PC Driver Relay Board Kit (RoHS)

39.50 CAD

With this kit you can use your PC as a dedicated controller for lights, motors & other devices. Two complete software programs included to drive it. One for DOS and one for Windows 3.1 Plugs into any printer port, uses separate 12V supply for the relays.


QK74 is now RoHS Compliant
With the rapid progress in computer technology many people have a surplus 386 or 486 computer just gathering dust somewhere.
Thus these computers are free to be used as dedicated controllers for a variety of uses: turning on/off lights or other devices around the home, office, laboratory or factory come to mind. all that is needed is the interface to connect it to the real world.

This kit provides both the hardware and the software to do this. The hardware PCB plugs in directly to the parallel (printer) port of you computer. It carries 8 relays. Each relay is switched on or off by one bit of the output byte which usually goes to your printer to print a character. Two types of software are provided: one operates under DOS while the other is a more detailed program which runs under Windows 3.1. Also available assembled as QK74A.

Write your own software with a DLL from http://www.logix4u.cjb.net/.

The kit is constructed on single-sided printed circuit boards. Protel Autotrax and Schematic were used.


The PCB contains eight identical switched relay positions, power input positions to the relays (either 2.5mm jack center positive, or via the terminal block) and a DB25 connector to the parallel port of a PC> To keep the kit simple no input latches have been put on it. If your application is important then you should use a UPS to keep the computer operating in the case of a mains power supply failure. If the board is accidentally disconnected from the parallel port then the 10K pull-down resistors will turn the relay off. The diodes protect the transistors from the back-emf which is occurs when the relay is turned off and its magnetic field collapses.

Solder the resistors into place first. Make sure to get the right ones in the right place. The components for the first relay position are given in the Components listing. Add the relays last.

The relays are under direct control of the output byte (8 bits) from the parallel port. When a pin is high a nominal 5V is presented to the base resistor R1. Since there is a fixed 0.6V drop across the BE junction of Q1 there is 4.4V across R1 (5V – 0.6V0. So by Ohms law 1.33 mA flows through the 3K3 resistor. There is also 0.6V across the 10K pull-down resistor which draws 0.06mA. If we assume a hFE of 100 for Q1 then 127mA flows through the CE leads of Q1 when it is turned on. Since the 12V relay turns on at around 30mA this current is more than enough to turn on the relay. The relay has a coil resistance of around 400 ohms and a coil power consumption of 30mW.

Note that this analysis assumes 5V is available from the parallel port pin. In more modern computers this may not be so. If the LED turns on but the relay does not then you will know that the transistor is not saturating. Reduce the value of R1 (and all the similar resistors) to 2K7 from 3K3 will probably fix this. Measure the voltage on the collector. When the relay is on then the voltage should be around 0.1V. If it is 3V – 4V then the transistor is not fully on.

The relays are rated at 10Amps for 125 volts but the PCB and connectors would never handle this. The relays were selected to be at least twice what the current flow would be in order to have a long life span and good reliability. In order to have the PCB carry up to 5 amps of current you would want to add capacity to the PCB traces.The kit not modified would be able to carry 2- 3 amps. You could scrap of the solder mask and tin the copper trace or use copper wire from point to point. If it keeps its cool, it will last longer and be more reliable. In order to drive even larger loads use a relay specific to your load. Electrical supply houses sell large relays with double poles for switching large AC loads safely. Use the contacts on the QK74 to supply the coil voltage to the larger relay. Safety first, make sure you do not make any short cuts where large loads are concerned.

Here is a new app for use on Vista or XP with Dot NET installed. Converts from all sorts to all sorts and then lets you write the value to your QK74.




OutPort for QK74

OutPort II for QK74 has count function added as well as a random output function. Works with Dot Net on XP and VISTA, perfect for your Halloween lighting effects, on a budget.


We have provided two independent sets of software with the kit. First is a set of DOS-based utilities. If you have some experience writing batch files then you can easily write a program to control the relay board. A sample test program is provided. The second is a fully-featured Windows-based utility. It was written especially for this application.

Of course, you can write your own programs in any language to output a byte to the printer port and the bits which are high will turn on the corresponding relay. The overlay on the PCB shows which bit in the output byte turns that particular relay on. For example, output 00010001, or 11 in hex turns on relays 1 and 5. Each relay number is also marked on the overlay for easy reference.

DOS-based Utilities.

There are three utilities in the set:

– RELAY outputs a hex byte to the designated parallel port.

– DELAY waits for a user defined number of seconds.

– WAITFOR waits until a specified time (HH:MM)

Batch files using these programs can be written to control the operation of any of the eight relays on the board.

The programs were written using Borland Turbo C/C++ 3.0. The C source code is also provided on disk.


Syntax: RELAY , to LPT1, or

RELAY/1 , to LPT1, or

RELAY/2 , to LPT2

Output the hex byte to the specified parallel port. The value of hex byte can be in the range 00h (all relays turned off) to FFh (all relays turned on.)

The parallel port can be specified following the command name (as in the last two examples above.) If none is given then LPT1 is used by default.

Here’s an example:


Outputs the hex value 5A (binary 0101 1010) to LPT1, causing relays RL2, RL4, RL5 and RL7 to turn on.