Unless you need them, it's pretty much a case of ignore the LEDs. Back to back like that they'll just act like a 1V (or so) drop in either direction.
Black & green to ground, red and yellow to your inputs for active low triggering.
Hi all,
I've bought on a Chinese e-commerce site a tool setter model JS-58RG. (picture attached)
It looked rugged and well built, and the item description boasted a very good accuracy. It has two separate NC contacts for the touch signal (two wires) and for the overtravel safety (two wires).
The problem I've encountered is about the touch contact wiring. From the probe's schematic (picture attached), it can be seen that the NC contact has two diodes in counterphase (one is an LED), so I cannot see how to get an output signal from it, to send towards my CNC controller.
I would have expected rather a "clean" contact or a transistor output, but instead these two diodes in between are puzzling me.
I know, I could devise some circuit able to detect the current flow and transform it to a suitable control signal, but still the doubt remains about the reason for such an awkward design.
Did someone use a similar device, and be able to shed some light on this issue?
Apologies for my bad english, greetings from Italy
Roberto
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Unless you need them, it's pretty much a case of ignore the LEDs. Back to back like that they'll just act like a 1V (or so) drop in either direction.
Black & green to ground, red and yellow to your inputs for active low triggering.
Hi, and thanks for your observations.
But unfortunately the voltage drop I get is that tipical of an LED, thus around 1.7V . This is confirmed also by my diode tester.
The other reversed diode instead, although shown on the (very poor) schematic, seems to be missing, since the diode tester does not detect the junction (open)
And, considering that the probe input of my CNC board is as follows:
there's no way I can possibly drive low the TTL input with this tool setter, being 1.7V far above the required low-state voltage threshold. I've tried also to add the 470 ohm resistor just to light up the LED but without any improvement.
And the LED, being in series with the probe contact (embedded and sealed within the probe housing, thus non-bypassable) in my opinion renders this gizmo practically unusable without some additional level shifting circuit. I have still to understand what's the logic (if any) behind this awkward design. What if, for instance, the LED should fail?
I'm possibly missing something, but can't tell where...
Last edited by RobMar; 09-29-2016 at 03:05 PM.
Hi Roberto
the tool setter will work with a Geckodrive G540 or a breakout boards (like CNC4PC's C31) or PLD designed to work with 24V logic
levels
the only simple way without building an interface
is to short circuit the LED so you just have a N/C switch
John
Hi John,
unfortunately this is not possible. As I was saying in my previous post, the LED is sealed within the probe case. Only the two end wires are available. No way of bypassing LED without heavily tampering with the probe. This is exactly the weird issue of the probe.
Regards,
Roberto
Last edited by john-100; 09-29-2016 at 06:22 PM. Reason: add G540 & C31 input circuits
Thanks again John for your suggestions!
About the zener circuit, I had in mind something similar, the only drawback is that this way the logic becomes inverted.
That is, the CNC input is expected to be at a low level when active (switch closed) being the signal subsequently inverted by the Schmitt trigger gate.
Regards,
Roberto
Hi Roberto
I would build your interface in post 6
my simple 2 resistor circuit will be susceptible to interference if the tool setters cable is very long
I added the zenner to protect the IC's input ( 74HC14?) just incase the LED failed and was short circuited
with the values of the resistors I used ,
the IC input would of been taken up to 5.7V if the IC's internal protection could not handle the current
if I remember correctly
the IC's protection will conduct when the pin is taken to 5.5V
John
You can usually select whether the probe should have NO or NC contacts in the SW.
Cheers
Roger