Digital RPM Meter

[Functional Artist]

Over the winter I ordered a

Tachometer RPM Speed Meter 4 Digital Blue LED + Hall Proximity Switch Sensor (~$10.00)

https://www.ebay.com/itm/New-Tachom...m43663.l10137&nordt=true&rt=nc&orig_cvip=true

Specification:
Power requirement: DC 8-15V
Measure range:5-9999RPM
Clear zero:Automatic
Clear zero time:about 10s. the greater the RPM value,the longer the clear zero time
Refresh frequency: 0.2-0.5S(120-1200RPM);0.25-0.06S(2400-9999RPM)
Measure indication:RPM<5000,±2;RPM>5000,±3
Display:Blue 0.56" LED
Sign: Pusle signal,NPN 3 wires normally open
Response frequency:100HZ
Tachometer dimension:72x36x20mm,panel cutout Dimension:68x33mm
Hall proximity model:NJK-5002C
Sensor dimention: M12x10x37mm
Detection range: 1mm-10mm
Proximity output current: 200mA
Detected objects: Magnet
Sensor wiring:brown-Power+;blue-Power-;black-signal
Operating temperature: 0 to 50°C.

...to get some "real" data, on the "actual" RPM's, that some of these small electric motors, that we have been working with, are producing

...unloaded & also loaded

 

[mckutzy]

This is cool... Ive been waiting for someone to do a writ up on this... I actually forgot about these.....
I was going to get one of these awhile ago... I was going to see if I can rig it to the blower housing to take reading off of the flywheel magnet..... but the HE sensor needs to read a certain magnetic polarity for use...

I found that itll work based on the measurements given.. in theory that is... but Im curious as to your input of these....

In the search on these devices... I found that people have used them for all sorts of rpm reading things, cause its really self sufficient; mount, apply magnet, maintain gap, apply power.....
A Cool one I saw, was for a drill press, just mounted to the quill and was able to get a really good reading...

---------- Post added at 09:08 AM ---------- Previous post was at 09:06 AM ----------

A secondary question to above......
With this device how much of a fuse value would you need to use? say in a stand alone or even automotive use?

 

[Functional Artist]

This is cool... Ive been waiting for someone to do a writ up on this... I actually forgot about these.....
I was going to get one of these awhile ago... I was going to see if I can rig it to the blower housing to take reading off of the flywheel magnet..... but the HE sensor needs to read a certain magnetic polarity for use...

I found that itll work based on the measurements given.. in theory that is... but Im curious as to your input of these....

In the search on these devices... I found that people have used them for all sorts of rpm reading things, cause its really self sufficient; mount, apply magnet, maintain gap, apply power.....
A Cool one I saw, was for a drill press, just mounted to the quill and was able to get a really good reading...

---------- Post added at 09:08 AM ---------- Previous post was at 09:06 AM ----------

A secondary question to above......
With this device how much of a fuse value would you need to use? say in a stand alone or even automotive use?

Hey mckutzy

Looks like it
...but, I dunno, were gonna find out.

As far using as a fuse,

According to the specs:
..."Power requirement: DC 8-15V
...& Proximity output current: 200mA"

Kinda small values
...& the diagram doesn't show a fuse
...so, I gonna skip it (for now)

But, to answer your question, in an automotive situation it looks like 12VDC will power it fine
…& probably a 12V 1A - 3A automotive type fuse should be adequate protection

 

[mckutzy]

In this case what would be the proper power supply, wire gauge to use? 12ga?

 

[itsid]

the meter itself draws maybe 200mA (I'd say closer to 100 tbh)
the proximity sensor doesn't need 300mA at all
(maybe for 300kHz but that'd equate to 18 million rpms and I haven't seen anything that spinny yet )
It might still draw the full amps though (totally unneeded for the task at hand)
I use Honeywell 495As hall sensors and they can be powered by the chips output pin (20mA or less) and are still quite responsive .. at least good enough for something like such tach
something with an internal comparator makes more sense for such task,
but I only have smd parts (salvaged off a bldc cd-rom motor)
5mA good for the 30k rpm the bldc is able to run at [model escapes me atm]

Anywhoo, four seven segment led ~10mA per segment I guess..
makes 70mA per digit.. 280mA max..
usually they use pov so no more than three leds are on at once (30mA per digit... makes 120mA) especially since that saves a led driver chip and all leds can be powered of the microprocessor directly (which I guess can provide no more than 200mA in total)

Sooo at worst that thing draws 500mA;
I'd say the chip and stuff draws 15mA maybe 20.. the display should be good with less than 150mA and so it all comes down to the proximity sensor... if it actually draws 300mA or is cool with 50mA [as it should])

So in total I assume something like 250mA or less (depending on the number displayed)
So awg 20 should do really; 16 if you want to dial in on the safe side

Fuse.. a 500mA quick should protect all internals but I doubt it'd be needed really.

'sid

 

[mckutzy]

Im really not that knowledgeable in the electronic field, I just know everything is needs to have certain things in a fashion or things burn up... But thanks for breaking it down....
I kinda understand...

This is cool... So even a/couple of 9v batteries could do the trick with out a fuse???

FA- what kinda things were you thinking about using this for???

 

[Functional Artist]

Im really not that knowledgeable in the electronic field, I just know everything is needs to have certain things in a fashion or things burn up... But thanks for breaking it down....
I kinda understand...

This is cool... So even a/couple of 9v batteries could do the trick with out a fuse???

FA- what kinda things were you thinking about using this for???

Yup, that was a really good/in-depth analysis, Thanks Sid.

I gonna set this (1) up as a mobile/hand held style meter
…& run it off of a (1) 9V battery

Kinda like the hand held Amp/Volt meter I assembled during/for use with El Turbo
http://www.diygokarts.com/vb/showthread.php?t=36894
(the meter part starts @ post #51)

I mainly want to use it to get some RPM data on these MY1020 & ZY102 motors
...then, ???

 

[Functional Artist]

Here is the meter (front & back)

I did some measuring (it looks like ~2 3/4" x 1 3/16")
…& marking (used a pencil & a square)

* I'm using another piece of plastic from an old TV back
…& gonna use a small plastic box for the back cover

** I did all of the marking & cutting on the (smooth) back side
...so, if I messed up/slipped & scratched the surface it wouldn't be visible on the (textured) front side

*** I also put a scrap piece of cardboard on the work bench
...as to not scratch the front side, while I'm workin' on the back side

 

[Functional Artist]

The back cover/box is ~2" x 3" x 2"
...so, the display, a 9V battery & an on/off switch should fit inside.

Below are the diagrams I drew up

First, I just redrew the (factory) diagram (for quick reference)
...then, I drew a diagram including the battery (power supply) & the (on/off) switch

Notice, the wire colors are no where near consistent
...Chinese ingenuity?

Positive (+) :
red (meter) to brown (sensor)

Negative (-):
black (meter) to blue (sensor)

Signal:
yellow (meter) to black (sensor)

* The switch that I'm using is "technically" a (3) way, on/off/of switch
...but, I'll only be using (2) poles (on & off)

** There wasn't room for the switch on the "face" & to still fit inside of the box
...so, I just installed it (inconspicuously) on the side

*** The display will fill, most of, the front of the box
…& the battery, switch & wiring/connections will reside in the back

 

[Functional Artist]

Industrial Velcro secures the 9V battery in place
...but, didn't get any pics

Wiring
I wired the positive (+) battery lead (red wire) to (1) pole of the switch
…& then, connected another piece of (red) wire from the other pole to the red/brown wire-junction

The negative (-) battery lead connects straight to the black/blue wire -junction

The signal circuit wires (yellow/black) simply connect together

After shrink wrapping all of the connections I gently tucked the wires into the box
...& then, bolted the face plate to the box, with a couple of 3/16 Allen head bolts/lock nuts


Here is our hand held digital RPM meter

* I mounted the box ~1/2" above the bottom of the face
...so, the meter will sit nicely & it will also hold the display so it's facing upward, just a bit

** Also left some extra plastic, above the display for maybe
...a hole (to hang it on stuff)
...or a clip (to clip it on stuff)
...or ??

 

[Functional Artist]

Picked up some more magnets
...for testing different stuff (motors/karts)
...may have to "glue" 'em on

These magnets are a bit smaller
...but, should be strong enough to activate our sensor.

* just noticed that the sensor has a small red light that's lit when the meter is turned on.

So, for a quick test (to see if this thing even works) I stuck the "stock" magnet to the rear "driven" sprocket on the El Mini (electric mini bike)
…& just held the sensor (by hand)

** The 10T "drive" sprocket on the motor is too small to attach a magnet to

So, we'll just take our readings off of the bigger 75T rear sprocket
...& then, do some math

Here is a quick video of this first (unloaded) test

https://www.youtube.com/watch?v=1iEcG92Q-rg

 

[Functional Artist]

I came up with a quick, simple (maybe temporary, depending on how well it works) sensor mounting bracket
...just "zip tied" it to a small, plastic, squeeze clamp

This way, I can simply "clamp it" on to a bar, somewhere close to the tire
…& it's easily adjustable

So, here is a video of my "loaded" RPM test

https://www.youtube.com/watch?v=8OcsGzFQ7Pw

 

[Functional Artist]

Here are the RPM tests results for El Mini with the ZY-1020 (GD) 48V 1,000W motor & a 7.5:1 gear ratio

Since were taking our readings from the rear sprocket, we will have to do some math
...& multiply the RPM reading by the gear ratio, to get the "actual" motor speed)

Unloaded Test
470 (on the meter) x 7.5 (gear ratio) = 3,525 RPM's


Then, if we divide the RPM reading, by the voltage were runnin' at
...we will get the RPM's per volt that this motor is producing
3,525/48V = 73.4 RPM's per volt

Loaded Test
430 x 7.5 = 3,225 RPM's
3,225/48V = 67.1 RPM's per volt

* The label on this ZY-1020 (GD) 48V 1,000W motor lists 2,800 RPM's
(most MY-1020 48V 1,000W brushed motors are rated @ 3,000 RPM's)

According to these tests, they seem to produce a bit more than that
...even under "load"

 

[itsid]

you sure?

Since the usual 12V battery has a 13.5V output when fully loaded
we talk 54V fresh and 48V semi depleted..

that'd be 3133 rpm unloaded and 2866 rpm loaded [@48V]
So my guess is, you tested with fully charged batteries and w/o your voltmeter attached

'sid

 

[Functional Artist]

you sure?

Since the usual 12V battery has a 13.5V output when fully loaded
we talk 54V fresh and 48V semi depleted..

that'd be 3133 rpm unloaded and 2866 rpm loaded [@48V]
So my guess is, you tested with fully charged batteries and w/o your voltmeter attached

'sid

I just posted what the tests seemed to show

I recorded these tests (did ya watch the videos?)

I did the unloaded test twice & got same results
...the loaded test was a several block ride
(it looked like 430 on the meter, several times)


Yes, these are brand new, fully charged batteries 12V 12AH batteries

No, I did not have any other meters attached


What factors did you use to get "3133 rpm unloaded and 2866 rpm loaded [@48V]"?

 

[itsid]

Yes, these are brand new, fully charged batteries 12V 12AH batteries

No, I did not have any other meters attached


What factors did you use to get "3133 rpm unloaded and 2866 rpm loaded [@48V]"?

simple.. assuming 54V (13.5V per battery)
dividing your results by 54 (rpm/volt to be 65.278 unloaded and 59.72 loaded)
then multiplying by 48 again to get the rated value

That's why I was asking about the voltmeter..
since again a fully charged 12V battery does NOT have 12V it's 13.5V or thereabouts.

'sid

 

[vybrano]

How did you attached magnet? Is it possible to measure speed with it? (I mean display actual speed, not calculate it out of RPM or something)

 

[itsid]

sure you can measure speed with it..
"actual speed " is a relative term tbh..
but if you know the effective circumference of the wheel* you can calculate it with the
measured wheel rpm.

and no, that is the ONLY way to measure road speed.. wheel rpm times circumference;
unless you want to use a GPS speed (which is less accurate most of the time)

attaching... superglue works well but IDK what Kevin did to attach it permanently.
(or if at all)

'sid

* effective circumference is calculated by the height of the wheel under load (center axle to road with you in/on the vehicle) and Pi, it's NOT the nominal nor measured circumference at rest!
ideally, you hop on/in and use some chalk marker across the tread, then roll (legs up) and measure the markings on the garage floor

 

[Functional Artist]

simple.. assuming 54V (13.5V per battery)
dividing your results by 54 (rpm/volt to be 65.278 unloaded and 59.72 loaded)
then multiplying by 48 again to get the rated value

That's why I was asking about the voltmeter..
since again a fully charged 12V battery does NOT have 12V it's 13.5V or thereabouts.

'sid

Those are some very good points, glad your around
...so, I checked for ya, the battery pack on El Mini sits @ ~53.4V fully charged.

New questions:
For calculating purposes is it "standard practice"
...to use 53.4V? (3,525/53.4=66.01 VPR) (volts per revolution)
...round it off to 53V? (3,525/53=66.5 VPR)
...or to also factor in the voltage drop (~1V while running) & use 52V? (3,525/52=67.7 VPR)

 

[itsid]

I think that depnends on how low your standard is

standard practice for an industry conform check that require battery operation
is to add a voltmeter to the system and log the volts under load at speed (ideally as precise as possible)
some companies work with just two decimals of precision, some with three or more
(using strictly calibrated meters with known drop).

So don't worry about it too much,
with your tools you cannot achieve a perfect result anyways
and the result you get is certainly good enough for what we do around here
since none of that is "solid truth" anyways
as long as we don't know the load specification for testing you cannot emulate that
and your load might be off by whatever unknown value.
(say a 2% error in the tach and a 1.3% error in the voltmeter)

even an ever so miniscule road inclination throws your values one way or another
so the only thing that'd make it a good result would be an average over a few runs on a dyno;
or precisely several different dynos to compensate their "errors"
that'd be an insane amount of effort for a home gamer.

So what you'd need is an average over several runs:
take a patch of road and do ten runs a day over ten days..
going up and down that same patch of road back and forth like a madman.
and keeping constant track of voltage and rpm (over the entire run)
but logging would be a pain, right..
(you can build such logger for a few dollars[<10 I'd say] if you want to I'll explain that later)
And then average out the results to get a really superb estimation
that should be within 1-2% of the proper lab-result.

'sid