user15180: I am going to do a project with a lot of Arduinos connected via I2C, where like of them can became master for a moment to announce an event. So I would like to see how much is the bus used. As there will be a central point, I thought about putting a LED there, from +5V to clock line, so it would shine when the transfer occurs and be black when the line is free. And so I would be able to guess the utilisation level of the bus. (The clock is effectively PWM going to zero often while in transfer, while being plain +5V when no transfer is active).
Of course direct connection is not the best idea, as it would disrupt the pull-ups and mess with the protocol (as it would drain a lot of power to clock each time it went down), there should be some separator/amplifier.
The question is - does anybody know a simple setting, where the LED would be driven by a small current to zero (that is inverting amplifier as simple as possible with high enough input impedance to not disturb I2C communication)?
Thanks for all hints :)

user4143: It can be done with two N-channel MOSFETs or one P-channel MOSFET (depending on what you have locally / can get / is in your bits box):

simulate this circuit – Schematic created using CircuitLab
In the left schematic the first MOSFET acts like a NOT gate, and the second is a simple low-side switch. The pair together make an active-low low-side switch.
The right hand schematic, which is somewhat simpler, uses a single P channel MOSFET as a high-side switch. This is normally driven with an N-channel MOSFET to invert the logic, since it is naturally active-low. You don't want that, so you can just connect direct to the gate.
Note that all MOSFETs are "logic level" with a threshold voltage below 3V (or -3V in the case of the P-channel).

15180: <p>I am going to do a project with a lot of Arduinos connected via I2C, where like of them can became master for a moment to announce an event. So I would like to see how much is the bus used. As there will be a central point, I thought about putting a LED there, from +5V to clock line, so it would shine when the transfer occurs and be black when the line is free. And so I would be able to guess the utilisation level of the bus. (The clock is effectively PWM going to zero often while in transfer, while being plain +5V when no transfer is active).</p>

<p>Of course direct connection is not the best idea, as it would disrupt the pull-ups and mess with the protocol (as it would drain a lot of power to clock each time it went down), there should be some separator/amplifier.</p>

<p>The question is - does anybody know a simple setting, where the LED would be driven by a small current to zero (that is inverting amplifier as simple as possible with high enough input impedance to not disturb I2C communication)?</p>

<p>Thanks for all hints :)</p>
 4143: <p>It can be done with two N-channel MOSFETs or one P-channel MOSFET (depending on what you have locally / can get / is in your bits box):</p>



<p><img src="https://i.stack.imgur.com/jHFxS.png" alt="schematic"></p>



<p><sup><a href="/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2fjHFxS.png">simulate this circuit</a> &ndash; Schematic created using <a href="https://www.circuitlab.com/" rel="nofollow">CircuitLab</a></sup></p>

<p>In the left schematic the first MOSFET acts like a NOT gate, and the second is a simple low-side switch. The pair together make an active-low low-side switch.</p>

<p>The right hand schematic, which is somewhat simpler, uses a single P channel MOSFET as a high-side switch. This is normally driven with an N-channel MOSFET to invert the logic, since it is naturally active-low. You don't want that, so you can just connect direct to the gate.</p>

<p>Note that all MOSFETs are "logic level" with a threshold voltage below 3V (or -3V in the case of the P-channel).</p>
