Proximity Sensor issue, never trips? < solved, used NPN>

[Allthewatts]

Hey all, making alot of progress converting over a G0619 manual Mill. Its moving around, and I can run simple Gcode, but stuck getting my home proximity sensors to work.

I'm following the wiring diagram here: https://www.centroidcnc.com/dealersuppo ... 955.r5.pdf, and I have the sensors configured in the inputs as NC, however I cant seem to get the sensors to work.
I have a hunch there is some electrical issue, in that I get a different response from the sensors when the black lead is connected to the acorn, or not connected.
When the black lead is connected to the acorn, the red status LED on the proximity sensor is always on, regardless of its proximity to ferrous material. When I remove the black lead from the acorn, the sensor works again, where the LED is off when in proximity to metal.

Any one have an idea here?

The Proximity probes are NC:

[cncsnw]

Your Amazon link does not work.

Can you post a picture of the circuit diagram on your sensor or sensor cable?

Are you 100% positive that what you got were NPN sensors? Acorn inputs require NPN sensors.

[Allthewatts]

I'm 100% positive that its a PNP NC sensor! There's my issue. Dek keeping the gate open because of this. New ones on the way!

[Richards]

Mistakes happen. Years ago, I bought some PNP prox. sensors instead of NPN sensors. That was before parts were available in two days on the Internet. I wasn't uncommon to have to wait for weeks for parts to arrive, unless you were willing to pay expense rush fees. Instead of spending $100 for three NPN proximity sensors I built a small circuit that allowed me to use PNP sensors.

Here's a schematic that will work with the Acorn.

When the PNP sensor turns ON, it SOURCES current to the NPN transistor. The NPN transistor SINKS current from the Acorn's Input terminal. The diode is a safety shunt that is optional. The two resistors form a voltage divider that assures that the transistor is OFF unless the PNP proximity sensor is ON. Values are not critical. 4.7k and 10k 1/2 watt resistors are common and easily found.

I use EasyEDA to create schematics and JLCPCB for PCB fabrication. EasyEDA can be downloaded at no cost. PCBs are at little as $5 for five boards plus $15-$20 shipping.

[kb58]

The 1N4148 is unnecessary unless driving an inductive load, but regardless, is shorted as-presented. I'm not picking on you, just pointing out the issues with the above schematic in case someone tries using it

[Richards]

KB58, no offense taken. However, did you notice that the In4148 is reversed biased, which is the correct orientation for its use as a shunt to protect an Acorn Input. The Acorn input is basically an opto-coupler. The Anode of the opto-coupler is connected to +24VDC internally. The Cathode of the opto-coupler is shorted to Ground by the sensor/switch. Current through the opto-coupler is limited by a resistor internal to the Acorn board. The Cathode of the 1N4148 is connected to the Cathode of the opto-coupler in parallel with the NPN transistor's collector. When the NPN transistor turns on, it SINKS current that flows from +24VDC, through the internal resistor, and through the opto-coupler. With a Cathode to Cathode connection, no voltage will flow through that connection; all of the voltage will flow through the NPN transistor. A $0.03 part is cheap insurance.

[cncian]

KB58, no offense taken. However, did you notice that the In4148 is reversed biased, which is the correct orientation for its use as a shunt to protect an Acorn Input. The Acorn input is basically an opto-coupler. The Anode of the opto-coupler is connected to +24VDC internally. The Cathode of the opto-coupler is shorted to Ground by the sensor/switch. Current through the opto-coupler is limited by a resistor internal to the Acorn board. The Cathode of the 1N4148 is connected to the Cathode of the opto-coupler in parallel with the NPN transistor's collector. When the NPN transistor turns on, it SINKS current that flows from +24VDC, through the internal resistor, and through the opto-coupler. With a Cathode to Cathode connection, no voltage will flow through that connection; all of the voltage will flow through the NPN transistor. A $0.03 part is cheap insurance.

Mike, no offense, but kb58 is right! The 1N4148 diode is shorted, and it does not really matter if it is reversed biased or not! The current will bypass it either way! Can you please revise the schematic, and re-post!

[kb58]

Even if the short is removed, the poor transistor is in series with it. The diode should be located right across the inductive load it's protecting the circuit from, with no intermediary circuits in series with it.

[Richards]

Maybe if I drawn the circuit as a generic snubber, you'll see that the diode is not shorted. In the circuit, current passes through the resistor, through the coil, and then through the NPN transistor. The diode suppresses induced current when the transistor is turned off by shunting that current back through the resistor to +24VDC. If the coil were replaced with a wire, current would pass through the resistor, through the wire and then through the diode. At no time would the diode be shorted because it blocks current when it is reverse biased; its cathode points towards a higher voltage. You'll note that on the Acorn, there is NOT a connection that allows a flyback diode to be placed in series with the opto-coupler's internal diode such that the Acorn's internal resistor can still limit current through a flyback diode; therefore, I had to place the diode in my original circuit as I did. Note that the 1N4148 is a SCHOTTKY diode, meaning that it is extremely fast, in most cases much faster than an NPN transitor. it will shunt transients back through the transistor as the transistor is turning off. Hook up an oscilloscope to the circuit and you'll see what I mean. I'm not trying to belabor the point. A schottky flyback diode is simply a best practice design that the military required when George Bauding, lead engineer at Kaiser, instructed me to always add a schottky flyback diode when using an NPN transistor to drive any kind of DC load. If you feel more comfortable leaving out the schottky diode, then leave it out. The circuit will still turn the INPUT on/off.