led dimly lit, I know why, how to fix?

[alan_3301]

I am trying to wire up hall sensor homing switches to my cnc, as discussed in the thread "Electronic home switches made easy".

The sensors are wired 12v supply, ground, signal out (tied to 5v pullup inside breakout board). I have a LED wired between 12v through a 1k resistor, and the signal out pin (when hall switch is active, signal out goes low/ground)

This all works, but when the switch is not active, the led is lit dimly. Because it has 12v on one leg, and 5v from the pullup on the other.

Should I try to add more resistance where the 1k resistor is now? Will this bring down the 12volts to closer to 5 volts for the LED.
Or should I add a pullup on the 5v line to bring it up to 12volts? I guess this is the preferred way. Any chance of damaging the breakout board??

I hope this is understood. Thanks for any help.

[Al_The_Man]

I am not sure how you are wiring this? But you should not mix and match/combine two supplies for the same application?
Show diagram.
Al.

[alan_3301]

Thanks for taking time to help me.

Im not sure why the breakout board has a 12v supply with a 5v pullup on the signal wire.

I guess my question is what is the most acceptable way of fixing the problem.
The problem being, that I have I should have the same voltage on both sides of the LED, untill I want it to light up, then one side should have 0v.

Do I tie the 12v wire to the signal wire through a resistor, adjusting the resistor value until I get 12 volts on the signal wire.

Or do I add more resistance between the 12v and the LED, until I read 5 volts after the resistors. With a 12v supply and 1k resistor I am gettings 9 volts before the LED, and 5 volts after. This 4 volts difference Is what is causing the dim glow when it should be dark. ( I Think)

[Al_The_Man]

Do you know the nature of the BOB input? You may be able to simply put the LED in series with the output to the BOB input if it is a Opto input for example, no resistor needed.
Al.

[lwill]

I would suggest wiring the LED to 5V instead of 12V, would eliminate the problem.
Another option may be to put a small diode between the BOB and the LED/opto to prevent 5V pullup from looking like ground. Anode to BOB, cathode towards LED/opto.
This could cause a problem if the BOB needs a solid ground signal since the diode will add ~0.6v drop.

[john_100]

Hi Alan ,

what breakout board are you using exactly ?

if the breakout boards input terminal is connected to a TTL logic IC the input (like the 74hc14 used on many boards)
the maximum voltage for a valid logic 0 is 0.8v
the 0.6V drop across a diode could be a problem

when the limit switch is off the input will be clamped by the internal protection diodes to +5.6V .
(this could be the reason the LED still glows )

you can either try using the diode or use a 5V supply and reduce the resistor in series with the LED to 330 ohms
I'm assuming its a Red LED and the volt drop across the LED will be 2V

if you have to use a 12V supply ,you can connect the LED in series with a 330 ohm resistor and a 6.8V zener diode eg 1N4736A or BZX85-C6V8
(the zener diode needs to be reverse biased and have its cathode connect to the positive supply )

John

[alan_3301]

Thanks for the help everyone. It is a cncrouterparts breakout board, I don't think they sell it anymore.

It uses rj11 jacks for each home/limit input with a dip switch for each to switch between normally high (5v) or normally low on the signal line. It supplies 12v,ground, and signal input as described above.

It would have made it easier if it the supply was 5v, or the signal pullup was 12v. But oh well.

I ended up using another 1k resistor to bring the signal line up to 12volts (or close enough) The LED still glows very dimly, but I can live with that.

[mike_Kilroy]

ya, get off the 12v.

u can put the LED in series with the 5v pull up and it will fix it with no other resistor required.

if u r not happy cutting into the BOB board pull up resistor leg, then just put the led in series with 480 ohm resistor from the hall output to ground and it too will fix it.

KISS wins in the end