WX200 / WM918 Temp / Humidity Sensor

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Inside The Sensor

General Temperature / Humidity Sensor Information


Temperature Sensor The Temp / Humidity Sensor - :

The sensor is built around U1, a 64 pin IC type MSM64162-288. The MSM64162 is a low power 4-bit microcontroller that employs Oki's original CPU core nX-4/20. The MSM64162 has 2 RC oscillation type A/D converters, an LCD driver for up to 80 segments which is not used here, and buzzer output port, also not used. It is best suited for applications such as low power, high precision thermometers and hygrometers such as this sensor.
More data including a full     312kb PDF   file can be found here.
The manufactuer's (OKI) web page for this is here.

Features -

There are a couple of other 0.1 uF capacitors connected to the uP and ground, these are decoupling capacitors for the several different supply rails used inside the micro controller.

Humidity Sensor
Inside the Temperature Sensor
Humidity sensor HUM1 is a small, white ceramic (I think) based element in a small plastic enclosure. It has a 1/8 watt padding resistor R2 across it, and judging by the soldering at this point and R2, R3 and R7 which are connected in a series - parallel arrangement, appear to have been selected under test, and added after manufacture as the solder on other surface mount components have a much duller appearance about them. Variable trimpot VR2 is in series with this network, so it would appear that the Humidity sensor is calibratable to some extent. My unit is / was working fine so I haven't tweaked it (If it ain't broke, don't fix it)

Temperature Sensor
Temperature sensor bead TH1 is a tiny brown / black blob on the end of 2 fine wires. This also goes to a pair of 1/8 W resistors, R9 and R10, but no variable trimpot is in the circuit for adjustment.

Blank PCB Area
A number of option solder pads are also on the PCB - P1, P2, and P11 thru P16. No idea what these are for, as I haven't tried joining them to see what effect it would have (It's taken me long enough to work this lot out and write it up)

There is also space for several more surface mount components, and an LCD connection on the reverse side. Judging from the way the PCB tracks are laid out, it would appear that there is the possibility of a version where the 1.5 volt voltage regulator is integral to the sensor, and not part of the Interconnect Box.

There also appears to space on the PCB for a reset generator consisting of an NPN transistor (Q2) and a PNP transistor (Q1) with several biasing resistors (R 11, 12, 13, 14) for those types of uP which do not have Schmitt triggers on their Reset Input

There are 4 wires connected to the PCB, they are -

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Updated 02:39 17/12/2000.

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Temperature / Humudity Sensor Schematic by Kent Bradshaw KB1ESG