[EL34]

I am running XP pro on a gateway notebook computer which is running mach3 and mach3 is controlling my 3D printer with 4 stepper motors
The motors and 3D printer all work fine
I have the notebook stripped down to bare bones and have not experienced any pulsing problems so far.

What I want to do now is use M7 and M9 to turn on and off a relay board that turns off 12 volts to the hot end temp control board.

So basically, a relay board will turn off the 12 volts going to the hot end after a job finishes and shut down the extruder

I have a relay board that works fine on my CNC machine desktop computer running xp pro.
The relay board uses a mosfet to control the relay coil
All it needs is a high/low voltage to trigger the mosfet/relay coil.
On my desktop cnc machine I can read a steady 5 volts on the parallel port output pin when I enter M7 in mach3

On the notebook I get a wavering voltage figure that is not steady at all
It wavers between 2.2 volts and back down, then up, and down, etc.
This is not enough to trigger the mosfet/relay coil

It's a plug in pcmia parallel port card and it works fine for all 4 motors, but there does not seem to be enough steady DC voltage to trigger the mosfet/relay coil

I am scratching my head as to why the pcmia card is good enough to operate all the steppers but I can't get a decent hi-lo voltage on an output pin
Output #2 LED does light up in Mach3 when I enter M7 and it goes out when I enter M9
Output #2 is assigned to pin #1 on my breakout board but I tried pin 14 also with the same results.

So how can a pcmia parallel port card work fine in Mach3 to run 4 motors but not be able to supply a steady DC voltage on an output pin?

thanks for any help

[neilw20]

Maybe it needs a pullup resistor. Say 10K. It might only be pulling down.

[EL34]

I tried using a 4.7k and a 10k pullup resistor across the ground and output pin
Still no good, the output pin voltage wavers all over the place

I have tried all 4 remaining unused output pins which are 1, 14,16 and 17

Same results on all four pins when assigned to output #2 in Mach3

M7 lights up the output #2 LED in mach 3
If I touch my finger to the gate on the 2N7000 FET, the relay coil will engage
M9 shuts off the relay coil just fine

The whole problem is the wavering voltage coming from the pcmia card does not seems to be steady or strong enough to trigger the gate on the 2N7000 FET

this is the circuit I am using and it has always worked fine on my Desktop XP CNC machine

[neilw20]

Try this. The mosfet looks like it needs more drive voltage.

Us a common base NPN transistor circuit.

Base of transistor junction of 2 10k resistors.
top resistor goes to 5v.
bottom resistor goes to 0v.
That will give 2.5v on the base.
Connect port output to the emitter.
Connect the collector to the gate of mosfet, with a 10K pullup.
You might need to make the 2.5 higher or lower by changing value of one of the base resistors.

This formum is shooting blanks again. Nearly did a double post because it appeared not to complete.

[EL34]

That sounds good but then I have to find +5 volts from somewhere
My 3D printer power supply is 12 volts
The only 5 volts I can think of is the USB port on the notebook

[neilw20]

A 10K from 12v and a 2.7K to 0v will give you 2.55v

Bottom resistor. A.
Top resistor B.

12 x (A/(A+B)) = 2.55v

[EL34]

Ok, thanks, a voltage divider
I'll see if I have a NPN trans laying around to experiment with

[EL34]

Ok your suggestion of adding a NPN transistor to the gate of the FET worked great
The Relay now clicks on and off with M7 and M9

Here's the Relay circuit I am using
Attachment 203352

[neilw20]

EL34,
That was nice to draw a circuit for all to see.
Call 2K7 the A and 10K, B.
We aim for 2.5v at the junction so other than 12v requires changes in the value(s).
See post #6 above for the clues on how to calculate.
You may need to change the values of the A resistor to get off the shelf value for B.
As long as the ratio is correct, the value of A and B could be 27K and 100K.

[EL34]

My 7amp 12volt Dc power supply is a bit higher than 12 volts

I may have to adjust the values a bit because I have closer to 3 volts at the junction of the voltage divider

[neilw20]

Use a 12K instead of a 10K. That should fix it.
If it works, measure how close the margin is to be sure.

[EL34]

A 2.2k for resistor A ended up working perfectly
I have 2.51 volts at the junction of the voltage divider
So I have a 10k at the top of the voltage divider and a 2.2k at the bottom

Connect the collector to the gate of mosfet, with a 10K pullup

I forgot to add this is my test and it still worked
So a 10K from the junction of the FET gate & NPN collector down to ground, correct?

Here's shot of the relay board I cut on my CNC this morning

[neilw20]

What voltage are switching on the output of the relay?

[EL34]

I am switching 12 volts
The load is around 3 amps

I am turning off the 40 watt heater cartridge to the extruder on my 3D printer
When the print job ends, Mach3 issues a M9

[neilw20]

That's fine.
Just checking because if you were switching high voltage you would need 3mm clearance between the relay contacts and 6mm between relay contacts and the PC side of the circuit.
This is for safety, and is creepage distance.
Have a look at layout on the bottom of old switchmode power supply to see typical isolation used.
Often there are slots in the PCB where the distance of 6-8mm can't be achieved.