Disabler Project

By Robert Sprowson

Originally published in EUG #33


If there is some gadget or addon that you want control over, but don't want to waste the User Port or clog up a ROM socket, then this simple circuit is ideal for you.

It can be used to turn on/off any device (up to the rating of the transistor) including mains appliances (via a relay) such as a monitor, or used to supply logic levels to elsewhere on the board. This might, for example, be used to control the READ lines on a block of Sideways RAM to remove it from memory if a bad image has been loaded.

The board will be activated by BREAK-key combination in a similar way to the Disk Filing System being booted.


The circuit diagram accompanying this article shows how to assemble the device electronically. It requires the following parts:

  1x74LS32 or equiv. (OR)
1x74LS00 or equiv. (NAND)
1x10 micro Farad capacitor or near match
1x3.3k pull up resistor
1x4.7k limiting resistor
1xNPN transistor to suit use (eg.ZTX300)
Stripboard, wire, solder


  • No additional driving software required
  • Easy to fit
  • No loss of valuable User Port or other IO space (so no address decoding required)
  • Load needn't be on the +5V rail as shown in the diagram, any other (even external) PSU will do.


  1. Assemble the parts as efficiently as needed to fit onto the board you have. It is, in fact, sufficiently simple to use point-to-point wiring and a bit of insulation tape!
  2. The +5V and 0V required by the logic can be taken from anywhere on the main board where it is present. Noteably, the flyleads from the PSU or on pin 14 and 7 (respectively) of most logic chips.
  3. Solder a wire to the not RST of the BREAK key on the keyboard.
  4. Solder the terminals marked "A" and "B" onto the chosen key combination.
  5. Connect up the load as appropriate. The supply can be any DC supply, as long as its ground is common to the BBC's ground rail.
  6. The capacitor and 3.3K resistor are needed to ensure a known reset condition occurs on powering up the BBC. Their values aren't critical.


  • I used this to turn off a fan that I added to my machine, where the supply of the fan was taken from the 12V line inside the PSU that is usually used for the disk drive.
  • By the addition of more OR gates (There are still two spare inside the LS32), you can make it into a BREAK,key,key,key combination if you have enough fingers to press all the buttons!
  • In its present state, the transistor is on by default on power up. i.e. the output of the RS latch made by the LS00 is high by default. If this is not as desired then swap it for the other output of the LS00.
  • Try swapping the RS latch for a divide by 2 counter, to give whatever it is you're controlling a toggling action.

Happy soldering!

Robert Sprowson, EUG #33