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I need help getting started.

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MrBugs

New Member
I'm another noob when it comes to electronics and my main hobby is Slot Cars. However I want to combine the 2 hobbies by building a tach that will measure the rpm at the wheels. I want to change the gear ratio and measure the changes can anyone point me at a web site where I can get some ideas?

Thanks everyone
Brent
http://slotcars.carlsoncomputers.com[/url]
 

John Sorensen

New Member
What a great project! But aren't those cars little-itty-bitty? How big are we allowed? What power is available (if any)? My first impression is some sort of opto-- you'd have to put a little reflector on the wheel I think. Then count the pulses. How would you get the info off the car? RF transmitter? Put the info on the rail? What a neat project...

j.
 

MrBugs

New Member
The scale that I'm into is 1/32, the cars are about 4 to 6 inches long 10cm to 15cm. The voltage they run on is from 12 to 18 volts DC. I was thinking about using a piece of reflective tape on the rear wheel or some sort of roller jig that the rear wheels turned and the jig was connected to a measuring device. I would measure the rpm on the bench not on the track these cars have very little space to mount any electronics so a test bench tool is what I'm looking for. I thought I might use one of the 3 pin fan headers from my motherboard, MBM measures them but Im not sure if they provive enough current. I would maybe write a VB program to read the data from the lpt port. How do I create the data and get it to the lpt port?
 

MrBugs

New Member
Basically all I'm looking for is a simple Tachometer circuit that I can build or a Tachometer kit I can purchase.
 

John Sorensen

New Member
If you can mark the tires with a piece of reflecting tape (I don't really know how that would affect balance/performance) then you can probably detect rpms easily enough using a photosensor/emitter pair (just picking one out of the blue, like Digikey p/n QRD1114-ND, you'd have to check the datasheet to see if it is really suitable for the frequency you're looking at.) Once you've played around with the distance/LED current/Tranistor load you really could just read the frequency with a DMM, for example, the Fluke 87, if you have or can borrow one. If not, counters are fairly easy to make too. The hardest part is going to be messing with the tape and the sensor positioning.

If you have an old mouse, you could probably use the IR photosensor out of it, if you're handy enough.

j.
 

MrBugs

New Member
I have several old mice and computers around. I'm a systems analyst so computer parts are quite common around my house. I know how to use a soldering iron but I don't own a very good DMM (10 yr old Radio Shack 22-195) so I would have to build a counter. I'm removing the optics from a mouse now :D
Do you have a schematic for the counter?
 

John Sorensen

New Member
I don't know nuttin bout programmin no puter, but a systems analyst like you might be able to use this:

http://www.e-insite.net/ednmag/contents/images/42999di.pdf

Since you don't have a "modern" DMM, are you going to be able to get a hold of a scope? Because getting a good signal from hacked mouse parts is going to take some testing, if the phototransistor is up to the job at all. A scope is the only way to know if you're headed in the right direction or not.

j.
 

MrBugs

New Member
I used to have an old analog scope quite some time ago but I'm not sure if I can get my hands on one now. Thank you for the PDF it is exactly what I needed. I'm thinking about making a special wheel for the rear axle of the car being tested. I have some solid aluminum wheels that I could paint black except for a small patch that could be used to reflect a signal back to a sensor.
 

stevez

Active Member
Seems to me that those little motors make quite a bit of RF noise and that the noise frequency is proportional to RPM. If there were an impulse or signal of sufficient amplitude that you could pick off the rails you might be able to do something with it - some filters and a F/V convertor or processing via your PC.
 

John Sorensen

New Member
If you can make special wheels, then why not make one with a hole or a notch in it, and use the photosensor as a through-beam instead of reflected? Probably give a better signal that way.

j.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Have you considered using a tachometer for a model plane?, they work off reflected light from the propellor - perhaps you could paint stripes on a wheel and pick it up that way?.
 

MrBugs

New Member
I didn't know they made Tachometers for other hobbies. How much is a Tach for a model airplane go for and what is the max rpm it will read? Thanks everyone for the help!

Merry Christmas & Happy New Year
Brent
 
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