Limit Switches with LED's

[D.C.]

correct me if i'm wrong as i am also new to this cnc lark still but i thought limit switches were protection to stop the machine from travelling to far.. so if they are tripped the machine would just stop at this point hence making it pretty obvious which one has tripped?

Things like e-stops and limit switches are a little bit more involved then 'just stop'. Think about doing 50mph in a car and for some reason you want to stop quickly, do you cut all the power to the engine and roll to a stop or do you hit the brakes hard?

Computers are very, very stupid so not only do you have to hit the brakes hard you need to make sure that nothing else is telling the motors to do anything else and the switches/e-stop can't be overridden by any other tech trying to ensure that the machine keeps position.

[JAZZCNC]

Problem here is that really you want limit switches wired Normally closed and active low so if a wire breaks the signal drops, machine stops because it's no longer safe.
Also to save inputs you'd have them wired in series. Which surely means each LED will need it's own supply to be of any use has an individual switch indicator.?

In Mach3 Limit Home switch's can be used has home switch's if needed.
Without limit switch's set has Home switches or individual home switches then the software can't be used to home has it doesn't now where to home too.? Instead the software just zeros out the machine coordinates where ever the gantry or moving table is rested.

Home switches are very very useful and I'd drop limits before home switch's any day.

[Robin Hewitt]

Problem here is that really you want limit switches wired Normally closed and active low

Okay, attempt #2...

[manofgresley]

Hi Robin

Thanks, i'll give it a go, can you recommend a particular NPN transistor, and what is the Resistor rating for the one in front of your 5v mark.

Regards

Ray

[Robin Hewitt]

can you recommend a particular NPN transistor, and what is the Resistor rating for the one in front of your 5v mark.

I am a dinosaur when it comes to transistors, how about a BC547.

Assuming 20mA to drive the LED and a 1.2 Volt drop across it, about 190 Ohms for the resistor.

I drew it up to minimise the component count, when the transistor switches on the Volts it has to play with fall away, may need a third resistor so don't lay any PCB's until you have tried it.

[HankMcSpank]

Assuming 20mA to drive the LED and a 1.2 Volt drop across it, about 190 Ohms for the resistor.

Personally I'd have gone with a resistor for each LED (to get their respective brightnesses balanced right)...therefore I'd connect the common anode direct to 5V & then place a current limiting resistor in each of the the LED cathode legs (the fwd voltage of a red led is about 2V, whereas a green led is about 3.2V ...therefore to get the right level balanced you'll likely need different value resistors).

Also 20mA - depending on LED - is gonna be very bright, I'd have aimed for 10mA or less.

Edit: Actually the way Robin has drawn it, for the red led you'll get a drop across the tranny itself, which pretty much balances the fwd voltage difference out for the red & green LEDs, so you may get away with a 180R single resistor for both....a 180R resistor will yield about 10mA through the LEDs

[Robin Hewitt]

Personally I'd have gone with a resistor for each LED (to get their respective brightnesses balanced right)...

That would get around the Volts falling away when the transistor comes on problem nicely and let you tinker with individual brighteness. Good idea.

[manofgresley]

Hi Robin
My apologies if my protocol is incorrect.
I have attached a PowerPoint Drawing, which is my interpretation of your drawing, which contains everything i know so far. Would you be kind enough to go over it, and advise me if it would work or not?

Regards

Ray

[Robin Hewitt]

So you don't have a seperate ground.

That means every LED up stream of the one that goes red will go out.

If you accept that limitation, dump the transistor and the 10k resistor, not required.

Connect one LED cathode to NO, one cathode to NC and the common anode to your 190 ohm off 5V.

[Shinobiwan]

Okay, attempt #2...

Sick sick drawing skills there Robin!