Help with LM3914 Tachometer w/ shift light

Thread Starter

blockbuster4664

Joined Sep 16, 2021
5
Hello, I'm currently working on an LED bar tachometer using an LM3914, with the alarm flasher functionality being used as a shift light.

I followed a circuit diagram from this post (https://forum.allaboutcircuits.com/threads/led-tachometer-lm3914.37656/) about setting the input to be 0-5v, however I adjusted the R1 & R2 values so that my input range is ~1-8.5v. This is acceptable for me within the range of the PWM converter I am using that I purchased off of amazon (outputs 0-10V analog from PWM).

My circuit is mostly functional however the flashing only occurs in a small window of voltage values, while the original example circuit flashes at all voltage values above the threshold where the last LED turns on. Additionally, the LEDs will only flash when the voltage starts at the upper end of the region. ie voltage will rise through the "flashing" region, but the LEDs actually flash only when voltage is dropping back down through it (see video for demo). It appears to be an issue with the values of the resistors and capacitors associated with the flashing circuit (470ohm resistor, 47uF cap, 1k resistor, and 100ohm resistor) but I do not have the analog circuits knowledge to diagnose which are at fault and how fix it it. If anyone is able to help me identify what changes I need to make to the flasher circuit to get it functional, that would be amazing.

I have attached a complete circuit diagram of my design from ECU signal to LM3914, as well as a chart for which LEDs turn on at what LM3914 input voltage, and the equivalent ECU duty cycle. One thing to note about the circuit is that there are resistors inline between the LM3914 pins and the LEDs (330ohm for blue and green LEDs 1-7, and 220ohm for LED 8 cus it's red). I have also attached a video demonstrating the weird flashing behavior.

LED Pin12345678Flash (to V-)
Vactive1.142v3v4v5.05v6.1v7.15v8.17.83-8.15
DUTY% (active low ECU PWM)90%81%71%60.5%50%39%28%18%18-20%

 

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eetech00

Joined Jun 8, 2013
2,644
Hello, I'm currently working on an LED bar tachometer using an LM3914, with the alarm flasher functionality being used as a shift light.

I followed a circuit diagram from this post (https://forum.allaboutcircuits.com/threads/led-tachometer-lm3914.37656/) about setting the input to be 0-5v, however I adjusted the R1 & R2 values so that my input range is ~1-8.5v. This is acceptable for me within the range of the PWM converter I am using that I purchased off of amazon (outputs 0-10V analog from PWM).

My circuit is mostly functional however the flashing only occurs in a small window of voltage values, while the original example circuit flashes at all voltage values above the threshold where the last LED turns on. Additionally, the LEDs will only flash when the voltage starts at the upper end of the region. ie voltage will rise through the "flashing" region, but the LEDs actually flash only when voltage is dropping back down through it (see video for demo). It appears to be an issue with the values of the resistors and capacitors associated with the flashing circuit (470ohm resistor, 47uF cap, 1k resistor, and 100ohm resistor) but I do not have the analog circuits knowledge to diagnose which are at fault and how fix it it. If anyone is able to help me identify what changes I need to make to the flasher circuit to get it functional, that would be amazing.

I have attached a complete circuit diagram of my design from ECU signal to LM3914, as well as a chart for which LEDs turn on at what LM3914 input voltage, and the equivalent ECU duty cycle. One thing to note about the circuit is that there are resistors inline between the LM3914 pins and the LEDs (330ohm for blue and green LEDs 1-7, and 220ohm for LED 8 cus it's red). I have also attached a video demonstrating the weird flashing behavior.

LED Pin12345678Flash (to V-)
Vactive1.142v3v4v5.05v6.1v7.15v8.17.83-8.15
DUTY% (active low ECU PWM)90%81%71%60.5%50%39%28%18%18-20%

The LM3914 is designed to operate in a linear fashion. It expects to light the LEDs by measuring equally spaced voltage divisions at its input. For example, Led=1v, led2=2v, led3=3v, led4=4v....and so on, are "linear" divisions. So you'll need to make a compromise. What linear divisions can you live with?
 

Thread Starter

blockbuster4664

Joined Sep 16, 2021
5
The LM3914 is designed to operate in a linear fashion. It expects to light the LEDs by measuring equally spaced voltage divisions at its input. For example, Led=1v, led2=2v, led3=3v, led4=4v....and so on, are "linear" divisions. So you'll need to make a compromise. What linear divisions can you live with?
the absolute range has to be within ~0-9V to fit within my PWM-voltage converter's output. So a voltage delta of 7-8v across the 8 LEDs would probably be ideaI. I am also only using 8 of the 10 LED outputs so i can "shift" the voltage range up or down by changing which LED outputs are being used (ex leds 0-8, vs 2-10). I can always change values in the ECU's PWM table, so this is more about getting the correct flashing behavior, then having exact division values.
 

Thread Starter

blockbuster4664

Joined Sep 16, 2021
5
I'd recommend the LM2917 to convert the signal from the points to the analogue signal for the LM3914
I've tried in the past to use an LM2917 however the setup i am using is not a standard tachometer signal. The signal does not use distributor points either. I am using the PWM table functionality of my PE3 ECU to map PWM duty cycle to RPM via a digital output pin. The ECU does offer a "tachometer" signal wire but I found PWM to be easier for me to understand the signal. I was never able to get an LM2917 to properly function so i've since given up on using the tach wire.
 

eetech00

Joined Jun 8, 2013
2,644
the absolute range has to be within ~0-9V to fit within my PWM-voltage converter's output. So a voltage delta of 7-8v across the 8 LEDs would probably be ideaI. I am also only using 8 of the 10 LED outputs so i can "shift" the voltage range up or down by changing which LED outputs are being used (ex leds 0-8, vs 2-10). I can always change values in the ECU's PWM table, so this is more about getting the correct flashing behavior, then having exact division values.
See below.
The LEDs will light in 1v increments.
The 0-10v input is divided in half for a 0-5v input signal, so LED's light in 0.5 volt increments (actual). This means when the input signal is 1v-LED1 will light, 2v-LED2 will light, etc. R1/R2 should be 1%, or R2 can be a Pot. The LM3914 outputs are current limited, so resistors aren't needed.

1634087282083.png

1634087326027.png

EDIT: Added R7
 
Last edited:

Thread Starter

blockbuster4664

Joined Sep 16, 2021
5
See below.
The LEDs will light in 1v increments.
The 0-10v input is divided in half for a 0-5v input signal, so LED's light in 0.5 volt increments (actual). This means when the input signal is 1v-LED1 will light, 2v-LED2 will light, etc. R1/R2 should be 1%, or R2 can be a Pot. The LM3914 outputs are current limited, so resistors aren't needed.

View attachment 250099
thank you so much, I will try this once i have access to my equipment again (currently on break from school). Also do you have a copy of that LTspice model for the 3914, I've been looking into using LTSpice but not having a model was one of my main hesitations to learning the software.
 
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