As you can in the first picture the PCB is divided into 5 sections

1 — Device power supply and regulator section:

Which is base on:

a — a 3.3V SMD voltage regulator with markings (4B2F) on it, probably TAR5SB33 3.3V voltage regulator

b — 1 NPN transistor (MMBT3904) with markings (1AM)

c — 1 PNP transistor (MMBT3906) with markings (2A), used as a high side switch for the LASER diode

2 — Microcontroller section:

This section is based on:

a — STM8L052C6 8-Bit Ultralow power microcontroller (datasheet), with up-to 4×28 segments LCD driver

I was surprised that they used the LQFP package of the Microcontroller and not a Chip On Board package as usual.

The microcontroller receives a square wave signal from the signal conditioning section, Turns on and off the LASER diode and the backlight LED and drives the device LCD.

b — programming header pins

c — 3.999 MHz crystal oscillator

3 — LASER Diode and backlight switches:

This section is based on:

a — 1 MMBT3904 NPN Transistor for low side switching of the LASER diode

b — 1 MMBT3904 NPN Transistor for low side switching of the backlight LED

4 — LASER Diode and photodiode PCB:

This is a small PCB that only has the LASER diode and the receiver photodiode soldered on it, and the tiny PCB itself is soldered perpendicularly over the main PCB

5 — Signal conditioning circuit:

This partition is based on the LM358 dual operational amplifier IC which converts the analog signal returning from the photodiode into a square wave with frequency equals to the received rotation speed in (Revolutions Per Second)

As you can see in the third picture, the top layer of the PCB only contains the LCD, the keypads and their rubber pads.

Be careful not to disconnect the LCD from its rubber zebra strip because it will be difficult to return them back as they were.

Leave a Reply