ramps 1.4 relay endstop wiring

Hi I have a rather odd setup which I've come up with to circumvent my inept electronics skills.

I have a Delta printer which I want to use a bed probe with, my bed is aluminium so I have chosen a induction sensor as they are cheap and simper than a mechanical sensor.

I have the output of the sensor wired to a DC Solid State Relay, I know that the actuation of the induction sensor closes the relay as I have tested this using LED's: when the probe is activated, the LED's connected to the output of the SSD light up.

My intention is to wire the output of the Solid state relay to my Ramps min Z endstop, in the same way that a mechanical switch is used as an endstop.

My SSD is Normally Open, that is when current is not flowing through the input side (when the induction sensor is not actuated), no current can flow through the output side of the SSD.

Do I need a pull up (or pull down) resistor for this set up, I have found a wiring diagram for Normally Closed mechanical switches but I'm struggling to find a similar diagram for Normally Open switches.

A long time ago I had mechanical switches as endstops for a Gen6 board, these switches had only two wires so I can only assume they used no pull up or down resistors, is there a reason they are now in common use?

My final problem is that I am unsure which pins on the Ramps board correspond to the +5V, Signal and Ground. I unwisely tried to figure this out by myself by plugging the output wires from my SSD into different pins but when I connected them between the pin closest to the edge of the board and the pin in the middle, when I closed the SSD, the board disconnected from my computer, luckily it seems undamaged, normally when I do things like this I end up with fried silicon for dinner.

I apologise if this post seems long-winded for such a simple query, but as I say I am electronically inept so I have tried to document my understanding as well as I can so that if there are any fundamental errors, you can easily point them out to me.

I suppose I could summarise all my above ramblings with "can you use a normally open switch as an endstop/z probe?"

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blabberjack
My SSD is Normally Open, that is when current is not flowing through the input side (when the induction sensor is not actuated), no current can flow through the output side of the SSD.

Do I need a pull up (or pull down) resistor for this set up, I have found a wiring diagram for Normally Closed mechanical switches but I'm struggling to find a similar diagram for Normally Open switches.

NO or NC doesn't matter. The pullup is needed to pull the signal and hold it high when the switch is open.

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A long time ago I had mechanical switches as endstops for a Gen6 board, these switches had only two wires so I can only assume they used no pull up or down resistors, is there a reason they are now in common use?

The pullup resistors were either integrated into the board, or integrated into the microcontroller. Pull-up and pull-down resistors are not new and have been around for as long as digital circuits have existed.

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My final problem is that I am unsure which pins on the Ramps board correspond to the +5V, Signal and Ground. I unwisely tried to figure this out by myself by plugging the output wires from my SSD into different pins but when I connected them between the pin closest to the edge of the board and the pin in the middle, when I closed the SSD, the board disconnected from my computer, luckily it seems undamaged, normally when I do things like this I end up with fried silicon for dinner.

It's printed right on the board. The three rows are labeled S, -, and +. - is ground, + is +5v and is used to power any active switches that require power (opto, hall effect, inductive, etc). Mechanical switches don't need/use the + pin. S is signal that needs to be shorted to ground when the switch is closed.

I would strongly suggest revising your sensor so that it operates in a NC mode. If you presume that it's always NO, then if the sensor becomes disconnected or nonfunctional, it will never close even when it's at it's minimum clearance above your bed. It could cause your hotend to collide into the bed, damaging either. Design things so that if/when they do fail, they fail in a manner that will cause the least amount of damage.

Edited 1 time(s). Last edit at 09/23/2014 07:40AM by cdru.

Err, just a note:

The + pin on the endstops is 5V, and it's derived from the 5V supply on the Arduino Mega.

This is why you can easily fry the Arduino Mega's 5V regulator if you manage to short the + and - pins on the endstop header.

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Cefiar
The + pin on the endstops is 5V, and it's derived from the 5V supply on the Arduino Mega.

Doh. That's what I originally thought, but then I double checked the circuit traces and mistook the ground trace as supply, hence my error. I corrected my post. Thanks for the correction.

thanks, I got it hooked up using the internal pullup resistor on the ramps board. I understand the point about designing systems to fail safe, just wish I had appreciated that before I bought the probe : )