How to wire output of rpm hall sensor to two different circuits?

[Rimbaldo]

Hi! I'm in the middle of a mini lathe cnc conversion, using the gecko g540. I already got the hall sensor working for rpm measurement, but not installed yet.

I know I can wire the output of the hall sensor directly on one of the gecko g540 inputs, grounding the sensor's output.

But I'd like also to wire at the same sensor output an RPM lcd display, to be installed on the lathe's case. It's got a DC +12V wire, a GND wire and an input (that is said to be able to read positive voltage or grounding on it's input.

So I'd like to wire the only hall sensor output at the same time to the gecko g540 input and the sensor input. Can I just connect the sensor wire to both inputs?

This is the sensor:

https://www.ebay.com/itm/152381471322

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[john-100]

a possible solution to your problem




John

[Rimbaldo]

a possible solution to your problem




John

That´s just about it!!!! All I need is a 2N7000 !!

English is not my first language.. I tried searching the forums, but maybe I didn´t use the correct keywords...

Thanks so much for your quick solution!!!

[john-100]

Glad I could help

John

[Rimbaldo]

Glad I could help

John

John.. I have one doubt though...

The tachometer measures the voltage on the input wire in relation to the ground wire? Because if the switch is open (magnet far away), then there´s the full, for instance (8-15V) on the tachometer, and the gecko g540 input get´s closed. If the magnet is near the sensor, then there´s 0V between the ground wire and the output wire of the hall sensor, and the gecko input get´s open.

So the relation is inverse? When there´s voltage on the tachometer input (magnet far away) the gecko input is closed. When there´s no voltage on the tachometer (magnet near), the gecko input is open.

Is that right or am I messing things up?

Thanks!

[john-100]

Hi

the tacho module has a 5V regulator on the board so the +12V supply is reduced to 5V for the counter IC



if you just connect the 12V supply you should find you have 5V between the input terminal and ground

looking at the Hall sensor ,
while your diagram in the first post shows the Hall switch powered by a 12V supply
the sensor circuit controls an NPN transistor with its collector connected to the Hall switches open collector output -


just think of the transistor as acting as an ON/OFF switch between the ground and output terminals
it can not provide any voltage from the output terminal , it can only sink an external current to ground

Yes , the 2N7000 acts as an inverter, if thats a problem
you can add a second FET and a resistor so the signal is inverted twice
(so its no longer inverted)




John

[Rimbaldo]

Thanks again John! Is very kind of you to even do a new diagram!

As the 2N7000 acts as an inverter, if I let the setup the way you sent before (without adding another FET), the only thing I should do is, in the MACH 3 interface, let MACH 3 know that the input is NOT active low? I just have to set-up the input behavior? If it´s normally HIGH or LOW?

I guess I read voltage between the output and ground in my sensor (OH137) because I already soldered a pull-up resistor between Vin and OUT, right?

Thanks one more time!

[john-100]

Hi Rimbaldo

as you expect
with a pull up resistor like this you will see the output switch between the positive supply and ground as the change in magnetic field is detected




yes , in Mach3 you can make the input active high instead of using the second FET (2N7000)

John

PS

what value of pull up resistor did you use ?

if its connected to +12V and too low a resistance
you can have too high a current flow into the tachometer inputs protection citcuit

very often IC input pins have two normally reverse biased diodes
one diode conducts when the input is pulled more than 0.6V above the positive supply (over 5.6V)
and a second diode that conducts when the pin is pulled more than 0.6V below the negative supply ( -0.6V)



John

[Rimbaldo]

Hi John!

I used a 1k resistor as a pull-up... is it too low? I could power the OH137 with 5 volts instead of 12 if it's better to reduce current...

Does the fact that there is a pull-up resistor already soldered (and inside the epoxy glue... ) changes anything on those diagrams you sent me? Or the resistor doesn't make any difference in the wiring?

Thank you one more time!

[john-100]

Hi Rimbaldo

the 1K resistor is low when you have it connected to a 12V supply

the problem is I'd expect current to flow from the +12V supply
through the resistor to the tachometers input terminal and via the IC to its +5V supply
so I'd expect the current to be 7 mA ( {12-5} divided by 1000 = current in amps)



I'd increase the resistor to 100K

By using a 5V supply for the Hall effect sensor the current will reduce to zero
( with +5V at each end of the resistor you get no current flow )

John

[Rimbaldo]

Hi!
Undestood....
Well... . as my hall sensor setup is already inside an acrylic/epoxy casing, would be possible to add a resistor in series to the output of the hall sensor? A resistor between the hall sensor output and the tachometer input, to decrease current? A 100k resistor then? Would it work?

Thanks again!

[john-100]

Hi Rimbaldo

how about re-arranging the circuit to connect the Hal switch to the G540 input and using the 2N7000 to buffer the signal going to the tachometer input




John

[Rimbaldo]

Hi! Yes!!

That's easier! So in this case, mach3 can be setup as Active Low, and the tachometer (correct me if I'm wrong) input works either way. Doesn't matter if the signal is inverted or not, right??

[john-100]

Hi Rimbaldo

I would not expect it to matter if the signal to the tachometer is inverted

John

PS
a third way using a diode


the diode enables the switch to ground the tacho input but
stops currents from the G540 input or the 1K pullup resistor
flowing into the tacho input

using the 2N7000 FET has the advantage of reducing the current the Hall swich needs to sink to ground

[Rimbaldo]

Hi Rimbaldo

I would not expect it to matter if the signal to the tachometer is inverted

John

PS
a third way using a diode


the diode enables the switch to ground the tacho input but
stops currents from the G540 input or the 1K pullup resistor
flowing into the tacho input

using the 2N7000 FET has the advantage of reducing the current the Hall swich needs to sink to ground

Thanks John!

But the current needed for the Hallswitch to sink to ground isn´t dependent on the pull-up resistor right? As the Out and GND are shorted when the magnet is near it... So it´s dependent only in the internal circuitry of the Gecko´s input? Is there a risk of using only a diode? To much current?

[john-100]

Hi Rimbaldo

looking at the data sheet for the OH137
the Hall switch can sink 25mA

with the switch output low it has to sink the current from the G540 input , 1K pullup resistor and the tachometer input

the current through the 1K resistor will be 12mA
if you assume the volt drop across the LED in the G540's input opto-isolator is 1V
when you ground the input pin the current = 11V /2K2 = 5mA

if the tachometer input is like the standard TTL 7400 series logic IC
the current that will flow to ground will be 1.6mA
(you will probably find it less than 1.6mA but I can find any data for the module )


the total the Hall switch has to sink will be 18.5 mA
that 6.5 mA under the Hall switches limit

if you connect the power to the G540 . Hall switch and tachometer without connecting the switch output , G540 input or tachometer together

you can use a multimeter set to mA to measure the current from the G540 input to ground

and the current from the tachometer input and ground to verify the current is what I expect

John

[Rimbaldo]

Hi John! It´s some months since our last posts!! I´ve been busy with work, so couldn´t test it all over my lathe...

But now I did!

I´m getting weird results though..

I´ve done all modifications inside my lathe, and it´s new control box. Hall sensor installed on the spindle, everything set-up. the only thing I haven´t done right now is connecting it to the Gecko G540 yet.

When testing the RPM using the Hall Sensor - 2N7000 - Tachometer module, I get stable results from 0 RPM to 1200 RPM more or less. When I put my lathe to it´s max RPM (Close to 1680 RPM), the tachometer gets crazy. It goes to 2000 RPM, 3000 RPM, gets back to 1000 RPM. Never gets to any stable measurement.

If I connect the output of my Hall Sensor (it has a 1K resistor between the Vin and Out) directly to the Tachometer´s input, not using for now the 2n7000 just for testing purposes, the measurements are correct through all the variations, from 0 to max, with no ripples, no crazy readings.

If I connect the tachometer´s input to the Drain, the Hall Sensor output to the Gate (and has also a wire from the gate to an unconnected Gecko 540 input yet), and the Source to the GND, the readings are ok until close to 1200 RPM. Beyond that it gets crazy..

Would it happen because the 2n7000 get´s lost in the switch-on-off-on-off so fast? Or it´s because I didn´t connect it yet to the gecko?

Thanks a lot for all your help!

Well I did some more testings.. I´ll edit my post...

I tried with the diode method and it worked worst..

I have a couple os tiny pcb voltage regulators, so I used one to get 5V to power the hall sensor, instead of using 12V. The tachometer still uses 12V.

But anyway the results were not stable.

I then mounted the 2N7000 soldered directly to the wires, instead of using jumper wires and it seems to have solved the problem. I think that the micro-vibration of the lathe may interfere with the electric connections... and maybe sometimes the signal wire is quickly disconnected, or any other wire powering the sensor or the tachometer, an then the crazy measurements happen. I´ll keep testing..

Hi Rimbaldo

how about re-arranging the circuit to connect the Hal switch to the G540 input and using the 2N7000 to buffer the signal going to the tachometer input




John

[john-100]

Hi Rimbaldo

you can try adding a 1K pull up resistor to the FET drain / module input



John

[Rimbaldo]

Thanks John! Will give it a try over this weekend!!

Will post the results then!

Thanks so much!

Hi Rimbaldo

you can try adding a 1K pull up resistor to the FET drain / module input



John

[Gandes]

You can try add resistor on the gate too. It is dependent on the safety of construction and working of jfet as they have capacitors building.