What is on the board
One chip, a capacitor, two pull-up resistors and a jumper on the back. The chip's three address pins are wired to ground, so it answers at 0x48 and nowhere else, and its thermostat output is wired to nothing, so the sketch does the deciding.
Five parts
The chip is an LM75B in an eight-pin package. The fitted part is made by UMW, as a second source for NXP's LM75B: the same pins and the same registers. The two makers' datasheets differ on some numbers, and How close to the truth says which ones this book uses.
The capacitor, 100 nF from VCC to GND beside the chip's supply pin, keeps the supply steady while it converts. The two resistors at the right edge are 10 kΩ pull-ups on SDA and SCL. The jumper on the back connects them to VCC.
There is no LED and no regulator. VCC goes straight to the chip.
The address is fixed
The chip's address is seven bits: 1001 is built into every LM75, and pins 5, 6 and 7, A2, A1 and A0, set the last three. On this board all three are wired to GND, so the address is 1001000, which is 0x48.
The chip could answer at any of eight addresses, 0x48 to 0x4F. This board answers at one. The page this handbook replaces said to wire A0 to A2 differently to fit eight on a bus; on the TK42 there is nothing to change, and One address, 0x48 is what to do instead.
The pin that goes nowhere
Pin 3 is OS, the chip's thermostat output. The chip compares every reading with a limit, 80 °C from power-up, and switches OS when it passes. It could drive a fan without a microcontroller at all.
On the TK42, pin 3 is connected to nothing. It still switches inside the chip, harmlessly, but no pin on the header can see it. If you want a fan to come on at 30 °C, the sketch reads the temperature and switches the fan itself.
When it does not work
No. A0, A1 and A2 are joined to GND in copper under the chip, with no pad to change. Many LM75 breakouts bring the three pins out; the TK42 does not. One TK42 per I2C bus.
The chip has one, pin 3, and on this board it is not connected to anything. The header carries GND, VCC, SDA and SCL only. Compare the reading with a limit in the sketch instead.
The chip, all of it. The sensor is inside the eight-pin package, so it measures the temperature of the chip itself, which follows the board and the air around it. The other parts only serve the bus and the supply.
What the chip sends, bit by bit, and the one line that turns it into degrees.
Degrees in two bytes →Edit this page — content/books/lm75/whats-on-the-board.mdx
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LM75 Temperature Sensor
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