HW projects - Parallel port relay board (and control using web browser)

Some history:

This simple LPT port relay board was constructed over 10 years ago and it was originally connected to a PC running MS-DOS and PCBoard BBS system with modem connection. I also wrote a PPL script menu for PCBoard that launched MS-DOS executables to control the board.

old PCBoard ANSI controls

Modernizing:

The idea was to resurrect this project by renewing things a bit and make it controllable from the internet. I happened to stumble across a ready made PHP / AJAX control script written by Tomi Engdahl that turned out to be perfect for this sort of thing. I just modified the script to suit my needs and made some minor changes and repairs to the old board. You can find the original script and huge amount of info about parallel port interfacing from www.epanorama.net/circuits/parallel_output.html.

Board details:

The relay board itself is really simple and was built on a protoboard mostly from "recycled" parts. The eight LPT data output pins are connected to ULN2803A Darlington transistor array chip that has built in input resistors and clamping diodes at the output for interfacing directly from 5V TTL logic to small inductive loads (like relays). The transistor chip simplifies things a lot if compared to building this from discrete components. The controlled 12V relays are rated for 16A / 230Vac operation, so they could be used to switch almost all kinds of mains operated single phase appliances (lighting, motors, heating etc.).

the board (in action)

The demonstration:

I have set up a page where you can switch the relays on and off and view it through a webcam. The loads, however, are just different coloured LEDs connected to the relay outputs. Only one user can can operate the board at a time to make the control more bearable (would work as a multiuser though, but it would be a mess). Javascript must also be enabled in your browser.

To the control page (limited to one user at a time):
[ LPT board control / webcam view page - still under construction, but it works ]

If the page is busy you can still view it here:
[ LPT webcam view page ]

• LPT board control
• LPT webcam view

Pictures:

(click for larger image)



the board