Datasheet error?

Would someone take a quick look at the data sheet here please

I've never used an LM2907, but was reading the above a couple of days ago (in connection with the thread '12v relay on 3v ??' in seb).

The +Vout figure of 67 Hz/V for the 'Minimum Component Tachometer' circuit shown at the top of page 8 seems wrong to me. Using the formula given on page 7, I make it 670 Hz/V. That would also be a lot more practical, at least for a car engine!

I'm curious to know whether it's the datasheet or me that's wrong.

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Terry Pinnell
Hobbyist, West Sussex, UK

Thanks, plainly my slip. Maybe I used 10k instead of 100k for R1. (I see manufacturer failed to include component designations, although should be obvious here.)

Also, assuming this was meant to be a typical car tachometer circuit, I thought 67 Hz seemed too low by about a factor of 10, reinforcing my mistake!

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Terry, West Sussex, UK

I didn't consider whether or not it is a good design. My first problem is an automotive design that has a regulated 15 volt supply and bills itself as a minimum compont count design.

But if we assume that this represents a tach for an 8 cylinder car and the pickup is the distributor pickup (that turns half as fast as the crank shaft does, then 5000 RPM would produce a frequency of

5000/60/2*8=333 Hz for a voltage aoutput at that speed of about 5 volts. That isn't so bad. If you designed it for 670 Hz per volt, the output would be about a half volt/ full scale.

Thanks. My calculation was badly flawed!

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Terry Pinnell
Hobbyist, West Sussex, UK