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I have an CNC router with 7 x SN04-NPN sensors connected in parallel with one control board input. The problem is obvious, i got random false endstop triggers due leaking of SN04. I know i should have one sensor per input but inputs are lacking and i can't connect them all individual, i don't need the individual control/info anyway. I come with an idea to overcome the problem but still connect them all in parallel by using optocouplers, schemantic below:

enter image description here

For simplicity i only have draw 3 pairs but there will be 7 of them. Is my drawing ok? Or there is any IC i can use to AND all inputs into one output? PS: Control board have optoisolated inputs, VCC is 24V Sensors use same power supply as control board (24V), power is shared

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  • \$\begingroup\$ Is that the same VCC on both sides of the opto-couplers? Do the boards and sensors all share the same ground, or are they isolated from each-other? \$\endgroup\$
    – user4574
    Dec 5, 2020 at 3:25
  • \$\begingroup\$ Sorry for not telling on op, edited now: Sensors use same power supply as control board (24V). So Yes \$\endgroup\$ Dec 5, 2020 at 3:27
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    \$\begingroup\$ What you have there creates an AND function (if any input is low the output goes low). Do you want an AND function or an OR function ? \$\endgroup\$
    – user4574
    Dec 5, 2020 at 3:43
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    \$\begingroup\$ Seems like it should work. To facilitate debugging, I recommend you put a visible light LED in series with R1, R2, R3 etc. Then if one of them trips you will be able to quickly see which one it is without getting out the volt meter. If VCC is 24V, it should be no problem having the opto and the LED in series. \$\endgroup\$
    – user57037
    Dec 5, 2020 at 3:55
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    \$\begingroup\$ I think it will be OK. The current will be (24V - Vf_led - Vf_opto) / 2.2k. The LED will be bright enough, but double-check if the opto datasheet has a range of current that is acceptable. You could go a bit smaller if you want or think you need to (to compensate for the added Vf). \$\endgroup\$
    – user57037
    Dec 5, 2020 at 7:24

2 Answers 2

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It appears that your setup will do what you want (make the output low if any input goes low).

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Is there any reason you couldn’t just use diodes instead of optocouplers? If the input signals might not pull all the way to ground there might be a problem, depending on the threshold voltage for board_input_1

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  • \$\begingroup\$ I already have one diode per sensor signal pin, problem is to many sensors in parallel that it hit the board input threshold at random times, the internal 2.2K resistor on board gives a wide window of operation 10V-30V \$\endgroup\$ Dec 5, 2020 at 4:16
  • \$\begingroup\$ Do you know what the threshold is? I can see that going below 2200 ohms would be a problem because the pull-up will be dissipating 0.25W as it is, and passes 10mA when pulled low. \$\endgroup\$
    – Frog
    Dec 5, 2020 at 4:51
  • \$\begingroup\$ board manual says 10V at minimum to activate the input, and i'm not sure about current it need, but as the inputs are optocouplers it will activate with few mA. Somehow i fell more "safe" going with my idea, if any sensor goes bad it will protect main electronics, i also have two servo signal (ALARM) in parallel with sensors, don't want to risk burning those. \$\endgroup\$ Dec 5, 2020 at 16:53

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