Great, @peerv !
That's a very good idea.
Do you have reasonable power consumption ?

@chbla, I'm quite puzzled.
As it works on the breadboard, of course, I would chase for bad soldering. For instance, the mosfet gate controled by pin 7.

Any picture of the arduino pro mini ?

@chbla, you are right. As mentionned on the schema, it should be connected to GNDT (the triggered GND). And now I do understand your point.

@chbla well, yes. As the level adapter AND the booster are triggered only when needed (during the measure time), I don't see where it closes the loop.
By maybe I misunderstand something. Where do you loose some current ? The trigger pin from the HC-SR04 ?

QQ.

Yep ! And that's why it's fun !
But my zc detector will work too. My first tests are OK

QQ.

@zboblamont : it's not exposed under the tank and totally shrouded on the boiler side

@gohan: right, by I really don't want to touch the wires. And neutral and phase are inside a single cable with not enough space to put a clamp..;)

From my experience, the transistor (or mosfet) is key to allow enough current to the DC-DC booster. Otherwise, it will not provide the appropriate voltage because max current getting out of a digital pin from Arduino is around 40 mA.

QQ.

@Nca78, actually, I did not want to touch the existing cables to avoid any claim from the maintenance guy And as I could not access one single wire, I had to find another way. Interesting enough, using the existing connecting screws was easier.

@zboblamont: the picture is quite not clear but the motor is very well protected to avoid any mechanical injury. And, probably as a consequence, there is not detectable magnetic field coming out of it.

@bjacobse: I have considered this as well (as well as some kind of a gauge based on pressure or anything else in the tank). But, unfortunatly, as this is a 7 tons / 11 m3 pellet tank, it is filled by a truck blowing pellets into it....making the life of the ultrasonic sensor really not long
Furthermore, ultrasonic sensor would be good to measure a level with a very large error margin. With my sensor, I will be close to a 1-gram quantum...measuring the current consumption very accurately.
For the story of it, I already have a way to measure pellet consumption in a rough approximation: but weighting the ashes

QQ.

With please. Here is a global schema of the pellet burner:

I have circled the part displaying the beginning of the pellet conveyor and the motor.

I'm following-up on this thread Sensor for pellet burner.
On my side, I have started to monitor my pellet burner about 1-year ago.
I'm measuring global power consumption of the boiler as well as departure and return temperature of the heating circuit.
Even though, I do that since one year only, I've been thinking about how to monitor pellet consumption for at least 5 years (yeah, failure is an option ).
What I have tested:

• IR sensor on the fan of the pellet convoying motor: it failed because it was too far
• Magnetic sensor on this same fan: same failure for the same reason
• Vibration sensor: the heater vibrates ...all the time

Now, I'm about to test something quite simple: mains presence at the convoying motor. This one is quite easy (I will show you how) to design and relates quite easily to the pellet consumption: 1 second equals x grams of pellet.

How do I know the time mains are activating the motor ? Well, a zero-crossing detector.

Beware, main resistance values are key: they depend on mains value, optocoupler characteristics and ..their power. Using the displayed value (56k) will keep the power under 1/4W. The optocoupler is a LTV-814 (cheap and efficient !).

Here is a quick schema of such a detector. I will keep you posted with the results.

Hope my experience will help others.

QQ.