Tachometer Design Help Needed

If you assmble a tachometer why not use a simple counter chip with one or several optical couple set? circuit is very simp;le and if you use a microchip you can save many trouble for digital display.and neddnt use the counter chip and less money spend. you can go far to get counterwiase and anticounterwise direction recording... the rotation speed is so low that I dont know if the F/V chip can work well? when you sample low pulse rate to supply the input. if you keep to use the chip, check out how low frequency it si? in otder that you can raise up your input frequency by add more samples, say more photo dioudes or hall device etc, ( I guess) to satisfy with its the lowest frequency requirement.

http://www.national.com/an/AN/AN-162.pdf

Look in the paragraph called "choosing R1, C1 and C2". If you don't know exactly what to do, just make some guesses and use the formulas to choose your parts. Then build it and see how it performs.

As for a display, I would just use a standard voltage meter. You can create a table to translate voltage to RPM, or buy a bare analog meter and label it accordingly.

Coo that's an old chip.... take apart any new (ish) car tachometer and it will have the same chip and components in it.

As has been said the data sheet tells you how to do the simple sumd necessary...

John.

Hello

This is an interesting problem to solve. Translated in electrical terms, the frequency is from 0.15 Hz to 3.75 Hz or puls time from 6 s to 0.27 s.

I think I should not use the LM2907 because it is old and expensive. Can I recommend the more known LM555 and an OP-integrator i.e. LM741 or CA3140 for the job.

The LM555 can trigg for every turn and give a pulse t_p. Designing the 555 mono-flop to a t_p of 0.2 s you will have a PWM signal from it. The PWM can be integrated in the OP-integrator to a DC or near DC and drive the voltmeter. This is only thoughts, I have not test it.

Are you interested of testing this solution you can write me. But today I am going for a 3 weeks holiday, so we can begin in the beginning of April.

Anders

g.a.gustavsson@tele2.se