The QK108A is the assembled & tested version.

This kit provides 8 relay outputs (mains rated) and 4 optically isolated inputs.   It is designed for control and sensing applications.  It can be used in various applications including load switching, external switch input sensing, contact closure and external voltage sensing. It is controlled via a serial port using a variety of methods of control software using a set of simple text commands. Connection to the isolated inputs and relay outputs is via "pluggable type" screw terminal blocks by Dinkle.

Using the serial port has several advantages:
1. Fewer wires are required (3 instead of 9 or more needed by the parallel port)
2. Serial cables can be a lot longer (up to 100ft) allowing "remote" control.
3. Serial ports can be used with any computer and operating system

The kit is constructed on a double-sided, through hole plated printed circuit board (PCB) and fits in a plastic case measuring 140(W) c 110 (D) x 35 (H)mm.   Screen-printed front and rear panels are supplied. 

The kit requires a 12V DC power supply center positive.  A wall adaptor rated at 500mA (minimum) is suitable. PS1205R

SPECIFICATIONS

CIRCUIT DESCRIPTION

This kit is an extension of our popular QK74, "PC Printer Port Relay Board".   The idea was to redesign the PCB to fit into a plastic case and add some inputs as well.  At the same time a "latch" was added so that the PC could be switched off without affecting the relays.  This proved to be difficult due to the PCs POST (Power On Self Test) function, which would operate and release relays during power up.  The easiest solution was to use the serial port. 

The brains of the kit is IC1, and 89C2051 micro-controller from Atmel (www.atmel.com).   This was chosen because it has the required number of I/O pins and built in serial port.  It is preprogrammed to process all commands received via the serial port, control the relays and monitor the inputs. 

IC8 provides conversion between TTL and RS232 signals.  IC2 is an octal relay driver, ULN2803A, and is used to drive each of the relays.  The opto-couplets, IC4-7, are used to provide electrical isolation between the inputs and the rest of the circuitry.

At power-up, all relays are off (released).  A reset switch is provided for manually turning off all relays. 

Note: during power up relays 1 and 2 momentarily operate.  Also, when using the reset switch, these two relays will be operated for as long as the reset switch is pressed.  This is due to a characteristic of pins P1.0 and P1.1 of the Atmel micro-controller used.

The QK108U upgrade kit is an upgrade to allow 2 x QK108(A) kits to be connected together to provide 16 relay outputs and 6 opto-isolated inputs.
QK108U Upgrade link in related products below