Automotive LED voltage indicator circuit diagram
To monitor the state of electrical equipment in the car, the driver is helped by various devices and indicators. If we talk about self-made such indicator, there are a great many such circuits,
The driver can keep track of the condition of electrical equipment in the car with the help of various devices and indicators. If we talk about self-made such indicator, there are a lot of such circuits,
both on a couple of transistors and on microcontrollers. But first let’s define what is required from such indicator, what readings and in what form it should show for convenient visual perception. After all, every day to represent the on-board voltage in numbers or a long LED scale is just not useful.
In my opinion it is not convenient to be distracted by numbers or LED scale, looking at the readings. In order to assess the condition of the electrical system of the car it is enough to have only a few thresholds that will signal to the driver about its condition. Exactly such a device is presented below.
The whole indication of the device is realized on just two LEDs of red and green glow. In total, the device distinguishes between four states of the power grid of the car:
- Voltage is below 12 V – the red LED blinks;
- Voltage is in the range of 12-13V – red LED lights up;
- Voltage is within the range of 13-14 V – green LED lights up;
- Voltage is above 14V – green LED blinks.
As you can see, the normal mode is the third. The two extreme ones are emergency, and the second one tells the driver to recharge the battery. I think that this is quite enough to assess the condition of the car’s electrical system and to take (or not) any action.
Input voltage from the battery through adjustable voltage divider R4-R1R2R3 goes to the buffer logic elements of the chip DD1 CD4049. A total of three channels are implemented: DD1.1,6; DD1.2,5 and DD1.3. The buffer elements, which are threshold devices, control the start of two low-frequency oscillators at the elements DD2.4, DD2.3 and DD2.1, DD2.2. The frequency of these oscillators is about 2 Hz. The outputs of the oscillators are opened by two transistors VT1 and VT2, to which LEDs VD1, VD2 of red and green color are connected. The circuit is powered through a stabilizer on the elements VD3, R9, C4.
When the on board voltage is less than 12 V, the output of DD1.6 will be a logic zero, respectively, and the oscillator DD2.4D2.3 will run. This will cause transistor VT1 to open periodically (2Hz) and LED VD1 to flash red. On the contrary, the output of DD1.3 will have a logic one and the oscillator of DD2.1, DD2.2 will not start and LED VD2 will be off.
At voltages between 12 and 13V the output level of DD1.6 will change to one, which will cause the oscillator DD2.4, DD2.3 to stop. But its high output level (10) will keep transistor VT1 open and this will keep LED VD1 continuously lit.
When the on-board voltage is normal (13…14V), the output of element DD1.5 will set one which will zero output 10 of DD2.3, transistor VT1 will close and LED VD1 will extinguish. At the same time, a logic zero from output DD1.2 will set a high level on output 4 of DD2.2, which means that transistor VT2 will be permanently open and LED VD2 will glow green continuously.
When the voltage exceeds 14V the oscillator DD2.1, DD2.2 will start and by analogy with the first oscillator the green LED will blink.
The printed circuit board for the circuit is shown in the figure above (download link). The board is double sided and there is no other way to make it work. You have to adjust the oscillator with a laboratory power supply by setting the appropriate voltage levels and adjusting the trim resistors R1-R3. When doing this you have to be guided by the logic levels of the elements.
Battery voltage indicator on LM3914
This device is a LED voltmeter (voltage indicator) of 12V battery, using the well known LM3914 chip (datasheet).
This device was necessary for me to know when the car battery is fully charged by the charger. The charger was of the old type and had no indicator arrows or digital readouts to measure the voltage.
For the LED bar indicator I chose a HDSP-4832 with 10 LEDs of three different colors: three red, four yellow and three green.
For proper voltage indication, it is necessary to determine the lower and upper level of the measured voltage, so that the first and last LEDs (bars) on the indicator light up respectively at these levels.
For a 12V car battery, the following ranges were chosen: the first LED lights up at 10V, and the last one at 13.5V, so the voltage indication step is 0.35V per LED. Of course you can also set other voltages with two trimmers. This makes it possible to use this indicator to measure the voltage of e.g. NiCd or NiMH batteries. The voltage limits in this case are set to Vmin = 0.9 * Ncells and Vmax = 1.45 * Ncellswhere Ncells – is the number of “cells” of the battery. Plus between + and – of the batteries a powerful resistor must be placed, designed for a current of at least 0.5A to simulate a real load.
The LM3914 chip can operate in two modes: the “dot” mode, which lights up only one LED, and the “bar” mode, which lights up several LEDs in ascending sequence. This circuit operates in the “bar” mode, for this purpose pin 9 of the chip is connected to the plus of the power supply.
When operating in bar mode, the power consumption of the LM3914 increases accordingly. When all 10 indicator segments are lit, the LM3914 consumes almost 10 times more than if only one LED (segment) were lit. To prevent the m/s LM3914 from burning out, care must be taken to ensure that the LED current does not exceed the maximum allowable current.
The maximum power dissipation of the chip should not exceed 1365 mW. And assuming that the maximum voltage is 14.4V, the maximum possible current is I = P/V = 1.365/14.4 = 94.8mA. Therefore, the current of each segment of the indicator should not exceed 94.8/10=9.5mA. In the circuit, the resistance of resistor R3 (4.7 kOhm) sets the maximum current of the LEDs. The LED current is about 10 times the current that passes through this resistor IR3 = 1.25 / 4700 = 266 µA. So the current per LED is limited to 2.6mA, which is much less than the allowable current.
Input stage: In order to read out the input voltage (and to feed the circuit with it) a 1:2 voltage divider connected to pin 5 of the IC is used in the circuit. The divider consists of two 10kOhm resistors, so the voltage taken from the divider is in the range of 5V to 6.75V, the input voltage will be 10V to 13.5V. The same values will be used to calibrate the LM3914.
Schematic diagram of the indicator
The circuit consists of two elements, one for the control circuit and one for the indicator board. They are connected to each other with a 11 pin connector.
The main setting elements of the circuit: R1 and R2 – voltage divider R3 and R4 – current limitation of LEDs and R5 – voltage lower limit setting
I talked about R1, R2 and R3 above. Now let’s take apart R4, which sets the upper threshold (pin 6 m/s): On pins 6 and 7 you must set the voltage at 6.75V (which is the input voltage of 13.5V after the divider, in case the battery is fully charged). Knowing the current through R3 and adding the “error current” from pin 8 of the chip (120mA), we can calculate the resistance of R4: 6.75V = 1.25V + R4(120mA+266mA) R4 = (6.75 – 1.25)/(386mA) R4 = 14. With a 22 kOhm trimmer we can control the voltage on pin 7 in the range from 1.25V to 9.74V, which makes it possible to set the upper voltage limit from 2.5V to 19.5V.
The resistance R5 sets the lower voltage limit: By substituting in the formula VO = VI * RB/(RA + RB) the following values: RA = 10 * 1K internal LM3914 resistors RB = R5 VI = upper voltage limit 6.75V VO = lower voltage limit 5V we get: 5 = 6.75 * R5/(R5 + 10K) R5 = 28.5K or more (we choose a 100kOhm trim resistor)
The printed circuit board
As already said before the device consists of two components, so two different printed circuit boards are used. This gives the possibility to use a remote indication e.g. on a car dashboard.
In the printed circuit board we have only one jumper (marked in red).