[Falcon69]

Thank You John. I will make that adjustment on the new boards. But, I think I will put the 100ohm resistors on the switch itself, there is room now, that I have taken the jacks off them for casting in resin. Would it be safe to do the same thing to the single switch? the one with the transistor on the switch itself? Connect the 100ohm resistor to each of the Red and Greed LED's cathodes?

[john-100]

Hi Jason

the 100 ohm resistors need to be on the main board so they are between the USB socket and the ULN2003 output
moving them to the limit switch would be like changing the existing 100 and 150 ohm resistor to 200 and 250 ohms

the only other thing to protect is the SS441 - not easy to do and keep the component count low

on the limit switch with one LED and the 3 terminal jack socket
the HALL swich has to sink the current for the led
plus the extra current from the circuit its connected - printer port , BOB etc

the resistor would need to limit the current to a few mA if the signal wire was shorted to +5V
a 2500 ohm resistor would limit the current to 2 mA
but the switch would then only switch a high input impedance circuit or CMOS logic IC
and is likely to pickup noise

with the 2 LED version , a 270 ohm resistor could connect the SS441 output to the USB connector
with the output wire shorted to +5V the SS441 would sink 18mA (5/270 = 18 mA )
plus the 2mA through the 2K2 pullup resistor on the switch PCB ( 5/2200 = 2.3mA )
the SS441 can only sink a maximum continuous current of 20 mA

if the switch output is connected to a TTL logic chip input the 0.4V dropped across the 270 ohm resistor could be a problem
added to the volt drop across the diode array on the two ULN2003 board
1V is higher than a TTL logic 0

I expect the the latest board will work with the addition of the 270 ohm resistor to the switch output

the input current to switch on the first ULN2003 transistor is limited by the 2K2 pullup resistor and the 2K7 resistor inside the IC
the extra 270 ohms is only about a 5% increase

as the extra IC provides buffered open collector outputs
the prevoius issues with the 12V inputs of the Gecko G540 or the Campbell B.O.B should be solved

John

[Mick Gannon]

Hi all
new to the forum, my machine is small 400x600mm working area, my limit switchs are RS normally closed reed type wired straight in to 5 axis break out board (UK design board) when tested the repeatability was with 0.001" cost cheap but it works.