easily be converted to an ammeter if necessary.
This simple design of a car voltmeter is used to monitor the voltage of the car's on-board network and is designed for a range from 10.5 V to 15 volts. Ten LEDs are used as an indicator.
The heart of the circuit is the LM3914 IC. It is able to estimate the input voltage level and display the approximate result on LEDs in dot or bar mode.
The LEDs display the current value of the battery or on-board network voltage in dot mode (pin 9 is not connected or connected to the minus) or column mode (pin 9 to the power plus).
Resistance R4 regulates the brightness of the LEDs. Resistors R2 and variable R1 form a voltage divider. Using R1, the upper voltage threshold is adjusted, and using resistor R3, the lower threshold is adjusted.
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 dot mode) or all LEDs (in column mode).
Then we apply 10.5 volts to the input and R3 achieves the glow of VD1. And then we increase the voltage level in steps of half a volt. Toggle switch SA1 is used to switch between dot/column display 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 lights up, indicating a low voltage level on the vehicle’s on-board network.
If the voltage is in the range from 12 to 14 volts, the zener diode VD1 unlocks VT1. HL2 lights up, indicating normal battery level. If the battery voltage is above 15 volts, the zener diode VD2 unlocks VT2, and the HL3 LED lights up, indicating a significant excess of voltage in the vehicle network.
Three LEDs are used as an indicator, as in the previous design.
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 forms the reference voltage for operation of the op-amp.
Electronic homemade products to help the motorist
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 it does require the ability to easily and quickly read the readings. A discrete LED voltage indicator best meets these conditions. Such devices have become very widespread for assessing voltage and power levels (in sound amplification equipment). They are usually implemented in two ways.
The first is described in detail in. 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, you have to pay with an unclear threshold for lighting the LEDs (as the author notes in). Such devices were once sold in the form of radio sets.
The second method is to use a separate comparator to turn on each LED, comparing part of the input signal with a reference one (as, for example, in), Due to the high gain of the comparators, most often performed on op-amps, the on and off thresholds are very clear, but the indicator requires many microcircuits . Quad op amps are currently still expensive, and one such chip can only drive four LEDs.
Finally, one cannot fail to note work (4), where the principle of analog-to-digital conversion is used. This design has many advantages, but still there are a lot of parts, and also uneconomical.
The voltmeter brought to your attention is optimized in light of the above - in it, clear threshold levels for LED ignition are obtained using a minimum of cheap, economical and widely available elements. The operating principle 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 is chosen to be 10...15 V in steps of 1 V. Both the interval and the step can be easily changed.
Threshold devices are six inverters DD1.1-DD1.6, each of which is a nonlinear voltage amplifier with a high gain. The threshold switching level of inverters is approximately half the supply voltage of the microcircuit, so they seem to compare the input voltage with half the supply voltage.
If the inverter input voltage exceeds the threshold level, a low level voltage will appear at its output. Therefore, the LED serving as the load of the inverter will be turned on by the output (inflowing) current. When the output of the inverters is high, the LEDs are closed and turned off.
From the outputs of the resistive divider R1-R7, the corresponding share of the on-board network voltage is supplied to the input of the inverters. When the on-board 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 resistors R1-R7 are calculated in such a way as to obtain a switching step equal to 1 V.
Capacitor C2 together with resistor R1 form a low-frequency filter that suppresses short-term voltage surges that can occur, for example, when starting an engine. The manufacturer of microcircuit stabilizers recommends installing capacitor C1 to improve their stability at high frequencies. 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 forces us to choose a current through the divider that is much greater 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 more than the leakage current. Then the total resistance of the divider RД=R1+R2+RЗ+R4+R5+R6+R7 (in kilo-ohms, if the voltage is in volts and the current is in milliamps) should be equal to: Rд=Uвx min/Imin = 10V/0.1mA = 100kOhm.
Now let’s calculate the resistance of each of the resistors under the condition Upor = Upit/2, i.e. in the case under consideration Upor = 3 V. With an input voltage of 15 V, 3 V should drop across resistor R7, and the current through it (equal to the current through the entire divider) Id=UBX/Rd=15 V/100 kOhm=0.15 mA=150 μA, Then the resistance of resistor R7: R=Upop/Id; R7=3 V/0.15 mA=20 kOhm.
At the input of the inverter DD1.5 there should be 3 V with an input voltage of 14 V. The current through the divider in this case is Id = 14 V/100 kOhm = 0.14 mA. Then the total resistance R6+R7=Upop/Id=3/0.14-21.5 kOhm.
Hence R6=21.5-20=1.5 kOhm.
The resistance of the remaining resistors of the divider is determined in the same way: R5=UporkhRd/Uin-(R6+R7)-1.6 kOhm; R4-2 kOhm, RЗ-2.2 kOhm, R2-2.7 kOhm and, finally, R1=Rд-(R2+RЗ+R4+R5+R6+R7) = 70 kOhm-68 kOhm.
In general, as is known, the threshold voltage of CMOS microcircuit elements is in the range from 1/3Upit to 2/3Upit. It is also known that elements of one microcircuit manufactured in a single technological cycle on a single chip have almost identical switching threshold values. Therefore, to accurately set the “beginning of the scale” of the voltmeter, it is enough to replace resistor R1 with a series circuit consisting of a trimmer with the calculated value and a constant one with a value half the calculated value.
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 temperature stability of no worse than 30 mV within the range of 0...100 °C.
The output voltage of the DA1 stabilizer should not be less than 6 V, otherwise the inverters will not be able to provide the required current through the LEDs. Inverters of the K561LN2 microcircuit allow an output current of up to 8 mA. AL307BM LEDs can be replaced with any others by recalculating the values of current-limiting resistors R8-R13. Capacitors can also be any with a rated voltage of at least 10 V.
To set up, the assembled device is connected to the output of an adjustable voltage source, which will simulate the on-board network. Having set the output voltage of the source to 10 V, and the resistance of the trimming resistor to the maximum, rotate its slider until the HL1 LED turns on. The remaining levels are set automatically.
The voltmeter parts are mounted on a printed circuit board made of foil-coated fiberglass laminate 1 mm thick. The board drawing 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 stands and another one of the same type, which simultaneously presses the DA1 chip to the board. Note that this microcircuit is installed with a plastic (not metal) edge to the board. A tubular stand is also installed between the chip body and the board, but it is shortened.
Before installation, the LED leads are bent by 90 degrees so that their optical axes are parallel to the plane of the board. The LED housings should protrude beyond the edge of the board and, during final assembly of the device, go into the holes drilled in the end of the box.
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.
O. KLEVTSOV, Dnepropetrovsk, Ukraine
Radio magazine 1998, number 2
Note from the editors of Radio magazine: 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 random voltage surges in the on-board network, the amplitude of which can reach 80 - 00 V, another capacitor should be connected in parallel with this capacitor - an oxide one. It must have a capacitance of at least 1000 μF and a rated voltage of 25 V. This capacitor will also have a beneficial effect on the operation of radio and sound amplification equipment for automobiles.
Quite a few motorists are faced with such a problem as unexpected battery discharge. It is especially unpleasant when this happens on the road far from home. One of the reasons may be the failure of the car's generator. Helps prevent impending battery drain car voltmeter. Below are some simple diagrams of such a device.
Automotive voltmeter on LM3914 chip
This car voltmeter circuit is designed to monitor the voltage of the car's on-board network in the range from 10.5V to 15V. 10 LEDs are used as indicators.
The basis of the circuit is integrated. This microcircuit is capable of estimating the input voltage and displaying the result on 10 LEDs in dot or column mode. The LM3914 chip is capable of operating in a wide power supply range (3V...25V). The brightness of the LEDs can be adjusted using an external variable resistor. The outputs of the microcircuit are compatible with TTL and CMOS logic.
Ten LEDs VD1-VD10 display the current value of the battery voltage or the voltage of the vehicle’s on-board network in dot mode (pin 9 is not connected or connected to the minus) or column mode (pin 9 is connected to the power plus).
Resistor R4 connected between pins 6,7 and the power supply minus sets the brightness of the LEDs. Resistors R2 and variable resistor R1 form a voltage divider. Using variable resistor R1, the upper voltage level is adjusted, and using R3, the lower one.
As mentioned earlier, this car voltmeter provides an indication of 10.5 to 15 volts. Calibration of the circuit is performed as follows. Apply 15 volts from the power supply to the input of the voltmeter circuit. Then, by changing the resistance of resistor R1, it is necessary to ensure that the VD10 LED (in dot mode) or all VD...VD10 LEDs (in column mode) light up.
Then apply 10.5 volts to the input and use variable resistor R3 to ensure that only LED VD1 lights up. Now increasing the voltage in 0.5 volt increments, the LEDs will light up one by one, and at a voltage of 15 volts, all the LEDs will light up. Switch SA1 is designed to switch between dot/column indication modes. When the SA1 switch is closed, it is a column; when it is open, it is a dot.
Car voltmeter using transistors
The following circuit of a car voltmeter is built on two. When the voltage on the battery is less than 11 volts, the zener diodes VD1 and VD2 do not pass current, which is why only the red LED lights up, indicating low voltage on the vehicle's on-board network.
If the voltage is between 12 and 14 volts, the zener diode VD1 opens the transistor VT1. The green LED lights up indicating normal voltage. If the battery voltage exceeds 15 volts, the zener diode VD2 opens the transistor VT2, as a result of which the yellow LED lights up, indicating a significant excess of voltage in the vehicle network.
Voltmeter on operational amplifier LM393
This simple car voltmeter is built on an operational amplifier. As an indicator, as in the previous circuit, three LEDs are used.
When the voltage is low (less than 11V), the red LED lights up. If the voltage is normal (12.4…14V), then the light turns green. If the voltage exceeds 14V, the yellow LED lights up. Zener diode VD1 forms the reference voltage. This scheme is similar to the scheme.
Automotive voltmeter on K1003PP1 microcircuit
This voltmeter circuit for a car is built on the K1003PP1 microcircuit and allows you to monitor the voltage of the on-board network by the glow of 3 LEDs:
- When the voltage is less than 11 volts, the HL1 LED lights up
- At a voltage of 11.1…14.4 volts, the HL2 LED lights up
- When the voltage is more than 14.6 volts, the HL3 LED lights up
Setup. After applying voltage to the input from any power supply (11.1...14.4V), variable resistor R4 must be used to make the HL2 LED glow.
This is a description of a simple pseudo-analog voltmeter. The measured value is read in the form of LED points, stylized as a pointer sensor (although it can also be done in the form of an LED ruler), but the measurement occurs in digital form, using a microcontroller. The voltmeter was created as a complement to the regulated power supply and was made from radio elements available on hand.
Schematic diagram
The voltmeter consists of two parts: a display and a measuring module. Here is a regular 5 V power supply, an Atmega8 MCU with an external reference voltage source and registers with 32 LEDs.
Simple LED voltmeter - digital part diagram The main voltage measurement range is 1-32 V with a resolution of 1 V, but it was decided to add an automatic range change of 0.1-3.2 V with a resolution of 0.1 V.
Simple LED voltmeter - indicator circuit The operating principle is based on voltage measurement using two converters ADC0 and ADC1. Converter ADC1 is used to determine the measuring range. The value from this sensor allows you to control and add resistor R9 through the PC2 port pin - forming a 1:10 divider, or turning it off. For voltages of 0.1-3.2 V, the input voltage from CON2 is supplied through resistor R8 and goes directly to the input of the converter ADC0. If the voltage exceeds the set value of 3.3 volts, it switches from the low range (the green LED33 diode lights up) to the high range.
To use such a voltmeter for a 15 V power supply, instead of a 1:10 divider, you can install a 1:4 divider, which gives a range of up to 16 V with a resolution of 0.5 V. Since not everyone will like switching ranges, you can refuse this and make one range by connecting R9 directly to ground, cutting the connection to PC2 pin, ADC1 unused, you can also connect to ground.
Diodes D2-D5 (together with R8, R10) represent the simplest protection of converters from supplying voltage higher than the Atmega supply voltage, that is, 5 V. Capacitors C7, C8 additionally filter the calculated voltage. The Atmega's internal voltage reference was abandoned due to its instability. The reference voltage is performed on TL431. The reference voltage was fixed at 3.3 V. Fine tuning is done using a potentiometer. Resistors R3 and R4 allow you to select the voltage adjustment range of the potentiometer.
The power supply for the analog part of the MK is also typical, using a 10 µH inductor and a 100 nF capacitor. Divided the mass digital and analog.
The measurement voltages are transmitted sequentially to the registers by signals labeled CLK, D and C., which are output to the CON4 connector.
Switching modes
The voltmeter can operate in “luminous point” mode according to the standard setting, or in LED ruler mode. Changing the mode is carried out by changing the state of contact PB0, pin 14. Connecting to ground is a point mode, disconnecting this contact from ground is switching to ruler mode.
Transistor T1, R6, R7 and LED1 form a simple current source, eliminating the need for separate resistors for each of the display's 32 LEDs. The current of such a current source is determined by the rating of R7. The voltmeter is made on single-sided printed circuit boards. Files and firmware - .
The good old way.
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 it does require the ability to easily and quickly determine readings. These conditions are best met by a discrete led indicator voltage. Such devices have become very widespread for assessing voltage and power levels. 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, you have to pay for the unclear threshold for lighting the LEDs. Similar devices were once sold in the form of radio sets.
The second method is to use a separate comparator to turn on each LED, comparing part of the input signal with a reference one. Due to the high gain of the comparators, most often implemented in op-amps, the turn-on and turn-off thresholds are very clear, but the indicator requires a lot of chips. Quad op amps are currently still expensive, and one such chip can only drive four LEDs.
The voltmeter brought to your attention is optimized in light of the above - in it, clear threshold levels for LED ignition are obtained using a minimum of cheap, economical and widely available elements. The operating principle 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 is chosen to be 10...15 V in steps of 1 V. Both the interval and the step can be easily changed.
Threshold devices are six inverters DD1.1-DD1.6, each of which is a nonlinear voltage amplifier with a high gain. The threshold switching level of inverters is approximately half the supply voltage of the microcircuit, so they seem to compare the input voltage with half the supply voltage.
If the inverter input voltage exceeds the threshold level, a low level voltage will appear at its output. Therefore, the LED serving as the load of the inverter will be turned on by the output (inflowing) current. When the output of the inverters is high, the LEDs are closed and turned off.
From the outputs of the resistive divider R1-R7, the corresponding share of the on-board network voltage is supplied to the input of the inverters. When the on-board voltage changes, its shares also change proportionally. The supply voltage of the inverters and the LED line is stabilized by the DA1 stabilizer chip. The values of resistors R1-R7 are calculated in such a way as to obtain a switching step equal to 1 V.
Capacitor C2 together with resistor R1 form a low-frequency filter that suppresses short-term voltage surges that can occur, for example, when starting an engine. The manufacturer of microcircuit stabilizers recommends installing capacitor C1 to improve their stability at high frequencies. 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 forces us to choose a current through the divider that is much greater 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 more than the leakage current. Then the total resistance of the divider RД=R1+R2+RЗ+R4+R5+R6+R7 (in kilo-ohms, if the voltage is in volts and the current is in milliamps) should be equal to: Rд=Uвx min/Imin = 10V/0.1mA = 100kOhm.
Now let’s calculate the resistance of each of the resistors under the condition Upor = Upit/2, i.e. in the case under consideration Upor = 3 V. With an input voltage of 15 V, 3 V should drop across resistor R7, and the current through it (equal to the current through the entire divider) Id=UBX/Rd=15 V/100 kOhm=0.15 mA=150 μA, Then the resistance of resistor R7: R=Upop/Id; R7=3 V/0.15 mA=20 kOhm.
At the input of the inverter DD1.5 there should be 3 V with an input voltage of 14 V. The current through the divider in this case is Id = 14 V/100 kOhm = 0.14 mA. Then the total resistance R6+R7=Upop/Id=3/0.14-21.5 kOhm.
Hence R6=21.5-20=1.5 kOhm.
The resistance of the remaining resistors of the divider is determined in the same way: R5=UporkhRd/Uin-(R6+R7)-1.6 kOhm; R4-2 kOhm, RЗ-2.2 kOhm, R2-2.7 kOhm and, finally, R1=Rд-(R2+RЗ+R4+R5+R6+R7) = 70 kOhm-68 kOhm.
In general, as is known, the threshold voltage of CMOS microcircuit elements is in the range from 1/3Upit to 2/3Upit. It is also known that elements of one microcircuit manufactured in a single technological cycle on a single chip have almost identical switching threshold values. Therefore, to accurately set the “beginning of the scale” of the voltmeter, it is enough to replace resistor R1 with a series circuit consisting of a trimmer with the calculated value and a constant one with a value half the calculated value.
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 temperature stability of no worse than 30 mV within the range of 0...100 °C.
The output voltage of the DA1 stabilizer should not be less than 6 V, otherwise the inverters will not be able to provide the required current through the LEDs. Inverters of the K561LN2 microcircuit allow an output current of up to 8 mA. AL307BM LEDs can be replaced with any others by recalculating the values of the current-limiting resistors R8-R13. Capacitors can also be any with a rated voltage of at least 10 V.
To set up, the assembled device is connected to the output of an adjustable voltage source, which will simulate the on-board network. Having set the output voltage of the source to 10 V, and the resistance of the trimming resistor to the maximum, rotate its slider until the HL1 LED turns on. The remaining levels are set automatically.
The voltmeter parts are mounted on a printed circuit board made of foil-coated fiberglass laminate 1 mm thick. The board drawing 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 stands and another one of the same type, which simultaneously presses the DA1 chip to the board. Note that this microcircuit is installed with a plastic (not metal) edge to the board. A tubular stand is also installed between the chip body and the board, but it is shortened.
Before installation, the LED leads are bent by 90 degrees so that their optical axes are parallel to the plane of the board. The LED housings should protrude beyond the edge of the board and, during 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 random voltage surges in the on-board network, the amplitude of which can reach 80 - 00 V, another capacitor should be connected in parallel with this capacitor - an oxide one. It must have a capacitance of at least 1000 μF and a rated voltage of 25 V. This capacitor will have a beneficial effect on the operation of radio receivers and car audio amplifiers.
Literature
