LM2907/LM2917 tach circuit

Thread Starter


Joined Mar 8, 2009
I know this subject has been discussed numerous times but I need some guidance on building a tach circuit. The circuit will have input frequencies between about 30Hz-500Hz.

Ideally, the IC will be powered from the car's 12V system but having done a bit of reading, I heard it was best to regulate the car's voltage using an LM7805 with filtering caps to about 9V.

I have scanned through the LM2907/LM2917 datasheets and I need some guidance on which chip to use. I'm not an expert in electronics and I know that the LM2917 has a built-in zener diode.

I have also downloaded some models of the LM2907 and LM2917 for LTSpice. I need help choosing appropriate values for the resistors and capacitors.

Ideally I would like to have an output of 0-5V so I can input it into an ADC port on a microcontroller.

How should I start???



Joined Jan 3, 2012
The best place to start is the datasheets for the parts. The datasheets from National (oops, now TI) for these parts is quite good, with about a dozen pages of applications information and example circuits. The best advise is for you is:

1) Read through the datasheet
2) If you have questions about what something means, ask.
3) Consider your application in detail, and take a look at the various applications shown in the datasheet to see what fits closest.
4) While you can mess around with simulating this circuit - you'll learn far more by getting some parts and building a prototype, and testing it. This will give you a MUCH better feel for the circuit and its performance.

I don't mean for this reply to sound glib - I just see it as a good opportunity to learn a lot. I've used these parts in the past, they are easy to use and work very well.

The choice of using an external regulator, or using the 2917 and the shunt regulator, is largely going to depend on what else you're going to be running with it. If you intend to run right into an ADC, then the ADC is probably going to need a regulated supply anyway, so the external regulator is probably a better bet since you can potentially use it to power both circuits.


Joined Jul 9, 2011
the IC will be powered from the car's 12V system but having done a bit of reading,
Are you planning to get the input signal for your circuit from the cars electric system/ECU? If so, this discussion will not be allowed on AAC according to the ToS. (You will know it when the mods close your thread)

You may find some help HERE.


Joined Jul 17, 2007
I had found a SPICE macromodel on the Internet that a Mr. R.L. Kincaid of Kincaid Engineering wrote a number of years ago, made symbols and exzamples for LTSpice, and uploaded them here. I don't remember which threads, but you should be able to find them using the Search feature.

I don't recall the specifics offhand, and I won't have access to my computer or notes for perhaps a week or more.

The opamp in the 2907/2917 does not have a rail to rail output, so if you try to operate it from 5v, you will get much less than that for an output.

There was a note in the macromodel or a .GIF that came with it to caution about a saturation when the output rose above Vcc/2; the real part exhibits this saturation while the model does not. This is important to keep in mind. It also means that if you want a 0-5v output, you will need to use a 10v supply.

Thread Starter


Joined Mar 8, 2009
Alright so I'm pretty decided on using the LM2907 (I still however do not fully understand the doifference between the 2917 and 2907). I'm going to give it an input of 12V (Vcc) so that I can get a maximum outout of 6V (Vcc/2). I have chose the following values for R1,C1 and C2


The issue I am having is verifying that the maximum frequency is satisfied (I2/C1*VCC). How do I get this I2 value?