Needing more than a spark test?

[rwm]

Crude:

 

[rwm]

What does this mean? "do safety related things like interlocks"

 

[WobblyHand]

Are you talking about making something like an ADC shield? Funny you should bring that up, I was thinking about that last night.

I've attached a PDF version of the current layout, with top metal, silkscreen and the ground plane. This is the design that's being fabbed and stuffed. The board dimensions are 75 x 35.2mm. I moved the two 8-pin headers to accommodate the mounting holes: but if it's turned into a shield it won't need mounting holes.

Right now I don't know if it would be possible to have a pinout that's compatible with both T4.0 _and_ 4.1. I now have one of each with the product card so it won't be too difficult to check that.

BTW it appears that EasyEDA has PCB footprints for both the T4.0 and 4.1. An adapter board is certainly possible but it would be best if we could avoid that. A complicating factor is that the choice(s) available for 8/16 parallel input pins are fixed so there's no wiggle room there. The control pins are a different story.

Is OVDD and OGND connected to Teensy's 3.3V power and ground? If OVDD is 5V, the ADC will destroy the Teensy.

 

[WobblyHand]

What does this mean? "do safety related things like interlocks"

If there were interlocks of some sort, something close to the microcontroller would be good. If someone used a PMT, you'd want interlocks on the enclosure, so the high voltage would be shut down if the enclosure is opened. Or the case was opened up and you could be exposed directly to the radioactive source. Some kinds of interlocks are good, as long as they can be bypassed for diagnostics. The interlocks prevent minor booboos from turning into issues, in my humble opinion. That's what I meant. Hope that is clearer. If not, keep asking more questions. I'm not looking for nanny stuff, which I dislike, but more reminders that doing X might have some hazards, and procede at your own risk.

 

[WobblyHand]

Crude:View attachment 432656

That's a possible (and reasonable) way to view the system. I'm sure Mark and Graham have formed opinions on this Some (software) functions which are not shown yet, could be partitioned various ways into the different processors, depending on how loaded one is compared to the other. Hard to tell at this juncture. Unless, this becomes a commercial product where we have to shave every penny, I usually like to see at least 50% spare capacity in each processing node in the beginning. Stuff happens along the way during development. Porting to a new platform isn't always easy, hence having capacity to add new features (or making stuff work at all) is important.

I've seen this commercially where an adamant management delayed successful deployment because of an insistence on using processor X, but X simply did not have the capability to do the task. X was cheaper, but just didn't have the resources or performance to do the job. After a one year delay, said management was removed, and engineering finished the job. Was a colossal waste of time and a loss of market share.

 

[homebrewed]

Is OVDD and OGND connected to Teensy's 3.3V power and ground? If OVDD is 5V, the ADC will destroy the Teensy.

No, the ADC board has a 3.3V regulator for OVDD.

 

[homebrewed]

If there were interlocks of some sort, something close to the microcontroller would be good. If someone used a PMT, you'd want interlocks on the enclosure, so the high voltage would be shut down if the enclosure is opened. Or the case was opened up and you could be exposed directly to the radioactive source. Some kinds of interlocks are good, as long as they can be bypassed for diagnostics. The interlocks prevent minor booboos from turning into issues, in my humble opinion. That's what I meant. Hope that is clearer. If not, keep asking more questions. I'm not looking for nanny stuff, which I dislike, but more reminders that doing X might have some hazards, and procede at your own risk.

We would need some sort of physical interlock if there's real concern regarding the smoke detector capsules, since we can't turn them on & off. Sliding in a lead shield through a slot would do it; but other than turning some sort of audible or visual alarm off, there's nothing that prevents the user from NOT using a sliding lead shield.

I suppose that's one argument for the considerable uptick in complexity and cost for using an xray tube.

My current setup has a lead shield on the interior of my aluminum box, except for the door. The door is .25" thick aluminum, which by my calculations reduces the x-ray flux by about half. A piece of 1/16" thick lead will drop the exit flux by a factor of 1.2E-4. I have the lead sheet but the other lead shielding in the interior is a bit too long so I can't close the door if I put the 1/16" sheet in there. I've been lazy and haven't trimmed the shielding bits yet.

 

[WobblyHand]

We would need some sort of physical interlock if there's real concern regarding the smoke detector capsules, since we can't turn them on & off. Sliding in a lead shield through a slot would do it; but other than turning some sort of audible or visual alarm off, there's nothing that prevents the user from NOT using a sliding lead shield.

I suppose that's one argument for the considerable uptick in complexity and cost for using an xray tube.

My current setup has a lead shield on the interior of my aluminum box, except for the door. The door is .25" thick aluminum, which by my calculations reduces the x-ray flux by about half. A piece of 1/16" thick lead will drop the exit flux by a factor of 1.2E-4. I have the lead sheet but the other lead shielding in the interior is a bit too long so I can't close the door if I put the 1/16" sheet in there. I've been lazy and haven't trimmed the shielding bits yet.

I'm just thinking out loud about some of this. It might be appropriate to have shields and perhaps an interlock in the "release" design. Obviously we cannot prevent folks from doing unwise things like sleeping on a pillow full of these capsules, but at least we can put in some warnings. Personally, I would shield everything unless it was in use. At that point, I'd open a door or aperture and allow the X rays "out" for a measurement.

 

[WobblyHand]

No, the ADC board has a 3.3V regulator for OVDD.

Thought so, but I didn't have access to the backside of your board, only the front. It was hard to tell how it was connected from the picture.

 

[rwm]

Can I ask about the PIN diode selection? We have selected the device from the pocket geiger (X100-7) however there seem to be a lot of choices available. What is the main reason for choosing this? I know dark current and sensing area are important factors. Could there be a better and cheaper choice? There are some diodes with lower dark currents but they seem to have smaller sensing areas.