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Picaxe tachometer

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paul1950
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Posted on Friday, 13 July, 2007 - 01:40 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I would like to build an optical tachometer using a picaxe and 3 x 7-segment LED displays.
This is to be used on an antique steam engine to monitor flywheel speed. There are 2 cut-outs in the flywheel which I intend to use to count the number of revs. Speed range is up to 999 RPM.
Can anyone help with a circuit and code ?
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boris
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Posted on Friday, 13 July, 2007 - 04:27 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

This should be a fairly straightforward project using a PICAXE. First look through the PICAXE interfacing docs, then go here: http://www.rev-ed.co.uk/picaxe/forum/default.asp
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john_becker
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Posted on Friday, 13 July, 2007 - 08:02 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Paul, I think this was covered in my Rev and Thrust counter I did for EPE about 2-3 years ago. Have a look at the software index of our downloads section.

My wind tunnel of Feb 03 also used an optical tachometer.

But both are in PIC assembly, not PICaxe.

J

(Message edited by john_becker on 13 July, 2007)
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paul1950
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Posted on Monday, 16 July, 2007 - 02:12 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks for your help,
I have some understanding of the program but am not so good with the circuit. Have checked the interfacing docs but am unsure of how to connect 3 x
7 segment LED's. Plan to use an LDR as an optical sensor. Can you help ?
Regards, Paul
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boris
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Posted on Monday, 16 July, 2007 - 05:47 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

If you use the 4511 7seg driver, as indicated in the docs, you can drive 3 separate 7seg units by multiplexing thus:

All the (common cathode) 7seg modules have all there lines paralleled, that is all Ďaí segments wired directly together, and all Ďbí segments wired directly together etc..

It would seem that this means all the 7seg modules would only ever display the same digit at the same time. Useless yes?.

Not so!. When you drive the displays, only one module has itís cathode connected to ground at any one time, so although all the Ďaí segments are driven, only one will light up. The cathode connections are all isolated from ground (0v) by transistors, one transistor per 7seg module. Each modulesí cathode is connected to the transistor collector, with the emitter to ground and the base driven (through a resistor) by one of the PICAXE outputs. Lets say outputs 0 to 2 drive the transistors.

With output 0 high and outputs 1 and 2 low, you send the 4-bit BCD data FOR THE FIRST DIGIT to the 4511. The first transistor is conducting, the other two are not, so only the first digit will be illuminated, the other two will be dark.

After a short delay, you set output 0 low and output 1 high, thus disabling the first 7seg module and enabling the second. Now, you set the 4-bit BCD data for the SECOND digit.

Then the third, and so on in sequence for how ever many modules you use. Although each digit is displayed on itís own, with the others blank, persistence of vision makes the eye see all the digits at the same time, assuming you repeat the process quickly enough.

One 4511 driver can thus service many 7seg modules, all you need to add is one extra transistor per module. Note. The transistors must be able to handle over 200mA collector current at a gain of at least ten. Any general purpose medium power NPN BJT and a 1k base resistor should do the job.

BTW. I do not recommend you use an LDR for the tacho sensor. These are sensitive, but they react relatively slowly and this might be a problem in your application. I would suggest a phototransistor. They are much faster (and cheaper).

There is lotís more experience and expertise over at the PICAXE forum. Give it a go.
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chippie
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Posted on Monday, 16 July, 2007 - 06:21 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Use a Hall effect device with a magnet for pulse generation....Allegro devices will send free samples.. guess how I know?
eccentric millionaire financed by 'er indoors
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john_becker
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Posted on Monday, 16 July, 2007 - 10:45 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Chippie, experience tells me that attaching magnets to a rotating surface can be problematic unless you are totally sure of the forces involved when a wheel, for example, rotates. On one occasion I thought I had fixed the magnet, only to have it fly off later, and it only just missed me :-(

J
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obiwan
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Posted on Monday, 16 July, 2007 - 11:20 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Boris is spot on,

Chippie is close, as John indicated, it's going to depend on what you're measuring. They use that method on bicycle tires for their speedometers soo..... But high speed? Forget it, you can throw it out of balance, throwing that magnet at high speed possibly killing somebody or just ruining the bearings, better be able to afford that equipment before you go attaching something to it!

But there are some Hall effect sensors that don't need an external magnet, it's just sensing the metal there. There's also a similar device that uses basically a metal detector.

Optical is always good. You can use transmissive or reflective.

But like Boris said, I'd try a photo transistor first, you may need some external light to get the full range of "on/off" though. And you'd probably need at least a buffer if not an amplifier to bump up the signal from it.

Personally, I like Hall effect sensors when I can use them. They just so simple in most cases, direct interface most times, all you need is +V/Gnd and output connects right to the chip.

(you can balance that wheel with one magnet on each side.... as long as it's balanced, and stuck tight!)

LDR's are best left for sensitive projects, or cheap sensors, slow changing light or comparing one level to another.

For something like a tach, you need a fast device, one that turns on and off really fast.

Transistors and diodes (photo) are going to be fast. They use photo diodes in communication circuits where the input is changing at a really high rate. (fiber optics)

Actually, almost any silicon device is photo sensitive. Got a regular transistor, like a 2N2222 in a metal can?? Cut the top off of it and you have a photo transistor! (not a very good one of course).

And it's been mentioned here many times that you can use a LED as a photo sensor.

Even regular glass diodes like 1N4148 or 1N914 are photo sensitive.

We used constant current diodes in some medical equipment to limit the current available to the baby. And those things were so photo sensitive it set the alarms off (on the device) all the time. So we had to spend thousands of dollars to recall the monitors and open them up and "paint" those little diodes and send them back out.

Learn something new every day eh??

Oh well, got to go get me some free samples I heard about! (God I love free!!!)
Do Not Hit The Fly That Lands On The Tigers Head.
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john_becker
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Posted on Tuesday, 17 July, 2007 - 01:02 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

By preference I would choose a Hall Effect, but there dont seem to be so many devices around now -unless someone knows otherwise??

J
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obiwan
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Posted on Tuesday, 17 July, 2007 - 02:55 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I've never had much of a problem finding what I want in the way of HE's.

I salvaged some keys from a Honeywell computer keyboard a while back an still use them as push buttons on my breadboards. They give out nice 37uS pulses, no bounce, no nothing. (little big though)

But just about any supplier, Digi-Key, Mouser, others should be able to provide a decent selection, variable or digital.

Tough to wade through the data some times, deciding if it's digital like the keyboard, or variable like a distance or just a field sensor.

If nothing else, I salvage them from old floppy disk drives now, there's normally at least two of them on there. And they're normally very small too.

(I wish I had more of those keyboard buttons!!, but just didn't have the foresight)

Oh, I just bought some not long ago, the TO-92 package for just sensors if you'd like the info on 'em. I can't remember where I got 'em, but it wasn't a place like Farnel, it'd probably be overseas shipping for you. But you can check the data sheet if nothing else.

(I think electronics people have GOT to be the biggest junk collectors there are....)

(Message edited by obiwan on 17 July, 2007)
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john_becker
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Posted on Tuesday, 17 July, 2007 - 11:12 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks Obiwan, but anything I design usually ends up in EPE, and so any devices I use must be readily available to all. Recently, the parts I wanted for something used to be sold by RS, but no longer, like LOHET II, and Schmitt trig switching HE sensors.

J
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davy
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Posted on Tuesday, 17 July, 2007 - 04:17 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

This is a common requirement for lathes to determine cutting speed.

If you do a Google search you will find many, mostly using optical sensors - I think for the reasons John has given.

I attach a link to a simple one using a spinning disk to be viewed by a fluorescent light, preferably. (Paste it on your fly wheel - also shades of John Logie Baird!)

http://www.gizmology.net/lathetach.htm
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davy
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Posted on Tuesday, 17 July, 2007 - 04:24 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Oh and the link is American so it will give a slight error on the speed (60 versus 50Hz - You use 50Hz in Australia?)
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paul1950
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Posted on Wednesday, 18 July, 2007 - 09:04 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks for the advise.
I will go with the photo transistor idea.
The flywheel is on an 80 year old steam traction engine, about 4ft diameter. The bearings are very worn, causing a lot of wobble and may not work with a magnetic pick-up. There are 2 cut-outs in the side which allows light to pass thru when it rotates.
Regards, paul
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boris
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Posted on Wednesday, 18 July, 2007 - 07:18 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

In that case, you may have problems with ambient light. I would take some precautions:

Put the sensor on the inner/lower most shaded area. Put an opaque tube around the sensor so that it can only be illuminated from one direction. Use biasing on the sensor circuit so that itís output will stabilise to roughly half VCC in various ambient lighting conditions. Use a capacitor on the output of the sensor circuit to block DC and only detect the pulses.

Additionally, you could use IR components.
"What's red and invisible?"
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obiwan
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Posted on Wednesday, 18 July, 2007 - 08:37 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

For something that large, (and old), balance probably wouldn't be that much of an issue. Adding a small magnet (if you can get it to stick), probably wouldn't be any more than the wheel is already out of whack (why the bearing is bad now).

Being that old, they couldn't balance them all that great to start with, and you can get very strong small mangets.

Having said that...

You could easily go with a small inductive circuit, basically a small metal detector. (I'll send you one if you want). With that much metal out there, a inductive sensor would be easy. And it's non contact.


If you go optical, there's several things you can do. One is like Boris said. Another is to modulate the light source and then demodulate it again on the phototransistor side. Basically a tone detector, in fact you can use the tone detector chip, the LM567 (Allelectronics.com had them for sale real cheap)

You can also use a strong IR light source and a good IR filter type photo "device", mostly photo diodes have them. Like the IR filters for your TV remote. In fact, for the IR source, you can get an old remote and yank off the plastic IR filter (if it doesn't have the IR filter LED's, you can tell because they're real dark. if it does the dar LED's, just use them).

Then you're basically over powering the IR detector with IR light, so the ambient doesn't affect it too much, same basic principle as the TV remote (although they use modulation also).

Let me know if you want one of those inductive pick-ups. It's a P&F (Ppperl & Fuchs) brand, here's the link:
http://www.pepperl-fuchs.com/selector/gui/show_product_detail.kly?selected_prod_id=3192
(best I can tell anyway)

The only problem with it is that you'll have to mount the sensor so it's very close to the rotating metal. They have a pretty short range. Only you'll be able to tell if it works or not.

In fact, if you wanted to learn something, you could experiment with building your own inductive pick up device. Won't be as small of course, but they're fairly simple and you can learn some things.

(Message edited by obiwan on 18 July, 2007)
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magnum4
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Posted on Wednesday, 18 July, 2007 - 10:24 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Paul,
You could bounce the ir light off the inside of the flywheel also, This will give you a return when the metal is there and no return when the hole is there. Make it easier to implement as the transmitter and reciever could be on the same board. :-))
Hope thats clear .
Regards,
Jim
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magnum4
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Posted on Wednesday, 18 July, 2007 - 10:49 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

IS471f Is a chip I used once. It has an ir modulator output to drive a ir led and a built in demodulator.
You could build a small detector that will give you a nice square output twice per revolution.
HTH
Regards,
Jim
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boris
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Posted on Thursday, 19 July, 2007 - 02:24 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

ď80 year old steam traction engine, about 4ft diameterĒ

Come to think of it. 999 RPM sounds like a lot for this application.

With 500 RPM max, a magnet + reed switch might suffice as a sensor. You could even use a cheap bicycle speedometer.

The fastest I have been on my bike is 42 MPH according to my cheap electronic speedometer. That works out at roughly 500 RPM. Approximately the maximum operating frequency of a standard reed switch.
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paul1950
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Posted on Thursday, 19 July, 2007 - 11:56 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks for the input.
You are right, 900 RPM is too fast for this old girl. Purpose of the tacho is to limit the speed to 350 RPM max.
Thought there might be a problem using infra-red sensors. Infra red is heat and the sensor is right next to the boiler which is very hot.
Originally intended to use the 4 holes on the side of the flywheel with a light sensor(see photo) but may try the reed switch idea.
Steam engine
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obiwan
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Posted on Thursday, 19 July, 2007 - 03:00 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Beautiful tractor. They have several around here that have been restored, and it's not uncommon to find something like that or similar, still sitting in the middle of an old field, all rusted up.

Just about any of the above methods should work well. But simplicity is the key.
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john_becker
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Posted on Thursday, 19 July, 2007 - 04:42 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

There are many in the UK and there are various expos and fairs of them in use during our summer months. Beautiful machines.

Didnt know you had them in Oz too, they are mainly UK & European so far as I knew.

Dont know how I would add a tacho to one though. A lot of options it seems.

J
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zeitghost
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Posted on Thursday, 19 July, 2007 - 04:47 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

You could use some sort of inductive proximity sensor.

That is fairly resistant to oil & crud contamination, which optical stuff isn't.
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boris
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Posted on Friday, 20 July, 2007 - 05:44 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

With a max of 350 RPM a reed switch will be the simplest and best solution I think. If you have ever used an electronic bicycle speedometer, then youíll know how simple it is to set up.

The cycle computer needs to know the circumference of the wheel in order to calculate the KPH. I have never tried this, but in theory, you could Ďcheatí and give it a fake number that will force your KPH display to actually show the numbers for RPM.

Simple example (note, Iím working in metric):

If the magnet passes the switch once every second, (60RPM), and the computer is set for a wheel circumference of 100cm, the readout will show 3.6 KPH. Since 3.6 goes into 60 roughly 16.66 times, a wheel circumference 16.66 times larger should read 60 KPH (the RPM reading you want). So you set the wheel circumference to 1666cm. Like I say, I have never tried this, so I donít know if your cycle computer can be set for a wheel that large (530cm radius !). If not. You could use a factor ten times smaller (1.666), setting the circumference to 166 cm. Iím sure the computer could handle that. But you will need to ignore the decimal point on the readout.

Might be worth a try.

If you wish to use some mechanical mechanism for limiting the RPM, or just an alarm. A PICAXE is probably the way to go. But the reed switch sensor should work fine. Simple, robust and oil-proof.
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obiwan
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Posted on Friday, 20 July, 2007 - 06:11 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I think if you're trying to use a standard bicycle speedometer/tachometer, you'll run into the problem when entering the wheel size. As I recall, at least on models in the states, you only had a limited selection, and those were standard tire sizes.

You have to remember, it's designed for a bicycle, and has a very simple data entry mode.

At least that's how it was with mine, may be different over there.
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john_becker
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Posted on Friday, 20 July, 2007 - 06:51 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

It is on my PIC Bike Comp I did in about 1997 Obiwan, as I allowed for any wheel size to be entered. I'm doing the same with something else I'm working on. Easy enough for a PIC to be told wheel dia and then for it to calc circum using 22/7 (PI). I'm using millimetres dia.

Boris, the problem with reed switches is their limited switching operations. Even a million may seem large, until you calc that in terms of wheel rotations in a given period. Stick with non-mechanical means is my advice.

J
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obiwan
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Posted on Friday, 20 July, 2007 - 08:09 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I was only talking about the commercial units as somebody had mentioned earlier in the post. But he did say he wanted to build it using a PICAX, so putting anything in there would be easy enough.

I totally agree with the statement about non-mechanical means. That's one thing I have learned early on. Anything, anything mechanical will eventually break.

Mechanical will always be my last choice in choosing a sensor.
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paul1950
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Posted on Saturday, 21 July, 2007 - 10:14 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks for the comments.
Traction engines are rare in OZ. Probably only 20 or 30 in the whole country. This one was built by John Fowler in Leeds and shipped to Australia about 1920.
It was restored by a friend of mine.
Had thought about the bicycle option but wanted to learn about Picaxes.
There is a lot of vibration when driving the engine, no suspension on solid wheels. Will this be a problem with reed switches ?
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obiwan
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Posted on Saturday, 21 July, 2007 - 05:00 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

It could be, depending on the switches. If you have a robust one (large), then it shouldn't be too much of a problem, but you'll have to mount it so that the motion of the contacts is counter to the vibration.

(if the vibration is mostly horizontal, the contacts would operate on a vertical motion).

But that means a bigger magnet to operate them.

Also, you'll have to debounce those contacts. With the other methods you don't have to worry about it, but since a reed switch is mechanical (with lots of vibbration as you said), they'll be bouncy, unless you can find some mercury wetted reed switches.

But in general, it won't be a problem as long as you build it with that vibration in mind and make it so it can stand up to it.

I built a lot of robotic stuff at AT&T, and learned a few tricks. Mostly for "bigger" stuff, but for what you're doing, any isolation you can add between your circuit and the source of the vibrations will help, a LOT.

like mounting with shock absorbing materials. But eventually, you can expect solder joints to fail somewhere down the line. (using a bit more helps, but not too much, because the added mass will simple cause it to fail faster. There's a happy point.

Mounting components with stress points and mounting them so they're locked down and can't move. Don't mount stuff so it can act like a pendulum, like board mounts far away from the PCB edge, that edge will just vibrate like crazy and anything at it's apex will break.

You want stuff "stiff", yet flexible. Too stiff and it will snap, too flexible and it will ... well, flex, and cause breakage. Wires are going to probably be the worst. Mount them with plenty of strain relief, like loops and so on. It you loop the wire before mounts, it will act like a little spring. (better that loop flexes than right at the solder joint, because that WILL break)

Encasing the whole thing in epoxy would help, but then you can't ever get to it if (when) it breaks. But you can add glue, hot melt or epoxy to certain flex points like wires and components.

Once you start fabbing it, let me know what it looks like and I'll offer any tips I can think of.
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boris
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Posted on Saturday, 21 July, 2007 - 07:03 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Apparently my information is out of date. Modern cycle computers DO use hall effect sensors.

I have never used them myself, but it looks simple enough. Like I said in previous post. PICAXE dedicated forum here: http://www.rev-ed.co.uk/picaxe/forum/default.asp

Well worth a visit. Prolly find all you need just by using the forum search.
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john_becker
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Posted on Saturday, 21 July, 2007 - 08:05 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I checked RS & Farnell cats for vibration effects info with reed swithes - not quoted that I could see. But the thought of using mechanical sensors still worries me considering the number of revs and switching operations that could be clocked up.

J
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boris
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Username: boris

Post Number: 228
Registered: 05-2005


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Posted on Sunday, 22 July, 2007 - 03:23 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Interesting information here: http://en.wikipedia.org/wiki/Hall_effect

I might just get hold of a couple to play with.
"What's red and invisible?"
"No tomatoes"
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obiwan
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Registered: 12-2005

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Posted on Sunday, 22 July, 2007 - 11:39 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I love Hall Effect switches/sensors. You can get linear sensors or digital sensors that are directly compatible with logic circuits. No bounce, no nothing except on/off when you need it. (if you get the right one). Solid state and last dang near forever as long as you don't abuse them.

I've been using a HE button that came from a "Cherry" keyboard for years on breadboards. Push the button and you get a single 57uS pulse out of it, every time.

I just wish I had more of them. Dummy me for throwing the things out before I knew what I had.

I also grab them from old floppy disk drives. They normally use them as a rotational sensor. They're small and free.
Do Not Hit The Fly That Lands On The Tigers Head.

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