tachcometer

HomeMade Tachcometer Project

Intro:

The idea here is to determine actual motor RPM. Yes Yes...The motors come with their RPM rating! But can you really believe everything you read?? With this simple project you can determine actual motor RPM and compare the performance of all kinds of motors without having to guess which is running faster. The idea came from a nice website at:
http://www.boondog.com/tutorials/irled/ir.html
It has a nice and simple Infrared emiter detector circuit with complete instructions and parts list. So I won't duplicate the information here unless one day that site is no longer available.

The Parts:

The above circuit....no more that $5
Multimeter with frequency feature.....??? I had this already1 :)
Old Ball Mouse..........?? had it too.....And they laught when I store junk Ha!

The Multimeter is the tricky part. unless you want to follow that site's idea and build a data acquisition board to interact with your PC... I'll stick with the Meter thanks. Heck You might as well invest in a good meter if you like this sort of stuff. Nice Christmas Gift ;)

Some Info

The circuit acts like a switch outputing a HIGH signal ~4v when the detector sees the emitter and a LOW signal 0v when it doesn't. the output will be a clean square wave that will repeat itself over and over...Since I used a wheel with a hole in it everytime the hole lines up with the encoder emitter you get a HIGH signal the rest of the turn its low. The signal will repeat with every revolution so since the meter measures Cycles per second, we take the number from the meter and multiply it by 60 to get revolutions per minute.

Everything you need to know on how to build the circuit is on that site. I followed the PCB Artwork Solder side diagram for my component placement and ran wires on the other side of the board. It helps to draw the componets on a piece of paper and draw lines representing the wires as you add the wiring, that way you won't miss any connections. Remember to always check your components data sheets for correct pinout. Is very easy to confuse the transistor legs, especially when working on the other side of the board were everything gets reversed.

CAUTION: The solder diagram doesn't show the components only the wiring you'll have to use the circuit diagram to draw the components on the Solder diagram. I noticed a small mistake on the Solder side diagram were the detector polarity was reversed. Pin 3 should be (+) and Pin 4 (-). Other than that all is good. I didn't see a mention for input voltage so I used 5v.

The Pics

Here's my handy SunPro Multimeter, this thing can handle lots of things, great for working on cars, It provides RPM, Temperature, etc. I LOVE IT!!!

The Two settings I used, I had to go to the 3200Hz for the faster motors...

BTW the RPM feature works by induction on the sparkplug cable so I couldn't use that.... ;)

The circuit and Test Platform, The Custom made platform was made of special insulating biological material...Easy to work with and reliable.
I used a emitter detector pair instead of the individual LED looking emitter and detector from the site which are bulkier. I've seen these at Radio shack! they work on the same principle but have a smaller footprint ideal for the scale we are dealing with. Sensor A is where I hook up the rig on the picture, Sensor B I use to hook the rig mentioned on the site (Phototransistor and IR emitter).

For de wheel I used the encoder from an old Ball Mouse. I covered all but TWO holes on the wheel with glue, then used black permanent marker to make sure no light could get thru., I removed the tab between the TWO holes to join them together into a nice square hole. The center hole on the wheel is a little larger than the motor shaft so I used a small piece of wire insulation to make a tight fit.

Before and After

All that's left to do is to HOOK the meter, Charge the CAR, Power the Rig and stepping on the GAS! well sort of! Pushing the Stick forwards on the remote and take the reading.

HERE ARE SOME NUMBERS!
The first number was taken on the car with the setup above, the second was done with an old AA 1.5v battery I had laying around to, I took the motors out of the car and wired the battery to them.

Motor(Color) (Rating)        Hertz and RPM            Hertz and RPM (AA old battery)
RED            21,500            328 = 19,680                  390 = 23,400
GREEN         16,000            260 = 15,600                  300 = 18,000
BLUE          10,000             185 = 11,100                  205 = 12,300

Interesting results, the higher we go on motor rating the more the actual number drops, Could it be that the car is not providing enough JUICE as the load increases??? When using a AA battery all motors performed 2000rpm over their Rated value. this might suggest that the motors are rated correctly...I was impressed that the motors are rated correctly.;)

I will keep Posting Numbers as I expand the tests to pager motors and Bits.

Again this only attempts to measure motor performance factors like gear ratio, weight friction, drag etc. are not taken into account and these can seriously affect final speed. But with this benchmark one could get a good idea on how voltage tweeks and other mods affect motor performance! :)

The Future
I worked on a mouse wheel that attaches to the outside of the rim, to measure wheel rpm...possibilities are endless.
The idea of having rollers so you would only have to put the car on the rollers and run it like real Dyno testing was mentined... Maybe if I have the time...The main problem I see is finding rollers with very little friction.

CloseUp pictures Tutorial