Wednesday, January 16, 2013

Siemens EPD-1D dosimeter teardown

Ebay can get you a lot of really exotic items really cheap. Period. This was no difference. While browsing through looking for a cheap (and yet still functional) NaI(Tl) scintillator, I came across an offer that looked like this:

(please excuse the violet background, I'm not the author of the picture...)

(© goodgadgets2u)

If you are still clueless about WTF is that, you are looking at the business end of a Siemens EPD-1D gamma dosimeter. Three PIN photodiodes (no idea on actual part number), each with a different level of shielding. The opaque one has a thick lead alloy shield around it (pictured above it), the right one seems to have a thin lead disc on top and the middle one is covered by thin aluminium foil supported by a polymer substrate.
The blue thing are three separate very sensitive FET amplifiers.

Instead of the sawed off guts, I got an entire unit (non-functional though).

© me (you can shamelessly steal these)

The unit did do something after being connected to power and restarted, but after performing a self-test and a few beeps, it always stopped reacting, while displaying an error.
So, autopsy time:

front cover, backside view
(note the shields for the diodes)
back cover, backside view

back cover, front side view

PCB, front
note the second piezzo buzzer (no idea why)
PCB, back
PCB, back (again)
(slight corrections so that everything is readable)
For some (to me unknown) reason, they decided to use a 4-bit MCU(the biggest IC on the picture), made by NEC in Ireland (Damn this thing is old). The small one labeled "X24C16S" is a 16K (2048 x 8 bits) serial E2PROM, most likely to store the dose measurements. (I might attempt to read the contents some day)
I have no datasheets on the other ICs though.

I'm just speculating, but the IC closest to the blue amp board is probably an ADC, the logo is Texas Instruments, but the search on their website returns nothing.
(Feel free to contact me if you know anything about any of these ICs)

Other than that, there are two crystals, one is for the CPU (1 MHz), the other is a real time clock (32.768 kHz). Note that the 1 MHz crystal has a ceramic package with a transparent window, so you can actually see the crystal itself (the thin rod in the middle).
That about wraps up the summary of interesting stuff  I can tell about this board. (for now, at least)

I'm still in the process of reverse engineering the PCB, since actually I need to finish building a capacitance meter first (SMD capacitors do not have the value displayed on them and the coloring is not standardised). I'm really only interested in the analog part of the detector, so I can rebuild it with a different MCU.
All will be posted in due time, so until then...