Thread: Mars Lathe DC motor conversion.

[QUOTE=Stustoys;1581245]

Hi Rob,
Something had been troubling me about your post 108 and people asking about the decimal point. I think you'll find your tacho is set to display Frequency and you've used the multiplier to make it display rpm. I've just checked and it doesn't seem to change where the decimal point.
600rpm=10Hz
1/0.0167=59.88
10x59.88=599rpm
Doubt it makes any difference at the end of the day but I can stop scratching my head about that number.

Stuart

Hi Stuart,

Looks like you've nailed what's going on.

Sounds like your explanation is spot on.

My tacho is going well. Still waiting for a treadmill that's not two hours drive away - been a couple that I passed up on.

A bit OT but I did have a little win the other day that I was almost going to put in tool gloat

I bought a 750 watt generator for $20. Yep it was that expensive.

I put up a video if anyone wants to hear me droning on - you might find it interesting.

My cheap Chinese generator - $20 - a bargain - YouTube

Been working on a slide hammer adapter for a set of locking pliers based on a discussion somewhere else in the forum.

Cheers all

Rob

Hi guys,

I need help, it's doing my head in!

After destroying my original induction sensor, I have been offered a replacement, in fact I have received two different replacements from two different people (Thanks Stuart, Thanks Techo1) They are both slightly different and I think have different frequency responses. I was hoping to try out both and return the one I don't use to the original owner.

Anyway, my luck has not improved too much. While I have managed to get one sensor to work, the other does not! Ahhh! Mind you I have no doubt that they working when sent to me, in fact they probably both do work!

The one that does work, I have connected to my tacho. I have set the divider to 3 (display shows 00003) and I have set the other selection to 1 or 3 (rotation speed)

But I for the life of me can't get (1) a stable reading or (2) a reading that even remotely resembles the actual RPM, even at lower RPM such as 80 - 100.

Am I doing something wrong?

The output of the sensor (black wire) I have going straight to pin 1 and the GND to pin 2.
Input voltage I have set to about 12V, display works fine but not showing what I want it so.
What am I doing wrong?

Simon

Hi Simon,

When you say the reading doesnt resemble the actual rpm. Would I be correct in assuming it's reading lower than it should?

Stuart

Originally Posted by Stustoys

Hi Simon,

When you say the reading doesnt resemble the actual rpm. Would I be correct in assuming it's reading lower than it should?

Stuart

No Higher.

Here are some readings:

Actual .... RPM Reading
8 ........... 213
16 ......... 153/319
32 ......... 175/360
50 ......... 193/238/354
80 .......... 225/160

Readings from 16 - 80 flashed multiple readings and they were reasonably repeatable. I know I have 3 pulses per revolution which may confuse the signal if they are not accurately spaced but I have physically measured them to be within 5/100 mm each so i can't see that that would be enough to create several readings from each revolution. Either way, there seems to be no pattern or trend with these numbers. I deliberately kept the RPM low so that sensor frequency response would not add to the confusion so It should not be that.

I have tried serveral combinations of numbers with both the divider and the tacho/frequency with little success. Do I have a dodgy tacho?

Simon

Hi Simon,
Does the sensor you have working have an LED in the back?
At those speeds you shouldnt have any trouble seeing the pulses.
If you trigger it manually at say 1 per second, does it display a stable 20rpm?

Wild shot in the dark, are you sure you have 12VDC? I'm wondering if maybe that switch mode you bought might be the issue?

Stuart

Hi Simon,

What do you have connected to pin 5 ?

Rob

It's a bit hard to know what's going on without the sensor part number and a circuit diagram, but do you need/have a load resistor? My tacho was giving crazy readings until I put a 10K load resistor in the circuit. The readings are now accurate and rock solid (I have an optical sensor with a digital output). The output impedance of my sensor is very high, so when it switches off it's essentially an open circuit. This means any stray signal picked up by the wire between the sensor and the tacho can trigger the tacho. A load or "pull down" resistor pulls the voltage down to 0V when the sensor output goes low.
Chris

Thanks for your help here guys. Im out atm and on my phone. Ok the sensor im using has an led at the rear and shows a very reliable output that can be seen. Im not using a switch mode supply atm as it is yet to arrive. Im using a benchtop dc power supply which i believe to be very clean dc. I don't have a pull up/down resistor on the output. Just got the output going straight to signal input. Both signal ground and- vcc. Are connected together since the tacho. And sensor share the same supply. Can't remember pin numbers so can't say what is connected to pin 5. It will either be 12v dc or signal from the sensor.I may try a resistor. Has everyone else used a pull up/down resistor?Cheers.Simon

Hi Simon,
It sounds to me like you have the setting on the tacho wrong. Do you have a scope? If so you can check the pulse timing and see if it is wrong or right.

Hi ueee.I had a look on the cro yesterday. The signal seemed pretty clean but low level stray signals may be present from a floating output. I think my first thing will be to put on a pull down resistor.Simon

Hi Simon,
I think its covered here.
https://www.woodworkforums.com/f65/wow-huge-price-difference-157501/index11.html#post1556305

Stuart

Thanks Stuart, I do remember that post.

OK. I just ducked out and tried both a pull up and pull down resistor, 330 ohm and 4.7K. The 330 ohm killed the signal and the tacho showed only 00000! This surprised me since the specs say that it's good to 300mA and 12/330 is only 36mA!

The 4.7K produced a signal but still no good. At approx. 40 rpm it produced an output that toggled between 152 and 315?

Don't know what next to do. I'm close to buying another one, at $11 it's worth a try. I'm a bit busy now but I think I'll have to have another look on the cro but I don't think the problem is the sensor......

Simon

Simon,
I think we are going to need a circuit diagram before we can comment any further. Also, what is the part number of the sensor?
Chris

Originally Posted by simonl

Both signal ground and- vcc. Are connected together since the tacho. And sensor share the same supply.Simon

Simon I think I know where you are going wrong.

The signal ground should be be connected to pin 2.

The input power supply voltage negative GND should be connected to pin 4.

You can't have then connected together as one is power supply negative and one is circuit board gnd - not the same thing.

Thats how I have mine and the tacho works fine.

Cheers

Rob