GCSE Project

[jamie_bkc]

Hi, i'm just starting my GCSE electronics project and this will be my first real project, I want to make like a multi-purpose tach cluster type thing, baisiacally an analog rev tachometer, LED segment display for tempereature and perhaps a digital or analog speedometer, all in one box. It will be used for a kart, the engine is a single barrel 2-stroke, 100cc. Any help with schematics, diagrams info etc. would be greatly appreaciated and if you want anymore information just ask.

 

[TillEulenspiegel]

I'm a yank ...whats a gcse?
Does your project have to be built from the ground up, or can you use modules?

 

[jamie_bkc]

ground up i'm afraid, GCSE's are the exams that last through the 3rd and 4th years of high school.

 

[hantto]

"General Certificate of Secondary Education" correct?

 

[jamie_bkc]

probably, I never knew!

 

[TillEulenspiegel]

hmmm.First project huh? Well you picked a heck of a doozie for it. Do You have any experience working with electronics? What are your time constraints? You mention a mechanical tach...Is that allowed?.

First thing You want to do is hash out all the expectations and constraints of the Professor's. When You've done that You can wrestle with a design...I think as your trying to do many things at once. I would approach the problem from an intelligent controller...PIC , with various sensors.The speedometer and the Tach are the same thing, it's just where You put the sensor and how the PIC does the math. To illustrate: a sensor on the flywheel will count how many revolutions there are in 60 seconds for the Tach , while the same sensor on a wheel will measure how much road is traveled I.E circumference of the wheel traveled in feet ( meters for you! ) are traveled in an hour ( well you have to do some maths along the way ) for the speedo. The temp sensor is just a resistor that varies over a temperature range.

The application is fairly straightforward, the implementation is .another question
I hope this at least gives you an idea of the scope of the project.

 

[jamie_bkc]

this isn't my first experience with electronics but previous ones have just been alarms and a capacitance meter, i've got about 7months to do it, the temp gauge seems pretty simple, but one problem is there is no flywheel, these karts are direct drive, so the input would have to come from the pulses generated by the spark plug cable, for the speedometer I thought i could use the bicycle speedo design, where a magnet is mounted on the axle and then a sensor senses when the magnet passes and tells the computer that the vehicle has covered the distance of the circumference of the wheel.

 

[bmcculla]

Another way to make a tachometer is to mount an electric motor (brushed DC) so it spins as the wheels spin. The little motor will generate current based on its rotational speed. You might not even need an amplifier just use a resistor to get a nice voltage for your display circuit.

Brent

 

[TillEulenspiegel]

Well sounds like you have a grasp of what your trying to do . The magnetic sensors are exactly what I had in mind Alternates are opto w/slotted disk or opto w/a reflective surface and an index mark. Their not as field worthy as the mag sensors tho.

There must be an output shaft somewhere , no? Is the kart chain drive w/ a friction clutch?If so any point before the clutch is usable for RPMs. Theres also a coil/magnet magneto trigger in most motors , is that accessible? I don't care to mess with Hi tension especially with CMOS devices just me I guess.

You can approach the Hi Tension P/U a few ways one is a coil around the wire ( dangerous isolate by Hi V diodes and a MOV) , a factory made inductive P/U ( used by strobe timing lights) or a neon bulb with 1 lead at ground and the other wrapped around the HI T wire, put that in a small case with a photo transistor or photo diode as a trigger source.

The neatest way would to put 2 disks with collets that use a screw to attach to the shaft/axel, 1 before the clutch for RPM and 1 after for speed. Attach a ridged bracket to mount the P/Us . Viola' next is to make a flow chart and implement a program to cycle thru the probes and output to a screen .

Smart displays can be had relatively cheap
here:https://www.mpja.com/listitems.asp?dept=361&main=122
example 20 x 40 Hitachi intelligent display w/ controller $10 US.

 

[jamie_bkc]

the engine iteslf is direct drive so no clutch, it's chain driven, thats why i wanted to make use of the spark plug cable for the tach, all i would have to do is convert the frequency made by the pulses in the cable to a voltage and connect that to an output display. It uses a very simple ignition system, where the coil is connected to the engines shaft so that when the wheels are turn the coil generates electricity which travels to the ignition system, this then sends the correct pulses to the plug to create a spark.

 

[TillEulenspiegel]

Gosh guy I'm lost . I don't know how you could even start the thing if there was no clutch?? I'm asuming this is a full sized kart , Yes? The clutch would be a doughnut looking thingie where the chain from the motor is attached to a sprocket and the case is attached to the liveaxle. Does it's movement depend on RPMs, in other words the thing sits there until you give it enough gas then it moves? Normally karts have a centrfical clutch it dosn't grab untill you raise the revs.
l ooks like this

Info on the motor would help, I.E. Briggs and Stratten 2 hp bla bla.......[/img]

 

[Nigel Goodwin]

Gosh guy I'm lost . I don't know how you could even start the thing if there was no clutch?? I'm asuming this is a full sized kart , Yes?

From what I can remember, the smallest class of karts don't have a clutch, you simply push them to start - they only have accelerator, brake, and a kill switch - that's it!.

 

[jamie_bkc]

right, ok let me explain, in england it tends to be that the slower karts have clutches with of course a few exceptions, many of the high power karts don't have clutches, theses are called "direct-drive", these karts just have a sproket on the output shaft and one on the axle with a chain connecting them, therefore the compression from the engine is sent straight to the wheels. So to start the kart you push it until the wheels turn enough for the ignition coil (also connected to the crankshaft) has enough kinetic energy to create electricity for the spark plug to fire and then the rest is simple, you just throttle it so that the fuel starts flowing and off you go. To cut out the engine you simply stop the kart, because the wheels are connected straight to the engine, putting the brakes on hard so that wheels stop turning stops the engine turning over. this is why i think using the ignition cable to detect revs is the best solution.**broken link removed** THIS is a very similar ignition system as the one on my kart. My engine is a Parilla tt75 racing engine, its 100cc and pushed around 13-15bhp (i forget the number exactly)

 

[fat-tony]

So there is no way to actually have the engine running and the vehicle stopped at the same time? That's kinda cool

If that's actually the case, then you only need one sensor. The RPM gauge and the speedometer will just have a different scale. Basically, you'll just have to do:

speed = tire circumference(in m) * rpm * 60(min/hr) / 1000 = km/h

Using a single magnetic sensor would probably work just fine. As was mentioned before, a simple bike sensor would be perfect. You *could* read it from the ignition wire, but you're probably safer just putting a little magnet on it.

Actually... Thinking back to shop class so many years ago , I seem to remember that little engines like that had magnets on them somewhere.... Maybe I'm dreaming, but I thought there was magnets on one of the rotating parts.

 

[TillEulenspiegel]

Tony's right! man that makes things way easier. 1 mag sensor , 2 conversion equations and a thermistor. Shouldn't take more then a few lines in pic basic . Get the pic proto board , interface your screen and your set.
You don't even have to use the hi tension wire, theres already a sensor in the motor ...if thats a picture of the motor you have. simply use the lo side of the coil..see those 2 wires going into the motor? 1 is to the magneto the other goes to points or a mag sensor use that.

 

[jamie_bkc]

yeah, the only way to run the kart while not moving is to put something under it so that the wheels are off the ground and turn the wheels over while they are in the air. One problem with using the axle for rpm is that it requires an equation dependant on what the axle sproket size is, and I change the sproket regulary depending what track i go to so i'd have to take the thing apart and re-program it for each different sproket. Would there be any way of having a menu on the screen to chooses the sproket size? for example, program the pic to do the math for sprokets ranging from 65 teeth to 95 and then choosing which sproket to use with a button?

 

[fat-tony]

If you plan on using an LCD, you could definately have a way to change conversion factors. Also, the little (1,2,4)x(16,20,40) LCDs are super easy to use with a micro.

 

[jamie_bkc]

so what sort of circiut am I looking at here? what sort od chips should I use and what do you mean by (1,2,4)x(16,20,40)?

 

[Nigel Goodwin]

so what sort of circiut am I looking at here? what sort od chips should I use and what do you mean by (1,2,4)x(16,20,40)?

Have a look at the code at **broken link removed** which is a bike computer - it could easily be adapted to do what you want, and is designed for variable wheel sizes. As already mentioned, you only need a single sensor as you don't have any gears or clutch, a simple bit of arithmetic will convert between speed and revs - although displaying both is really a bit pointless! - but I understand it's to look good on a GCSE project!.

The LCD numbers quoted are the size of them, 1x16, 2x20 etc. it was just a shorthand method of listing them.[/quote]

 

[jamie_bkc]

is that link right? ie crashes when i try to click it