NB8A:Frequency Divider for Tach input to AP60 Cruise Control

[wleung]

Hi All,

I've managed to install an AP60 Cruise Control into my NB8A and get it working off the Tachometer Signal from the ECU.

The AP60 was designed to run off magnets attached to the tailshaft. I was more Electrical than Mechanical oriented, so I preferred to skip the magnets entirely and use the tachometer signal. The beauty of connecting straight to an RPM signal instead of a signal related to speed is that you don't necessarily need the clutch switch (since the cruise control is controlling RPM, not speed, so it shouldn't over-rev too much should you forget and push the clutch pedal when cruising)
The models that could hook straight into the tach signal were $200 more than the AP60, so I wanted to make a custom solution to integrate the AP60 with the tach signal.

The big problem is the AP60 cuts out cruise control at 5000 pulses per mile as a safety measure (this is after you cut the black wire). Normally with a single magnet on the mx5 tailshaft, you'll be fine.
But this won't work for the tachometer signal - assumming I want my cruise control to work up to 4000rpm, the IG- Tachometer signal will be a square wave running at 133Hz.
I've managed to estimate that the 5000 ppm roughly equates to a maximum pulse rate of roughly 100Hz.

So I've just made a frequency divider of 2 by using a D-flip flop. Attached is a circuit diagram of my actual design using JayCar parts:

TachFreqDivide2.png

Originally I was going to route my Tach Signal into the diagnostic port in the engine bay, but in the end I just spliced into the wires behind the dash instruments as it was easier to get to the IG, 12V, GND wires there.
This means that there is probably no need for the Schmitt Trigger anymore (since the ECU outputs a rather clean signal and I'm not passing a wire past the ignition leads), so you could probably just route the tach signal straight into the CLK of the D-flip-flop. You can even buy D-flip-flops with a schmitt trigger CLK, but Jaycars CMOS one's didn't have those.

The 6.8k Resistor is also optional - I left that there as part of a backup plan.

Please test your frequency divider circuit on bench using at least the diagnostic port output and a multimeter Hz counter (or a Function Generator and CRO if you have access to one). I made a few mistakes in my implementation on protoboard.

Some good resources for the actual AP60 install on the MX5 can be found by googling for the installation instructions for ShiftI (to locate the instrument panel wires), and also for installs of the AudioVox Cruise Control (A similar American product that uses a vacuum actuator of the same shape as the AP60 and the same mounting bracket)

If you end up trying an AP60 install yourself, and you have any questions - don't hesitate to ask.

Cheers,
Wilson

[wleung]

An here's the board layout on the Jaycar HP9556 Protoboard (done via PEBBLE software)
Note the view is from the NON-TRACK side.
All the jumpers with a span of one was done by bridging solder across the two pads (instead of actually using wires).
I used 14-pin IC sockets for both ICs to allow for easy replacement, and I used three 2-way JST connectors for all the inputs and outputs (12V/GND, Tach, BlueOut/BLackOut)

Again - this was just my implementation. It could probably be made much smaller by skipping the Schmitt NAND and just using a CMOS D-Flip Flop.

TachFreqDivide2.png

BOM:
Jaycar parts:
3x HM3402 header with crimp 2way p=2.54mm $1.50
3x HM3412 header strt lock sgl 2way p=2.54mm $0.60
1x ZC4013 Dual D F/F DIP15 $1.15
1x ZC4093 Quad 2-input NAND Schmitt Trigger $1.65
2x PI6501 TIn/G 13pin/300MIL $0.80
1x HP9556 PCB Exp Mini 130x45mm $4.95
1x HB5204 Potting Box UL94HB $2.95

Total Cost $13.60
(Scrounged a 6.8K Resistor from stuff lying around my house - probably not necessary, could just replace it with a jumper or not scratch off the ground track)