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SWITCH
This page describes the simple four-channel input-output device, controlled trough PC parallel port. The main features are read inputs (pressed buttons), switch the relays’ pins according to the software logic or sequence of pressed buttons. It can be used offline when switching the relays manually by the knob. The character LCD is installed for the indication purposes. Initially I decided to build it as antenna switch, but actually this small device could serve as multipurpose switch, due to the software capabilities to apply any logic of using four outputs and inputs. The sample software project is written in Visual Basic 6 and uses Inpout32.dll for WIN NT/2000/XP. The set of the very simple circuits is presented bellow for the completion of the project. Always remember, that incorrect soldering could cause PC’s port damage! Disclaimer. These circuits are provided for your own study only. They all deal with electricity and electronics, a sufficient knowledge of both is assumed. In no event will LY2XW be liable for any consequential, incidental or direct damages suffered in the course os using the information provided on these pages.
This circuit shows how to connect a HD44780-based 16 character x 2 line LCD module with backlight option to the parallel port. It is quite simple to work with, as all the logic required to run it is on board.	This circuit handles relays (replicated four times for each relay). The opto-isolator is there to protect the port. Such arrangement prevents any currents on the external circuits from damaging the parallel port.
Four channel parallel port input circuit for switches was used in my project. When the switch is activated (+5V between parallel port ground and input pin), the pin goes to logic state 0. 3k pull-up and 330 safety resistors used to prevent PC port damage.	This circuit is very simple and easy to build dual power supply needed to swith relays (12V) and drive LCD or parallel port inputs (5V).Do not forget to put the heatsinks to the 7805,7812 regulators (big enough to avoid overheat because of the extra losses in the regulator).
Assembled it looks like this

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