might fill the gaping hole in my life.

I get that feeling a lot but usually I can fill it with red wine and cheese..

:-D

 

A DANIU DT2234C+ from Banggood measures revs well. That' works by shinning a light at the rotating item, works best with a black stripe on a wheel.

At under a tenner it's a bargain just for measuring revs, whatever it may or may not do to fill a hole in your soul.

Hall effect tachos pick up the magnetic field from the rotating item. Not tried that, it's not obvious how the wheel of a slot car would produce a rotating magnetic field for the thing to measure. Anybody else like to jump in and explain?

 

Hi Wankel

As per 300's advice I'd look for an optical tacho rather than something using hall effect. That is unless you have some tiny magnets to fit to things!

(the term "hall effect" refers to the sensing of magnetic fields, which produces an electric current, btw)

I bought an optical tacho from eBay, it works off a 9v battery and requires me to fit a black disk with a strip of (white) reflective tape to be fitted to whatever I'd like to measure. I made a disc out of black plasticard and the reflective tape was provided with the tacho. I've only ever used it on motors, both in H:O scale cars and 1:32.

I'm not sure on the absolute accuracy but the numbers are about right and it certainly good enough for 'this motor's faster than that one' comparisons.

This is the one I have:

https://www.ebay.co.uk/itm/LCD-Digital-Optical-Tachometer-RPM-Laser-Non-Contact-New-FREE-AIRMAIL/222384838436?hash=item33c72b5724:g:VscAAOSwxvxW8c34

(I don't remember what I payed, this one seems a tad high but it;s just the first one I found in search)

 

Thanks Wankel. I've been thinking of buying a tacho for ages and this thread has clinched the sale. I've just bought the DT2234C+ as recommended by 300SLR and LMP.

Ebay. £6.18 delivered (eventually) from Malaysia.

https://www.ebay.co.uk/itm/Advanced-Good-New-Digital-Laser-Photo-Tachometer-Non-Contact-RPM-Tach-Lot-W/282958687373?ssPageName=STRK%3AMEBIDX%3AIT&var=583009533933&_trksid=p2060353.m1438.l2649

 

I can also vouch for the DT2234C+

-g

 

For those interested here is something "old school" from the '60s.
Proper analogue- simple and fun to use

 

Hell, yeah! Scale horsepower readings! Now that's what I'd like! And in such a handsomely constructed body, too! Looks like you've really looked after that piece of kit. Genuinely, thanks for sharing.

Anyhoo, a fool and his money... Which is to say one will also shortly be on its way from Malaysia to the Wankel Technology Centre, here, in Oxfordshire.

NB, I could not bring myself to make the purchase from, snigger, banggood.com, just in case they plaster their packaging with their url - I'd never be able to look my postman in the eye again.

 

Fear not Sir Rotary. You can have one in plain packaging from the Bay of E instead.

https://www.ebay.com/itm/Digital-Photo-Laser-Tachometer-RPM-Tester-Handheld-Non-Contact-Tach-Tool-Black/123342483451?hash=item1cb7c8effb:g:SVIAAOSwfbpbiCkH:rk:2f:0

 

"The Hall effect principle is named for physicist Edwin Hall. In 1879 he discovered that when a conductor or semiconductor with current flowing in one direction was introduced perpendicular to a magnetic field a voltage could be measured at right angles to the current path."

 

As per 300's advice I'd look for an optical tacho rather than something using hall effect. That is unless you have some tiny magnets to fit to things!

Psst, there's a big rotating thing in the motor that makes a magnetic field I have a sneaking suspicion it measures the variation in magnetic field as the armature turns, instead of the optical variation that you have with the bicolour disc tachos.

 

that optical one shown is typical, and they work well. most come with a strip of reflective tape. I cut a small square out of it and stick it near the outside of the tire or roller and aim the tach at it.

I also have made a bunch of discs that will allow me to check motors- take a piece of plastic, cut a two-bit (~1"/2.5cm) round piece of it, make a motor shaft-sized hole in it and glue it to a spare plastic pinion. then put it on a motor, spin it up and file/sand it round. blacken with a marker for contrast and add a little piece of the reflective tape near the outside. then when you power a test motor from a fixed DC source, you can measure the rpm of various motors for reference.

that's a start. of course, the only thing that really counts is how it performs on the track.

 

I wanted a look at the insides of that sweet T.H.P Dynamometer, and found it in this great Gadgets thread started by Don. (Yet again, thank-you Don..

https://www.slotforum.com/forums/index.php?showtopic=146233

 

The southern engineering products dynotester...
I love that old school integrated analogue meter!
Very steam punk
AlanW

PS I used the Hall effect sensors on my mk2 rolling ( for the tachometer )
I found that the sticky refective tape (used on mk1) was always getting dirty and falling off into the inside of the unit.
AlanW

 

https://www.slotforum.com/forums/index.php?app=blog&module=display&section=blog&blogid=434&showentry=4474

 

Psst, there's a big rotating thing in the motor that makes a magnetic field

I have a sneaking suspicion it measures the variation in magnetic field as the armature turns, instead of the optical variation that you have with the bicolour disc tachos.

It would be interesting to hear any practical experience of that.
Great if it gives a consistent reliable reading (even if it's 3 times the actual rpm) but there's no way of knowing it'll work adequately without trying it.

It does take a little experimentation to get optical tachos to produce consistent reliable readings. Several posts above tell you how they make it work. I've found reflective tape isn't necessary, the tacho works reliably with black and white paint, An alternative to a disc on a pinion that also works well is a 2 mm bore wheel with a broad painted stripe on it on the 2mm motor shaft. I've also tried various ways using the tacho without discs or wheels - they produce a reading but not a consistent reliable one.

NB, I could not bring myself to make the purchase from, snigger, banggood.com, just in case they plaster their packaging with their url -

Hmm Baggood does sound a bit of an iffy name, but in my experience they are pretty good as long as you don't mind the 10-20 days delivery time. There's nothing embarrassing on their packaging!
On the very rare occasions they've sent a faulty item, they've given a refund or replacement no problem. That's in marked contrast to my bad experiences I've has with a well known auction site.

 

It would be interesting to hear any practical experience of that.
Great if it gives a consistent reliable reading (even if it's 3 times the actual rpm) but there's no way of knowing it'll work adequately without trying it.

I will have to get myself a hall effect sensor and see what I can brew on an arduino. Then I'll compare with someone who has an optical tacho. I like the idea of it being totally unintrusive - just placing a sensor near a motor and giving it revs, instead of having to fiddle with fitting something for the tacho to read.

 

Hi William

I look forward to that! Thinking through what might be expected, there will probably be a strong magnetic field from the magnets which will change a lot depending on exactly where the sensor is, the signal you are trying to measure will be a smaller variable field cycling 3 times per rev (once as each of the 3 poles rather than once per rev). Or my speculation might be wrong and you'll find something completely different!

Indeed totally unintrusive instead of having to fiddle with fitting something for the tacho to read would be attractive.

Adding discs or wheels to a motor shaft is clearly going to make some difference to the balance. That probably doesn't matter with low reving motors that are almost invariably someway out of balance anyway. For the quicker motors that are properly balanced to start off with that would be more of a concern.

 

The native sampling rate of a 16mhz arduino is about 9,600 hz (it takes 13 cycles to process a reading). So unless you use an outboard sampling chip the arduino will struggle to measure the sort of rpms we are talking about.

I know very little about arduino but I do know the cycle times are slow.

 

Thanks for the heads up. I imagine I would have come up against this quite quickly as the first thing I do when programming is google to see if anyone else has done it, and there's lots of talk of using ADC free running mode and fiddling with the prescaler to get around that limitation.

Though I'll look at dedicated hardware if it still poses an issue. I didn't have a strong opinion on tachos until today and now the idea of one that doesn't need any special hardware attaching to the motor/axle has captured my imagination...

 

I know nothing about Arduino and most of the other stuff you guys are discussing. I only know that electricity exists because I pull the trigger and the car moves but I'm sure glad that someone put a key on a kite string and started the ball rolling and you lot understand a bizzilion times more about it than I do. What I do know is that the highest revving motor of two identical looking motors does not always make a car faster. I usually just use sound and feel.

 

I know nothing about Arduino and most of the other stuff you guys are discussing. I only know that electricity exists because I pull the trigger and the car moves but I'm sure glad that someone put a key on a kite string and started the ball rolling and you lot understand a bizzilion times more about it than I do. What I do know is that the highest revving motor of two identical looking motors does not always make a car faster. I usually just use sound and feel.

..but gratifying to have the good feel of the car validated by some tech so that you can try to replicate that good car at home with some level of confidence before it hits the track?

 

The native sampling rate of a 16mhz arduino is about 9,600 hz (it takes 13 cycles to process a reading). So unless you use an outboard sampling chip the arduino will struggle to measure the sort of rpms we are talking about.

Yes. Exactly. It will surely struggle. A flux-capacitor on the other hand, would nail it.

 

Yes. Exactly. It will surely struggle. A flux-capacitor on the other hand, would nail it.

If any of you think I am going to donate the flux capacitor from my HB Viva you can think again.....

 

One way around having to stick a reflector on the motor or wheel would be to have a permanent roller. Using the motor to drive wheels that drive the roller would give a reading. If you know the diameter of the tyres and the roller and the gearing of the motor then RPM can be calculated. If sounds complicated but its simple if you have a spreadsheet and can set up the math. That way you could measure the RPM of the motor whilst still in the car AND the RPM would be under a realistic load.

Its on my project list. But according to my wife her list takes precedence 🤨

 

Yes. Exactly. It will surely struggle. A flux-capacitor on the other hand, would nail it.

Hi

but only if the car can hit 88 mph surely