Posted on Feb 7, 2014

The intelligent temperature sensor DS18S20 from Dallas semiconductor is a calibrated precise two resp. three pin device that interfaces direchtly to microcontrollers or PCs producing Celsius readings. The so-called 1-wire protocoll (better `one DATA wire. ) can be adapted to the RS232 port with minimal component count. Simple circuits are publishe

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d that do not work too well with new DS18S20 components. But improvements are on the way. Latest desktop or laptop computers do not provide serial RS 232 interfaces any more, but instead numerous USB interfaces. USB is an intelligent interface, in every application it requires an intelligent IC to signal the type of interface to the computer when USB gets connected. On the other side more power, up to 2 W, is available to operate the 1-wire bus circuit. The FTDI USB-to-serial interface chip provides the electrical interface to the computer as well as the functions of a "fake" serial protocol. When properly applied with suitable hardware the computer may still believe to operate a serial interface when in fact USB is used. Below a prototype board of such an interface is shown that uses a USB-to-serial chip to provide the "Transmit TXD" and "Receive RCV" signals that finally operate a 1-wire- hardware interface. The circuit uses an ready-made kit interface adapter board for the FTDI USB-RS232 chip in surface mount technology. The 1-wire circuit interface includes a voltage doubler circuit to provide "always" enough drive voltage and current for the 1-wire bus even in case when the USB voltage is lower than normal. The zener diode at the output node limits the drive voltage as well as external overvoltage or cable noise. Compared to a passive pullup resistor this circuit provides substantial loop current even at voltages just...

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