Motorcycle dyno tach

Hi, I need help on the circuit. I'm making fir my inertia dyno for motorcycle, using lm2907n for freq. to voltage converter. I'm using a variable reluctance sensor to count 70 teeth gear to 5000 rpm roller speed,and 15k engine rpm cdi sensor using lm555 and lm2907 also. Im a mechanic with little knowledge of electronics.
70X5000/60=5833RPS OR 5.83KHZ or 6khz = f in of my roller
Vcc=12v powered by cpu power supply.
Vo = 5v pp
C1=?
P1=?
C2=?
P2=?

The datasheet says that the 8-pins LM2907 or LM2917 is used with a variable reluctance pickup.

Camprofile,
What you've done is "hijack" an existing topic. Even though it's very similar to the original thread, you have your own unique requirements that differ from what the original posters' was.

This can cause a great deal of confusion. Expect your post and this reply to be moved to a new thread.

Your situation is more complex because you are using a variable reluctance sensor. Unless the VRS already has an amplifier that puts out a reliable signal that varies at least 1v p-p, you will need to assemble an amplifier circuit for it.

There are several amplifier ICs available.
These are Dallas/Maxim's offerings:
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/5822/t/al

ONsemiconductor:
http://www.onsemi.com/PowerSolutions/product.do?id=NCV1124DG

National Semiconductor: http://cache.national.com/ds/LM/LM1815.pdf

There's obviously more to it than just slapping in some resistors and capacitors.

There have been several other topics regarding the LM2907/LM2917 ICs over the last year or so. Not all of them have ended gracefully.

This is not a particularly easy IC to deal with. I don't have any of them to experiment with, which makes things a bit difficult.

Can you acquire an LM2907-8 or two? That will greatly simplify the variable reluctance pickup problem, but be aware that low-speed operation will be "iffy".

I'm using a variable reluctance sensor to count 70 teeth gear to 5000 rpm roller speed,and 15k engine rpm cdi sensor using lm555 and lm2907 also. Im a mechanic with little knowledge of electronics.
70 x 5000/60 = 5833RPS OR 5.833kHz or 6kHz = f in of my roller

Click to expand...

OK, if you want any chance of accuracy, it's good to know the details.

Vcc=12v powered by cpu power supply.

Click to expand...

This means you'll want the LM2907-8; as it's designed for operation up to 28v and can handle variable reluctance pickups; it has input protection where the 14-pin version does not.

Vo = 5v pp

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OK, a voltage on the output, 0v signifying 0 RPM and 5v signifying 5,833 RPS, right?

C1=?
P1=?
C2=?
P2=?

Click to expand...

We'll get to that. I'd like to verify the above info first, please.

HI sir! Thanks for the reply! I can only get lm2907n and lm2917n sir.


yes sir ! 0v signifying 0 RPM and 5v signifying 5,833 RPS.

We're pretty informal around here. "Sir" isn't necessary.

Tell us more about your variable reluctor sensors' output signal. Does it already have a signal amplifier on it?

What signal output do you get? It will be very helpful to know if it is a low voltage AC that varies with speed, or a DC signal that is constant with speed. Using an oscilloscope will help you to determine that. If you don't have an O-scope, you might get an idea using a multimeter set to AC or DC.

SgtWookie said:

We're pretty informal around here. "Sir" isn't necessary.

Tell us more about your variable reluctor sensors' output signal. Does it already have a signal amplifier on it?

What signal output do you get? It will be very helpful to know if it is a low voltage AC that varies with speed, or a DC signal that is constant with speed. Using an oscilloscope will help you to determine that. If you don't have an O-scope, you might get an idea using a multimeter set to AC or DC.

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No sir I don't think it has signal amplifier on it. Using my digital V O M It can detect low AC volt that varies with speed.

camprofile said:

I don't think it has signal amplifier on it. Using my digital V O M It can detect low AC volt that varies with speed.

Click to expand...

OK, that's going to make things a tad more difficult, but at least you can measure a signal.

There will have to be some added circuitry on the front end so that:
1) The LM2917 will be able to "see" the crossings of it's threshold voltage.
2) The AC input voltage must be limited (clamped) to prevent damage to the LM2917 during higher RPM operation.

[eta] Your image link doesn't work.

Would you please attach the image to your post?

Use the "Go Advanced" button below the text box, and then click the "Manage Attachments" button. Navigate to the image on your HDD, select it, and then upload it. Image size is limited.

This is my roller with pickup.

Interesting; I haven't seen a dyno roller using a belt like that before.

The dynos I've seen use two rollers. The front roller is just an idler roller to keep the wheel basically centered fore and aft.

You'll have somewhat of a time keeping that wide of a belt centered on two wide rollers. It will help a great deal if your two rollers have a crown in the center (ie: larger in diameter in the center than on the ends).

Just some preliminary numbers:
For C1, 0.01uF (10nF)
For C2, 0.001uF (1nF)
For R1, 71.43k Ohms. This will need to have a +/-10% adjustment range due to the tolerance of C2.

You will need a method to calibrate the output voltage for 5.0v exactly with a 5,833Hz input. The calibration signal must be quite accurate, or your output will be way off.

Here's basically what needs to be done to process the variable reluctor's input. Refer to the attached schematic/simulation.

The LM2907 is represented by the dashed box. Only the inputs to the internal comparator are shown; the rest is to make certain that your variable reluctance sensor does not exceed the input voltage rails (R4, D1, D2), shifts the DC level to the voltage reference level (C2, R3), provide a reference level that is 1/2 Vcc (R1, R2), and stabilize the reference level (C1).

The signals at various points (A), (B), (C) are shown in the simulated O-scope display at the bottom in their respective colors.

If you had an LM2907-8, most of this wouldn't be necessary.

SgtWookie said:

Interesting; I haven't seen a dyno roller using a belt like that before.

The dynos I've seen use two rollers. The front roller is just an idler roller to keep the wheel basically centered fore and aft.

You'll have somewhat of a time keeping that wide of a belt centered on two wide rollers. It will help a great deal if your two rollers have a crown in the center (ie: larger in diameter in the center than on the ends).

Click to expand...

Hi SgtWookie, I'm not using any belt on this roller, this is a solid 4140 shafting , it will directly in contact with rear wheel of the motorcycle. What you might think as belt are knurled surfaced of the shafting for friction to prevent the wheel from slipping.
Thanks again to your effort, ill be buying new components from your circuit design.

Do you have a way to generate a 5,833Hz signal for calibration of the output voltage?

SgtWookie said:

Do you have a way to generate a 5,833Hz signal for calibration of the output voltage?

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No I don't have but if the output voltage is near 5 v pp , I can adjust it on the software fine tuning program. Base on the actual motorcycle speedometer.

Ahhh... but I thought motorcycle speedometers were connected to the front wheel?

SgtWookie said:

Ahhh... but I thought motorcycle speedometers were connected to the front wheel?

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We have one with digital speedometer sensor that is connected at the engine sprocket.

I think if you could take your circuit to a frequency generator at a nearby college or something, you could most likely calibrate it with a much more accurate signal and have a lot more confidence in your readings. It would also verify the circuit works as expected and you don't have any poor connections or shorts before trying to troubleshoot any problems when it is installed on the dyne.

Have you considered how the non-linearity of the disk brake as it heats up will affect your readings as you load the motorcycle?

OK, I think we have the roller portion more or less covered.

But in your first post, you also said:

camprofile said:

... and 15k engine rpm cdi sensor using lm555 and LM2907 ...

Click to expand...

I guess you're also going to need an LM2907 circuit for the engine tach portion, right?

That part is going to be a bit more involved, because motorcycles can have anywhere from 1 to 4 cylinders, and their engine RPM range could vary quite a bit.

Or, is this dyno just for bikes with a certain number of cylinders?

Anyway, tell me as much as you can about the CDI output of the ignition. Obviously, it can't have much loading on it.

SgtWookie said:

OK, I think we have the roller portion more or less covered.

But in your first post, you also said:


I guess you're also going to need an LM2907 circuit for the engine tach portion, right?

That part is going to be a bit more involved, because motorcycles can have anywhere from 1 to 4 cylinders, and their engine RPM range could vary quite a bit.

Or, is this dyno just for bikes with a certain number of cylinders?

Anyway, tell me as much as you can about the CDI output of the ignition. Obviously, it can't have much loading on it.

Click to expand...

Yes you right ! this dyno is especially design for single engine motorcycle. I'm planning to use 555 timer to get the engine RPM signal from the high tension wire.
By the way I got the parts today and I was not able get the zener diode in your circuit design (1n746) , instead I bought 1n4728A 3.3v do you think this will work?