Tachometer mods

[mousewheels]

This thread is for modifications to tachometers.

Anything goes!

A few ideas to focus upon:
Getting automotive tachs to work with our scooters
Re-scaling tachometer ranges. Want to convert an 8k tach to 10k? 12k?
Tachometer signal pickups
Using DMM's as a tach
What about those bike computer tach mods?
Building your own tach etc...


There's is current interest in automotive tachs - low price, reasonable looks, what can be done with these?
So we start with an automotive tachometer seen below:

--- Sun Mini Tach ---
This tach is available from just over $30 with shipping. It has a 0-8k RPM range and a brand name from long ago.

Here's a pic of it's internals. It's a through hole board design. That's good for us - easy to work on! A 14 pin IC sits under the meter movement. '8/6/4' switch changes resistors used to calibrate for different numbers of sparks/rev.

The resistor standing up with the yellow flylead is a mod in test for 1 spark/rev operation. The resistance value selected is scaled exactly as a factor of 2 from the 4 cylinder range. A 4 cyl/4 stroke engine produces 2 sparks per revolution. To match our scooters we need a tach calibrated for 1 spark/revolution.

[mousewheels]

Calibration check of the Sun tach on 8 cyl and modified range (1 cyl) compares well with 8cyl range for accuracy. Sun tach by itself has both positive and negative errors depending upon RPM.

Sun_Tach_Cal_Test_Compare_1cyl_8cyl.JPG (37.31 KiB) Viewed 10907 times

Cal test uses a signal generator and frequency counter to set accurate input signals.
Pic below is the showing the best cal point for the 1cyl mod. 100.028hz x 60hz = 6002RPM in. Tach reads 6000 RPM.

SunTach_modified_100hz.jpg (68.71 KiB) Viewed 10907 times

[gyromaniac]

what all mods have you done to your tac or is it completely stock

[mousewheels]

what all mods have you done to your tac or is it completely stock

Only tach mod is one resistor is changed. It is visible and described in the photo posted earlier. Hold off on getting and modding a tach, until this one is running on a scooter.

It may take external circuitry to get a clean signal for the tach input. We could double ignition pulses as part of the tach signal processing. We could then use the tach stock.

[gyromaniac]

How would you double the engine pulses so that you could use the tac stock?

[mousewheels]

How would you double the engine pulses so that you could use the tac stock?

The Gyro ignition drops breadcrumbs. There are two distinct pulses per spark from the spark trigger coil.

Gyro Ignition scope pic -
The trace with neg and positive humps is the spark trigger pulse - about +/- 10v AC. The other trace is a high amplitude signal which charges a cap in the CDI. It is hitting about 325V DC. At the center of the trace, on the falling edge of the trigger pulse, the capacitor in the CDI is being dumped into the coil, making the spark. The positive going pulse does nothing for the ignition and can be used to create a 'second' tach trigger pulse for each spark.

[mousewheels]

Ed: posted from another thread on tachometers. This is good input regarding an option for tach signal pickup. Folks with Diesel engines have used the alternator and other methods as well.

Marine tachs all run off the alternator pulse and have an adjustment switch for the number of pulse involved. Some have only 4-6-8-10-12 pulses per revolution. But some go clear down to 2 pulse per revolution. Check with a Johnson or Evinrude dealer or a mercury Marine dealer. Oh and some tachs only have 6,000 RPM limit but there are 7k and 8k tachs and some go as high as 10,000. Most are analogy tachs.

[noiseguy]

The nice thing about this is that you should be able to use that resistive value to retune the tach to a higher total RPM... in fact, that would be an "easy" way to rescale the whole thing. I think this tach tuned to a top RPM of 10K-11K would be a lot more interesting.

Looking forward to how you plan to hook in the pickup on the engine. Some of the tachs I've seen use an inductive loop around the spark plug wire... as do timing lights.

[mousewheels]

The nice thing about this is that you should be able to use that resistive value to retune the tach to a higher total RPM... in fact, that would be an "easy" way to rescale the whole thing. I think this tach tuned to a top RPM of 10K-11K would be a lot more interesting.

That's true - and member Breezy is ahead in posting results, nice work. http://www.hondaspree.net/phpBB3/viewto ... 6&start=43 .

The fun in this is we can use low cost tachs on our scooters and they can be modified to fit our needs.

Looking forward to how you plan to hook in the pickup on the engine. Some of the tachs I've seen use an inductive loop around the spark plug wire... as do timing lights.

Will work on this. It is attractive on installation and 'can do no harm' standpoint.

[mousewheels]

This is a finding triggered from Bear5-70's posting that marine tachs utilize alternator pulses.

The Elite AF16 (94-01) stator output yellow wire - (headlight drive) outputs 3 pulses per spark. 3 sparks per rev is equivalent to the auto tach being set to '6 cylinder' setting. This will allow use of an auto type tach without internal mods. The following oscilloscope traces show the waveforms.

Top trace is the stator yellow wire - AC output to headlight, dash lamps and bystarter. The lower trace is the ignition spark trigger coil

Elite_SA50_Ignition_vs_Stator.JPG (37.79 KiB) Viewed 10801 times

-- Tach signal input --
The standard auto installation for this tach is on the primary side of the coil. The ignition module (or points if its a very old car) alternately open and short the primary side of coil to ground. The nominally sets the range of signal expected by the tachometer input to 0-12v.

Our scooter AC headlight drive is about +/-15 from peak to peak. Converting the signal to a 0-12v pulse adjusts the signal to match up with the tach. A schematic is presented at the end of this post.

Generic_Tach_Diagram.jpg (30.95 KiB) Viewed 10779 times

-- Results--
Automotive tach installed on AF16. Comparison run with Trail Tech Vapor. Tach readings track between the to meters.

Vapor_4800_vs_Sun_4800.JPG (83.52 KiB) Viewed 10801 times

Vapor_8200_SunAutoTach_8100.JPG (77.29 KiB) Viewed 10801 times

-- Tach driver info --
Edit: There is a simplified circuit which follows (mousewheels, 05/02/2012)

Breadboard:
Not much to wiring up this board. Breadboard has a component substitution for D1 - seen as a second transistor.

Tach_InputSignalBoard.jpg (48.89 KiB) Viewed 10780 times

Schematic:

Tach_InputSignalConditioningSchematic_sm.JPG (36.62 KiB) Viewed 10780 times

-- Update: Another approach --
Original driver board was meant to present a low load to the input signal. The spark trigger pulse coil has less output than the stator. Also, it has an output strictly between +12v and ground. Allowing a greater input load, and that the tach input signal can go to about -0.6v, lets us use 2 components and no perf board.

Notes: (05/02/2012)
The Sun tach in this thread has an internal diode which blocks any negative voltage at the input. The -0.6v neg limit of
this circuit is not seen by the chip in the tach.

This circuit is an example, it will be built and tested this week.

Tach_Input_Signal_Simple.JPG (21.44 KiB) Viewed 10646 times

[mousewheels]

Couple notes before closing the Elite SA50 94-01 tach mod:
1) The comparison photos are on an actual Elite SA50, this is verification the circuit works over the intended range on a real bike.
2) If anyone wants to build a tach interface board, I will make up to 25 parts kits for available for $4.00 shipped (US only). All components, perfboard, interconnect wire, solder. Just add wires to connect into your Elite harness to complete. I'd build a few assembled/tested boards for $12 but these are easy to build. Many hopes that members building their own.

[mousewheels]

Rescaling Tachometer Ranges -- 8k --> 10k/12k/15k

The nice thing about this is that you should be able to use that resistive value to retune the tach to a higher total RPM... in fact, that would be an "easy" way to rescale the whole thing. I think this tach tuned to a top RPM of 10K-11K would be a lot more interesting.

Yes - after getting the tach to trigger reliably, re-scaling is a good follow on. For the Sun tach in this thread, the initial mod to create a 1 cylinder range calibrated at 8kRPM full scale came out exactly to 2x the resistor used for the 4 cylinder range. This information was used to calculate theoretical resistance at other full scale RPMs.

Results are about dead on accurate.

Calculated resistances for various modified full scale tach ranges:

Calculated_and_Verified_rescaling resistors.JPG (20.28 KiB) Viewed 10764 times

The calculated values were mapped to standard commercially available 1% tolerance resistance values. For location of the resistor being changed, take a look at the photo on the first posting of this thread. It is the vertical resistor with the yellow 'fly lead' attached.

Results for 10k/12k/15k rescale of the Sun tach to 1 spark/rev operation. For the photos below, the reader needs to 'imagine' mouse created new artwork for the tach scales for the proper max range. For now, the pointer sitting dead on 8k means the re-scale was successful. You may compare the Calibration Frequency shown above with the HP Frequency counter showing the input frequency required for full scale operation. Results are pretty much dead on.

10k RPM calibration (theoretical full scale at 166.7hz) - 627 kOhms (measured)
Result: Passed, in calibration

Rescale_10kRpm_627k.JPG (29.16 KiB) Viewed 10764 times

12k RPM calibration (theoretical full scale at 200 hz) - 531 kOhms (measured)
Result: Passed, in calibration

Rescale_12kRpm_531k.JPG (29.13 KiB) Viewed 10764 times

15k RPM calibration (theoretical full scale at 250 hz) - 422 kOhms (measured)
Result: Passed, in calibration

Rescale_15kRpm_422k.JPG (33.47 KiB) Viewed 10764 times

[Breezy]

That stator pulse converter schematic is pure genius, no voiding of new tachometer warranties

[mousewheels]

That stator pulse converter schematic is pure genius, no voiding of new tachometer warranties

Thanks Breeze


-- Graticule replacement --
When rescaling our tachs, a new graticule should go with the change. I'm working on an app to generate new scales at custom angles and diameter. Other uses might be speedometer scales, custom volt or amp meters etc. A degree wheel too

0 - 10k scale

Tach_Graticule_Maker_1.JPG (57.94 KiB) Viewed 10712 times

0 - 5k RPM scale (2 scales overlayed to demonstrate size and meter sweep adjustments)

Tach_Graticule_Maker_4.JPG (49.46 KiB) Viewed 10712 times

[Breezy]

Wow, and here I was measuring it out manually, that app is awesome, will it have numerals as well?, I was looking for some small number stickers for my gauge, but have found nothing yet.....

One thing that I am working on right now is that some Tachs have what I would call a lazy 1K, like my gauge, on my gauge the measurement between 0 RPM and 1 RPM is 27.3% shorter than the rest of the scale.
now that I have shrunk the scale down to fit more numerals, I am wondering if some of this lazyness has transferred into the 2k range setting the scale accuracy very slightly off, or if with the recalibration the lazyness will shrink down too....
I will test this I hope next week

Just some food for thought........