Been building auto DC monitors with LM3914 .
Want to use LM3914 or LM3915 for 8 cylinder engine tachometer .
Maybe use LM2907 ?

 

Been building auto DC monitors with LM3914 .
Want to use LM3914 or LM3915 for 8 cylinder engine tachometer .
Maybe use LM2907 ?

Do you have any kind of sensor to sense the speed of the axis and you want to amplify its signal?

or Do you want to build a tachometer entirely from the beginning and you want some ideas?

 

An LM2907/LM2917 seems like a good starting point.
You'd need at least a couple of decades of LM3914's just to get a rough idea of the engine's RPM range. If the RPM range of the engine was 0 to 7000, two decades would give you a resolution of 350 RPM, which is pretty doggone rough. Three decades would give you a resolution of 233.3 RPM per segment.

What's your RPM range, redline, and what resolution do you need?

 

I'm looking at National Semi May 2003 LM29x7 pdf page 10 top :
Voltage Driven Meter Engine RPM .
Has anyone built similar ?
Apparently one wire from "ignition" to pin 1 of LM29x7 .
Zener in LM2917 improves 0.3% accuracy ?
First breadboard for 4 cylinder 2002 Ford Escort ; max RPM 5000 ?
Why 400 Hz ~ 6000 ERPM - 8 cyl ?
Will decide LM3914 or LM3915 , 1 or 2 decades later .

Thanks , Jim D

 

Hi Jim,
An LM3914 will produce a linear display, which is what you want.
An LM3915 will produce a logarithmic display, which would be of limited usefulness for your application.

The "ignition" lead would be from your ignition system's tachometer lead.

Don't know offhand why a Zener diode would improve accuracy.

For your engine specifications, consult your owner's manual, a repair manual, or your local Ford dealer.

Estimated RPM: In 4-cycle Otto engines, the crankshaft makes two revolutions per ignition. So, 6000/2=3000. Now we convert from RPM to RPS. 3000/60=50. In the example, 8 cylinders are given - so we multiply by 8. 50 x 8 = 400 Hz.

In the LM3814 datasheet that National Semiconductor provides, there are a number of examples of connecting the IC for display of multiple decades of LEDs.