Open drain question

Other Hardware General Electronics
1

Hi there,

I am a complete beginner here and my question may maybe seem a little odd.

I am trying to control an external triggering device with an Arduino. This device has two cables: one being ground and another having a voltage of some 3,3V. When these two cables are physically connected together, I get the the desired effect: the external device recognizes this as a trigger-action.

I would like to be able to control this 3,3V being pulled down to ground with an
Arduino and I have came across an old but closed topic in another forum where somebody mentions open-drain configuration for a similar task.

If I got it right my setup will be like this:

  • Ground cable should connect to the Ground pin on Arduino.

  • 3,3V cable should connect to a digital pin on the Arduino and switch between pinMode(INPUT) and pinMode(OUTPUT) . When this pin is in output mode it will pull the voltage of the pin down to ground. I may put a 1K resistor between the 3,3V pin and the the Arduino in order to protect the Arduino while still ensuring it can pull the voltage down.

Is this completely bonkers or am I on the right track?

2

Will it pull the voltage low enough with a 1K to ground?
Have you tried connecting a 1K between the 3.3 output and ground to see if it works?

3

Hi Jim, thank you for replying so fast:)
I haven't tried connecting a 1K resistor, but if you think that 10K resistor would be a better option than 1K I will definitely try that.
So you think that the whole idea could work?

4

Personally, I would use the circuit shown below. You don't need to worry about resistors or blowing-up you arduino

image
image699×432 5.57 KB

2 Likes
5

How much current flows when you do that?

If you use a 1K resistor to connect them, does that trigger the device? How much current flows then?

If the current is < 30mA, you can probably connect directly to the Arduino pin.

6

There are many ways of doing this. The simplest especially if you want isolation is to use a relay module. If you want to do it electronically use a MOSFET.

7

Ha can use a cheap, single channel optocoupler (as I remember, like an LTV815) too, if the switching frequency is like limited to a real ON/OFF pattern. That will provide real isolation.

8

That would be a good solution assuming there is not much current. Connect the opto emitter to ground, the opto collector to the 3V signal. The LED needs a few hundred ohms in series with it and then drive it directly with a port pin. I am assuming a UNO or other 5V Arduino.

9

Answers could be clearer if the identity of the "external triggering device " was known.

1 Like
10

That is why I recommended the optocoupler.
However, if I understood correctly it is a 3,3V signal pulled to GND.
With that, it is not needed to connect the two grounds galvanically.

11

Any other questions?

12

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