Having a gander inside a SMART SENSOR AS808 (an inexpensive, standalone 1xAAA-powered temperature and humidity sensor)

SMART SENSOR AS808 screen, while the sensor is in operation. — The screen of the (reassembled) SMART SENSOR AS808, displaying the ambient temperature and humidity as well as the date.

We have some small and accurate-enough temperature and humidity sensors (“SMART SENSOR AS808”). They run on single AAA batteries and the batteries last for months and months. Their biggest flaw is the lack of any way of easily hanging them. They have a small foot that folds flush with the rear of the case when not in use and a nail/screw-mount hole, but no way of running a bit of wire or strapping material through part of the case so that it can be hung or lashed to something firmly.

I had the hardware but wanted to open the case before hand-drilling the holes for the fasteners to ensure I didn’t accidentally damage something essential inside.

SMART SENSOR AS808 cracked open. — What you see when you crack open the AS808 case. Note the spring contacts (left half) for the piezo buzzer (right side). The humidity and temperature sensors are visible projecting upwards from the rightmost edge of the brown-back of the PCB. The black, barrel-shaped component is an inductor, I think the shiny metal-canned thing is a crystal oscillator. The silvery circle at the center of the PCB is the RESET switch button, accessible via the corresponding hole in the rear of the case (visible on the right side).

Resistive sensors are used for both temperature and humidity. For temperature, there’s an NTC thermistor and, for humidity, there’s an HR202. I found it interesting that the leads for the piezoelectric buzzer are little springs. In my limited experience, solder joins to piezo discs seem more liable to fail than other solder joins and spring contacts strike me as a clever way of making the device a little more durable. It may also make assembly easier.

Front of the SMART SENSOR AS808's PCB. — A front view of the AS808’s PCB. The black pinhead with two leads is a NTC thermistor used as a temperature sensor and the rectangular white thing with an intermeshed pattern is a resistive humidity sensor (an HR202, I think).

I haven’t counted the numerous traces to the epoxy blob, but the behavior of AS808 is complex enough that the black goop must be covering a microcontroller brain chip. The PCB, in addition to labels for components, bears this text: 3 9906 M807 03. I have no idea whether that would give anyone a clue as to the type of MCU used.

SMART SENSOR AS808 — Close-up of the right side of the PCB.

To fasten the loops, I used two little inner-hex, flat-head M3 machine screws and, on the inside of the back half of the case, two hex locknuts with nylon inserts. The batter panel door still slides out easily. You can see the foot and screw/nail-head hole as well as the orifice for the RESET button and a hole for the noise from the piezo disc.

Another tiny but inordinately satisfying mundane task completed. THE END.