Darlinton array for interfacing limits?

[Greolt]

I am building my electronics enclosure and wiring up homing and limits.

If I drive the switches direct with the logic signal, 5v, that is suseptable to noise and resistance on the contacts etc.

Have a spare 15v winding on the toroid which will rectify out to about 20v

My idea is to feed my switches 20v and use a high voltage darlington array chip (ULN2002A 14 to 25v) to interface to the inputs on a ncPod.

Thought this would be better than a board full of relays.

Now anyone with electronic smarts can tell already that I don't have them.

Is there any issue with the 20v side sharing a common ground with the ncPOd?

How about pullups on the pod side?

What do you think? Any gotchas? Is it just a stupid idea?

[pastera]

HCPL2601, 6n137 - Adds isolation for around $1 per channel

Your ncPOD does not seem to have any protection on the inputs (going from the pictures) so you really want to isolate it from the machine. Use your spare winding to supply power to the switch circuit.

Aaron

Aaron

[Greolt]

Optocoupler, Of course ..... You can tell I'm a carpenter can't you

Can you get them with multiple couplers in a dip package?

Never mind, here are diagrams of two that I can get easily local

The first one is half the price.

Can I connect both the Base and Collector to the input pin and the emiter to the ground on the pod?

Or do I need to provide a separate feed to the Base?

Maybe connect to base and emiter leaving collector unused?????

Sorry for dumb questions.

.

[Greolt]

I have done some reading and it seems I connect the base and collector to the pin and connect the emiter to ground.

Sounds good ????????

Also I will use a 12v regulated souce to power the input via the limits etc.
So that should mean a 620 ohm resistor will give just under 20 ma to the opto coupler diode.

Is that a bit much, or OK ???????

PS, I bought the type in the first pic.

.

[Greolt]

Well you learn by doing. At least I do. Never learnt much at school anyway.

Made up my little board with the opto couplers. Hooked it all up and the optos were turned on all the time.

Discovered that if I disconected the Base and left just Collector and Emiter it all worked just as it should.

Wont be long now and this machine will be a goer.

[pastera]

Sorry for the late reply - busy lately

20mA is a lot for the LED - 10mA should be more than enough.

You should add some pull up resistors at the collector as the phototransistor can only pull the pin low, not drive it high.

Aaron
BTW: great looking board

[Greolt]

Hey thanks for that Aaron.

10mA it is. I'll change out the resistors to the appropriate size.

What value resistor should I use on the input pins as pullups?

I had thought 1k which would mean 5mA.

I asked Arturo (his board) and he said he uses 4.7k on his latest board. But I think that is as "pull downs". Does it make any difference?

Thanks, Greg

[pastera]

Do you know what part number you actually have? I can assume a 4n25

You don't want to go too low with the resistor, as the transistor will not be able to drop the voltage to a "zero" for a TTL input - with a If of 10mA the transistor will come out of saturation at ~3mA but does not hit a Vce of 0.7 until ~9mA

If you go too high, there will not be enough drive to raise the voltage very fast, so you would get slow edges that are sensitive to noise.

Somewhere between 1K and 3.3k should give you a balance between power and noise sensitivity (if connecting to a single input). Going higher might just be asking for trouble in a noisy environment (CNC).

Aaron

[Greolt]

Yes 4n25 it is. OK I'll go somewhere in the middle and keep it in mind if I have noise issues.

Thanks Aaron