Digital RPM Display Problem

Hello Everyone,

I have one of those very cheap digital rpm displays from China. Mine is the blue four digit display.

I have read various threads on this forum, watched a number of you-tube videos and searched other web sources for information. I believe I have connected it the way the majority of opinion leads me to believe is correct.

The 12v power supply is from a wall wart. When connected as shown the display shows 0000. The light on the rear of the sensor is illuminated.

For testing I have the magnet (neodymium) fixed to my Fobco drill chuck controlled by a VFD and the sensor is aligned temporarily in a small vice on the drill table. When I rotate the chuck nothing changes! The display stays at 0000 regardless of speed.

Yes I have changed the magnet over and that made no difference. I clearly have power to both the display and sensor so can someone please tell me the blindingly obvious! What have I done wrong?

By the way the lead from the display is a five core ribbon cable, no distinguishing colours and I have assumed they are numbered left to right whilst looking at the rear of the display. If I connect it right to left the display is blank.

Just to confuse mere mortals like me on one of the suppliers listing it says:

1 Power source + (assume this is dc supply +)

2 Power ground (assume this is dc supply -)

3 Signal earth (assume sensor but which lead?)

4 Empty (understood!)

5 Measuring + (assume sensor but again which lead?)

Another supplier lists the sensor connections as:

Brown (Power +)

Black (Test +)

Blue (Signal)

Any ideas welcome please.

Display pins 1/2 ..... Connect your power source there as you stated...

Probe connect up as you did but, put Black to pin 3 and Blue to 5 ( on the display terminals)....you may need to link 2&3 on the display ( unless they are connected internally..)

Now try...might need to swap magnet back...

Can you provide a link to the actual unit please?

Looking at your circuit, I think Blue (signal out) should connect to 5 (Measuring+). That would explain why you don't count revolutions but not why the sensor lights up.

Black ('Test +'?), is probably DC- for the sensor but I'm guessing!

Anyway, with luck, you have Black and Blue the wrong way round. Your diagram looks correct apart from that.

Signal Earth and Power ground are likely to be connected together inside the box. Best practice is provide a signal cable with its own earth to avoid the possibility of picking up noise from the power line or spark leads in an engine. In this application you can probably use either.

Dave

PS John Rudd types faster than me, at least it's the same answer!

Edited By SillyOldDuffer on 16/07/2018 13:05:55

See here....more info inc sketches

https://www.model-engineer.co.uk/forums/postings.asp?th=112167

Thank you John and Dave for your replies. I have tried as you suggest and there is no change - I still cannot get a rpm reading.

I cannot explain why both sensor and display have power but it does not work. I have decided to put my retirement at risk by investing in another from another supplier. After all if others can get them to work maybe either the display or sensor has a fault. Like modern cars maybe substitution is the easiest way to find out!

The one I have at present was from eBay: Item ID 152633294042.

Thanks again for your help, I will post a result when I get a replacement.

hi

I had exactly the same problem if you look at my posts there is a posts that got mine up and running

hope this helps Geoff

What the gap between the magnet and the sensor?

Could be a faulty sensor. Does the sensor led flicker when the magnet passes it?

Looking at the unit you have listed from ebay :

Panel pins are : 1 positive ( 8-24v )

2 negative

3 signal gnd

5 signal +

on the sensor brown + blue - and black signal but the shcematic ( if one could call it that ) shows it connected to the + rail via what looks like a pullup resistor although idoesnt show a value .

If you hook up the sensor to power and rotate the magnet past it does the red led ( next to where the cable enters the sensor)   light up ? The pull up resistor keeps the signal pin high until the transistor inside the hall sensor turns on pulling the pin low try a 10k resistor between the positive and the black sensor wire .

The units i have didn't need a magnets just a disk with a cut out section ,

Edited to exterminate one of those winky things that keep chasing me ! Die you B***tard  !!!

Edited By XD 351 on 16/07/2018 15:53:06

Edited By XD 351 on 16/07/2018 15:59:53

Today I received another digital RPM display and connected it up as John and Dave suggested. Whoopee it works!

By substitution it appears the original sensor was faulty.

Thanks to all for your help and advice.

John (a genuine old) Crock(er)

Good to hear you got it sorted...

One problem I have had with these units is interference causing either no change in reading (as here) or a very rapid and unrelated series of readings, seeming at random. I traced this to the voltage supply (not a battery but an interrupted DC signal itself, in many cases), the workshop lights (LED floodlights with their own internal cheap electronics), and even the electronic voltage regulator on the vehicle itself.

So, I abandoned the idea, relying instead for my old car on 50-year-old advice from a friend who had a Velocette. He said 'Oh, I take it up to three amps and then change gear'. Those were, indeed, the days.

Cheers, Tim

Edited By Tim Stevens on 30/07/2018 22:04:03