Well it’s time for a new DMM. After the last pair of eBay El-Cheapo Chinese meters just didn’t last very well, I decided a proper meter was required. This one is a Tenma 72-10405, stocked by Farnell for under £60. Not quite as many festures as the cheapo Chinese meters, but I expect this one to be a bit more reliable.
Since I can’t have anything without seeing how it’s put together, here’s the inside of the DMM. (Fuse access is only possible by taking the back cover off as well. The 9v PP3 battery has a seperate cover).
He’s the input section of the meter, with the 10A HRC fuse & current shunt for the high-amps range. The other fuse above is for the mA/µA ranges. The back cover has a wide lip around the edge, that slots into a recess in the front cover, presumably for blast protection if the meter should meet a sticky end. The HRC fuses are a definite improvement over the cheap DMMs, they only have 15mm glass fuses, and no blast protection built into the casing.
There are some MOVs for input protection on the volts/ohms jack, the jacks themselves are nothing more than stampings though.
Not much at the other side of the board, there’s the IR LED for the RS232 interface & the beeper.
Most of the other components are on the other side of the PCB under the LCD display. The range switch is in the centre, while the main chipset is on the left.
The chipset of this meter is a FS9922-DMM3 from Fortune Semiconductor, this is a dedicated DMM chipset with built in ADCs & microcontroller.
Here’s a piece of medical equipment that in recent years has become extremely cheap, – a Pulse Oximeter, used to determine the oxygen saturation in the blood. These can be had on eBay for less than £15.
This one has a dual colour OLED display, a single button for powering on & adjusting a few settings. These cheap Oximeters do have a bit of a cheap plastic feel to them, but they do seem to work pretty well.
After a few seconds of being applied to a finger, the unit gives readings that apparently confirm that I’m alive at least. 😉 The device takes a few seconds to get a baseline reading & calibrate the sensor levels.
The plastic casing is held together with a few very small screws, but comes apart easily. here is the top of the main board with the OLED display panel. There appears to be a programming header & a serial port on the board as well. I’ll have to poke at these pads with a scope to see if any useful data is on the pins.
The bottom of the board has all the main components of the system. The microcontroller is a STM32F03C8T6, these are very common in Chinese gear these days. There’s a small piezo beeper & the main photodiode detector is in the centre.
There is an unpopulated IC space on the board with room for support components. I suspect this would be for a Bluetooth radio, as there’s a space at the bottom left of the PCB with no copper planes – this looks like an antenna mounting point. (The serial port on the pads is probably routed here, for remote monitoring).
At the top left are a pair of SGM3005 Dual SPDT analogue switches. These will be used to alternate the red & IR LEDs on the other side of the shell.
A 4-core FFC goes off to the other side of the shell, bringing power from the battery & supplying the sensing LEDs.
Power is supplied by a pair of AAA cells in the other shell.
The sensor LEDs are tucked in between the cells, this dual-diode package has a 660nm red LED & a 940nm IR LED.
Top removed from the mouse, the ball fits in the gap in the centre. The slotted discs are visible which contact the ball & move relative to the surface the mouse is on.
PCB removed from the shell. Pairs of IR LEDs & Phototransistors make rotary encoders with the slotted discs. The microswitches read the mouse buttons & wheel.
IC in the centre interfaces with the PC over a PS/2 connection.