# tachometer

#### mechtronics

Joined Jul 30, 2011
10
hi i was just wondering does any body know can a 0 to 10 volt output be achieved by using either the lm 2917 frequency to voltage converter or the lm2907? i know the lm 2917 has a built in zener diode which has a value of 7.56 volts. does this limit your output voltage in that you can only get an output voltage of up to 7 volts to correspond with a given frequency??? or can a bigger output voltage be obtained by using the lm 2907 without the zener diode regulation?

any help would be appreciated thanks.

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#### SgtWookie

Joined Jul 17, 2007
22,201
The LM2917 won't even get to 7v output, as the output amp has a transistor being used as an emitter follower.

If you want more than about 5v out from the LM2917, then you will need to use an opamp wired as a noninverting amplifier.

Don't expect the LM2907 to output more than about +V minus 3v; so if your supply is 12v, 9v would be maximum.

If you really want to "play it safe", don't expect to get more than 5v out of either of them, and use a noninverting opamp with a gain of 2 to amplify the output signal.

Have a look at the attached simulation of an 8-pin LM2907 with an opamp (U2) wired for a gain of 2 on the output.

In the plots on the bottom, the cyan trace, V(+in), is a simulated input from a Hall-effect sensor that is a frequency sweep from 0Hz to 2kHz.

The green trace, V(out), shows the output from the '2907 as being noisy. V(out2) is V(out) after going through a simple low-pass RC filter. The red plot V(out3) is V(out2) after being amplified x 2 by the opamp.

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#### mechtronics

Joined Jul 30, 2011
10
hi, so if i amplify the output signal of 5 volts by a gain of two that means i should get an output of 10 volts right?? because what i am trying to do is make a tachometer for an engine which revs to 9000 rpm so i wanted to have 1 volt to correspond to 1000rpm , 2volts to 2000 rpm etc all the way up to 9 volts, 9000rpm.would the op amp give me this range of voltage to do the job and if so the op amp output is then fed into the lm3914 to give an led display?

#### mechtronics

Joined Jul 30, 2011
10
hi, so if i amplify the output signal of 5 volts by a gain of two that means i should get an output of 10 volts right?? because what i am trying to do is make a tachometer for an engine which revs to 9000 rpm so i wanted to have 1 volt to correspond to 1000rpm , 2volts to 2000 rpm etc all the way up to 9 volts, 9000rpm.would the op amp give me this range of voltage to do the job and if so the op amp output is then fed into the lm3914 to give an led display?

can the op amp be connected to the output pin on the 14 pin version of the lm2917?? because that is what il be using i think.

#### SgtWookie

Joined Jul 17, 2007
22,201
hi, so if i amplify the output signal of 5 volts by a gain of two that means i should get an output of 10 volts right??
IF you have the rest of the circuit for the LM2907/LM2917 set up properly. One usually has to do some "tweaking" on it to get it adjusted/calibrated properly.

because what i am trying to do is make a tachometer for an engine which revs to 9000 rpm so i wanted to have 1 volt to correspond to 1000rpm , 2volts to 2000 rpm etc all the way up to 9 volts, 9000rpm.
What engine do you have that revs to 9,000 RPM, and how many cylinders is it?
If it's a 4 cylinder, then 2 fire per revolution; 6 cyl, 3 per rev, 8 cyl, 4 per rev, 12 cyl, 6 per rev.
You divide the RPM by the # of cylinders/2, and then divide the result by 60 to get revolutions per second.

You can use an audio signal and a meter to calibrate the output.
And yes, if you really have to, you can use an opamp to scale the output of the 29x7 up. However, you can also scale the input range of the LM3914 to anything within reason.

would the op amp give me this range of voltage to do the job and if so the op amp output is then fed into the lm3914 to give an led display?
Like I suggested, the LM3914 could turn various voltage ranges into a full bar display. For example you can scale 0v-5v in to be 0.5v=1kRPM, 1v=2kRPM, 1.5v=3kRPM etc - and avoid having to use an opamp in the middle.

#### THE_RB

Joined Feb 11, 2008
5,430
...hi, so if i amplify the output signal of 5 volts by a gain of two that means i should get an output of 10 volts right?? because what i am trying to do is make a tachometer for an engine which revs to 9000 rpm so i wanted to have 1 volt to correspond to 1000rpm , 2volts to 2000 rpm etc all the way up to 9 volts, 9000rpm.
...
You can make the whole thing with a 7810 (10v) regulator, and a CMOS 555 like a 7555.

The 7555 is a monostable, and you set the fixed ON period so that when it is triggered at 1000 RPM the output duty is 10% (1v) and 9000 RPM the output is 90% (9v).

Then just filter the output PWM to DC voltage by RC filter.

#### mechtronics

Joined Jul 30, 2011
10
cheers for the replies guys thanks.

just one more thing if i have to use the op amp to give me a 10 volt output will this let me correspond 1volt to give me 1000rpm 2volts to give me 2000 rpm etc all the way to 9volts 9000 rpm? and ya i was thinking of making every 0.5 of a volt to correspond with 1000 rpm i had that in mind too. oh and would the 741 op amp let me do this do u know??

#### SgtWookie

Joined Jul 17, 2007
22,201
The 741 is an antique; the design is over 40 years old. They don't work well at all on single supplies.

#### mechtronics

Joined Jul 30, 2011
10
its a 4 cylinder engine. wat op amp would u reccomend to use ?? and would it do the 1 volt to 1000rpm all the way up to 9000 rpm being 9 volts ??

#### mechtronics

Joined Jul 30, 2011
10
hi the RB i was just wondering do you have a circuit diagram for your method of the tachometer circuit?

#### THE_RB

Joined Feb 11, 2008
5,430
Hi mechtronics, It's not "my" method but is very traditional and standard. It was done in the old days with 2 transistors (as a monostable multivibrator), and was very common in vehicle tachos and in auto tune up meters. I repaired many tachos and tune up meters that literally only had 2 transistors in them!

And sorry, no I don't have a circuit, but it is pretty easy. A 7555 monostable is a standard circuit (there will be heaps already on this forum) and a 10v regulator is very simple (you can use a common 7810).

Tacho questions seem to be quite popular, if you start a thread on "building a tacho with a 555 timer" I can offer suggestions and I'm sure lots of other people will offer help too.