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USB-powered PC-Therm

Temperature shown on the Windows taskbar

In case you were unable to find the LM2936Z5 regulator, or just want to build a minimal component-count version of the PC-thermometer, you can follow James Maloway's schematic:

How it works

It works because the USB port includes 5V power for the USB peripherals, so we can "steal" a few milliamps for our circuit. James cut an ordinary USB cable leaving only the connector that plugs to the PC, and connecting the power (5V+ and GND) to the DS1631. The data wires (D+ abd D-) are unconnected.

The power from the USB port is regulated by the PC, so all the parts related to the power regulator are unnecessary, except the 100 nF capacitor that counteracts the inductance of the cable and must be placed as close as possible to the integrated circuit. The PC reads the temperature from the serial port, using a software-emulated I2C-bus as in the original circuit, therefore you can use the same software and the PC won't notice the difference between the circuits. The resulting circuit counts only 5 parts besides the DS1631, and works with the DS1621 as well.

Parts list

2 x 5.1 volt 1/4W zener diodes (1/2W and 1W diodes are OK)
2 x 4700 ohm resistors
1 x 100 nFcapacitors
1 x DS1621 or DS1631 digital temperature sensors (Maxim-Dallas Semiconductor, www.maxim-ic.com )
1 x DB9 female plug
1 x TYPE-A USB plug (from a USB cable)

Important notice

Please be very careful when modifying an USB cable and when using power coming form USB, because a wrong connection or short-circuit can damage the PC or cause the PC to power-off abruptly with potential loss of data. In fact, power on the USB connectors comes from the same power supply unit that feeds the CPU, so you are warned, you do it at your own risk!

Before powering the system, please double check every connection, solder joint, etc, against the schematic and the photo of the USB connector.


USB type-A plug

 

Back to the PC-therm main page (original circuit details, software, sources, tips).

 

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