Odd behaviour with a proximity sensor

[Alexanderfitu]

Hi all

Im tyring to run a tachometer from a proximity sensor.

The tack is setup to look for 2 pulses per revolution.

I have a pulley with 2 magnets on it and a PR-12 2DN NPN NO Proximity sensor.

There are 3 wires on the sensor, Brown (+12v), Black (load) and Blue (0V). With nothing connected up apart from power and ground, I get 0v on the black wire with no magnet in front of the sensor, and 12v with the magnet in front of the sensor.

If I connect the the black wire to the tach input and measure again I get 0v and 3.7v respectively. The tach is looking for pulses WRT 12+V

The whole setup used to work with a reed switch, which I am trying to replace. (the reed switch broke as its obviously a bit delicate for this application)

Any thoughts? Thanks in advance!

 

[MikeMl]

Have you read the data sheet to determine the min. operate time (max frequency) for the proximity sensor? The last one of these I played with had a lot of delay till the output changed state...

 

[Alexanderfitu]

Hmmm reading it now it says Max response frequency - 1.5KHZ so it should be ok?

1.5 khz 1500 times a second.

3600 rpm (which is max rpm on this engine) at 2 pulses per rotation is 7200 pulses per rpm which is max 120hz.

 

[MikeMl]

Does this sensor have an internal pull-up, or does it require an external one?

 

[Alexanderfitu]

Control output diagram shows an internal 10K pullup resistor between black and brown wires.

 

[Alexanderfitu]

Here is the datasheet:

**broken link removed**

 

[MikeMl]

So what is the value of the pull-up resistor, and what voltage is it connected to?

edit: What is the sensor connected to? Post a schematic.

 

[Alexanderfitu]

It says 10kohm, but its connected to a 12v supply.

 

[Ian Rogers]

I found that I really needed to feed the prox output through a PNP bjt so I could get reliability..

Also a 2k pullup worked... But as the prox sensors differed from manufacturer to manufacturer the follower works best... ( I can also use PNP prox switches aswell..

Seriously... Try a 1k pullup...

 

[Alexanderfitu]

So what is the value of the pull-up resistor, and what voltage is it connected to?

edit: What is the sensor connected to? Post a schematic.

Sorry, I dont have a schematic for the tachometer, its a closed source thingy.

 

[Alexanderfitu]

I found that I really needed to feed the prox output through a PNP bjt so I could get reliability..

Also a 2k pullup worked... But as the prox sensors differed from manufacturer to manufacturer the follower works best... ( I can also use PNP prox switches aswell..

Seriously... Try a 1k pullup...

I will give a 1K pullup between black and blue a go. I will report back.

 

[MikeMl]

Sorry, I dont have a schematic for the tachometer, its a closed source thingy.

Can you determine its input characteristics? Threshold voltage, Input impedance? Open circuit input voltage?

 

[dr pepper]

If its an industrial proxy there are 2 basic types, npn and pnp, the o/p is switched to ground or + rail.
Your tacho if an older one is probably looking for a switch to ground input, as in old fashioned points (or fets in ecu's).
If your proxy is a pnp o/p it might not work, try putting a pull down resistor 470 ohms or so from the o/p of the proxy to ground.

Edit: I have tried running telemecanique proxys from 12v, even though they say on the side 12v - 240v ac or dc they dont work from 12v, 24v was the least it'd work off.