Ionization smoke sensor circuit: "active guard" pins?

Hey y'all Arduino Community!

It's been a while since I've started building my smoke sensor circuit using the HIS-07 (which I bought about a week ago) and A5368 IC from Allegro, and progress has been painfully slow.

I consider myself somehow inexperienced in some major parts of electronics, and I would like to ask for help regarding my project.

I've followed the circuit schematic from the pics attached (Allegro's datasheet and product design for comparison of operation) and rebuilt them too many times, without any significant result.

Also, I tried replicating the "active guard" layout on my makeshift circuit (in the pics) compared to the product design PCB (in the pics), and I dunno if I'm doing it right. Can someone please tell me what can I do about this?

I'm afraid of putting it on a PCB (not the universal one), like on the product design because I might get errors and it seems very final. I'm planning for lower costs by buying discrete components and be able to modify the circuit to make it smaller. As for now I'm trying to test if it really works out. What is an active guard for? I really don't grasp its concept and as said, I'm still inexperienced with these principles like hysteresis, etc.

Lastly, in regards with the product design PCB, do I need to connect 9V to pins 3 and 13 of the IC even without the resistors there? Or could I leave it as it is?

Thank you very much. :slight_smile:

The device has a super high impedance - this means it won't work if even a few pico amps of leakage are
present in the circuit.

All common materials can leak that much if there's a voltage gradient across the surface, due to water
molecules adsorbed onto the surface, and other stuff like that. Typically the problem is orders of magnitude
worse in damp weather compared to cold winter weather, so it may or may not show up today, but one day
it will!

So you have to eliminate the voltage gradient across the surface to reduce the leakage current to an
absolute minimum - or use materials like teflon which repel water molecules, or take both precautions even.

A guard electrode or guard ring is kept at the same voltage as the signal (via an amplifier usually), eliminating
voltage gradients affecting the very high impedance signal itself.

Your "makeshift" board appears to be entirely coated in copper still!

An 'active' guard ring (not always a 'ring') is basically a fence around a circuit board trace that is actively kept at the same voltage potential as the trace itself.
So there is to voltage between the two, and no leakage current.

Hey guys it's been a while. I managed to get my "makeshift" design working by placing some short strands of copper wire that run parallel and in between the guard terminals. They should be placed really close to the wires though, now I understand too. Thank you! :smiley: