LPT Relay

A friend, concerned to monitor the temperature of his computer, found the need to trigger an external set of fans to improve the heat exchange between the environment where that computer is installed and the external environment.

Monitor the temperature and identify if the CPU is hot would be done via scripts. But how to drive an external device? A simple solution consists in using the parallel port of the PC. With this approach, one of the 8 bits of data can be collected to trigger a relay. If USB or serial ports were used, a microcontrolled circuit would be needed to interface with and interpret the data from these ports. In the case of the parallel port, the only additional concern was the use of an opto-coupler to promote electrical isolation between the computer with the rest of the circuit.

(click on the pictures to enlarge)