Protecting an Austin 7 crankshaft, RPM warning.

[Classic Microcars]

After talking to someone who has an Austin 7, it appears that the two bearing crankshaft suffers from breakage if the speed is kept between 2200 and 2400 RPM. This owner is aware of the upgrades available, but wants to keep the car original. With this in mind, he asked for my opinion on a warning system. There are various easy ways of achieving it, but I decided to investigate something that would be more versatile.

My idea is to use an Arduino and these are my results so far:

There is a small oscillator circuit on the top breadboard, using a 555 timer. That runs between 28 and 150Hz and provides a square wave to the Arduino, simulating an ignition system that runs between 850 and 6000 RPM. The Arduino reads the frequency, computes the RPM and then displays that information on the LCD. There is an alarm output that you can see on the red LED that is enabled when the RPM is between 2200 and 2400. Further enhancements are to add a temperature and oil pressure sensor. The idea is to have the LCD on a flylead so that it can be hidden when the car is on display, or a speech synthesiser to give relevant info.

Here are some pictures of my test bed. Currently using parallel LCD but will convert to I2C to reduce the wiring requirement. The keypad currently does nothing, it's just there for other development.

[JPB]

Neat idea, and of course a whole lot more discrete than simply fitting an unburstable 3 bearing OHV engine (with bonus 3-synchro gearbox) from one of the dozens of tipped tricycles that litter the hedgerows around the country at this time of year.

[pryantcc]

I love it! Can you please excuse my ignorance and teach me how the RPM sensing part will work? What input signal will be used on the car?

[Classic Microcars]

I love it! Can you please excuse my ignorance and teach me how the RPM sensing part will work? What input signal will be used on the car?

It's a conventional ignition system so it will come from the CB terminal on the coil. For a vehicle where it's not possible, you can sense from a plug lead, or use a Hall Effect sensor off one of the pullies etc. even a perforated disc off the crank pulley with an optical sensor would do.

[pryantcc]

I love it! Can you please excuse my ignorance and teach me how the RPM sensing part will work? What input signal will be used on the car?

It's a conventional ignition system so it will come from the CB terminal on the coil. For a vehicle where it's not possible, you can sense from a plug lead, or use a Hall Effect sensor off one of the pullies etc. even a perforated disc off the crank pulley with an optical sensor would do.

OK, so the computer somehow monitors the voltage and counts the jumps for each spark plug firing? That's the bit that baffles me, I understand how you'd program it once it has acquired that input signal, but I have no idea how you'd tell a computer to "watch" that input and count when the voltage reaches more/less than a given value.

[tractorman]

Remember that it isn't a PC-type of computer but a "PIC" system that has inputs that can accept all sorts of signal (digital or analogue) with a suitable input circuits (to make the inputs' voltage/current suitable for the PIC) . Make the input a nice simple "switch" and, with a little programming, it's dead easy to sense the input to the machine ("on" or "off"). The more difficult bit is to get a clean "on" (or "Off) signal in a "noisy" environment such as a car's electrical system! That's not to say that you can't use a PC to read such stuff - you just need the right interface (often to the USB these days). I have three or four such devices for the model railway (that were inferfaced to work around 150 outputs and a lot more inputs). I still intend to complete a major rebuild - but have only soldered two wites up there in the last three years (or so).

I won't go into the more clever things we did with current loops (where we sensed input as a current so that, if cables broke, the system knew there was a fault and not a zero volt input) or the other clever things that I was taught at Uni (I went to sleep in many of those lectures!).

[TerryG]

There are "PCs" with handy analogue interfaces built in, eg the raspberry pi. I was bored over Christmas so made the analogue outputs on mine in to a binary clock (i was REALLY bored)

[Classic Microcars]

OK, so the computer somehow monitors the voltage and counts the jumps for each spark plug firing? That's the bit that baffles me, I understand how you'd program it once it has acquired that input signal, but I have no idea how you'd tell a computer to "watch" that input and count when the voltage reaches more/less than a given value.

The computer sees the contact breakers open and close, just like any other switch. You time the on / off switching at that gives you the frequency, which is directly proportional to the RPM. To ask the Arduino to "watch" an input pin is quite easy, a command like this:


* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
int ledPin = 13; // LED connected to digital pin 13
int inPin = 7; // pushbutton connected to digital pin 7
int val = 0; // variable to store the read value

void setup()
{
pinMode(ledPin, OUTPUT); // sets the digital pin 13 as output
pinMode(inPin, INPUT); // sets the digital pin 7 as input
}

void loop()
{
val = digitalRead(inPin); // read the input pin
digitalWrite(ledPin, val); // sets the LED to the button's value
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

However, my design uses something called "interrupts". In a nutshell, the computer is constantly aware of a in configured as an interrupt and if it sees something happen on that pin, it will stop whatever it is doing and go to a subroutine that services that interrupt, before returning to whatever it was doing.

[tractorman]

Actually Terry, I'm wrong - there are analogue inputs on PCs (by which I really meant the "IBM compatible" type of computer rather than a stand alone personal computer). Though not on my notebooks, if you look around the back, you'll often find a game controller port. Indeed, I have some freeware somewhere for an oscilloscope that is supposed to work in the port - subject to input voltage considerations (ie it has a 0-5v range, so not a lot of good for mains stuff - though it would be possible to make an inferface for that).

I did think of buying a Pi, but decided that I have too many old computers and interface boards to play with - if I ever get around to it!

I was taught to consider interrupts like a telephone - you stop everything while you answer it (and get annoyed when it's an Asian who works for "Windows"). Yes, I got caught by one this morning (I was expecting a phone call, so didn't check Caller Display). The lad was shocked when I asked if he believed in reincarnation and that some faiths think, if you are bad in this life, you come come back as a lower life-form - like a slug of other nasty insect. He hung up without replying for some reason!

[TerryG]

It's been a really long time since I saw a MIDI port on a new PC. I suspect they are optional extras on higher end kit but the workstations I deal with at the moment tend to be aimed at the graphics market so no need for any audio beyond the ping noise outlook makes to tell you a new email has arrived.
If you would like one to play with I probably have some sound blaster PCI cards kicking about.