Why didn't Acorn put a TAB key on the Electron? And why was the <SHIFT LOCK> key left off too? It's not as if there wasn't enough room to accommodate these useful keys either mechanically or electronically.

To rectify this oversight, I have designed a couple of circuits. The first simulates the TAB key by using the <CTRL> <I> key sequence and the other toggles the <SHIFT LOCK> "on" and "off".

TAB

My design remit was to use a single key to simulate the <CTRL> <I> key sequence to produce a TAB character in a wordprocessor.

The first part of the design was to determine the sequence of events:

• Press <CTRL>
• Press I
• Release I
• Release <CTRL>

This translates into the electrical waveform:

 
                ______________________________ 
         ______:                              :____________  CTRL 
  
               :<-----------"t1"------------->: 
                              ____________ 
         ____________________:            :________________     I 
        
               :<---"d1"---->:<---"t2"--->: 

The circuit had to produce two outputs, CTRL & I, whose waveforms have the above relationship. I have included diagrams showing the circuits which can be accessed from the Articles Menu.

In Operation

Pressing and releasing S2 triggers IC3b (a retriggerable monostable) immediately which in turn switched on IC2b (a bi-lateral switch). This effectively closes the CTRL keyswitch for 300mS (t1).

When IC3b is triggered, R6/C2 delays the trigger of IC3a for 150mS (d1). After the 150mS delay IC2a switches on, effectively closing the I keyswitch for 75mS (t2). R6/C2 also debounces the S2.

IC2a pins 10 & 11 are wired to the "CTRL" keyswitch and IC2b pins 1 & 2 are wired to the I keyswitch on the keyboard.

<SHIFT LOCK>

IC1 (a D type flip flop) is configured to toggle its output once for every clock pulse it receives. The clock pulse is generated by S1 which is debounced by R1/C1. The Q output of IC1 is used to switch on an LED to indicate that the SHIFT LOCK is "on" or "off" and to switch IC2c, another bi-lateral switch to effectively latch open or close the <SHIFT> keyswitch.

IC2c pins 3 & 4 are wired to the SHIFT key on the keyboard.

Construction

The construction of the circuit was done on Vero board but a circuit board could quite easily be designed and produced. The boards mounting will depend on your system. I mounted the circuit on the left hand side of the keyboard next to the <SHIFT> and <CTRL> keys - this space is only available to those who have moved their Electron circuit board into a Slogger RX case or similar box. It could be mounted somewhere behind the function keys or alternatively in a separate box.

Parts List

R1	15K	0.25W	C1	10uF	16V	IC1	74LS74
R2	4K7	0.25W	C2	10uF	16V	IC2	CD4538
R3	470R	0.25W	C3	22uF	16V	IC3	74C4066
R4	47K	0.25W	C4	4.7uF	16V
R5	2K2	0.25W
R6	15K	0.25W	Q1	BC107		SW1,2	Keyswitch
R7	15K	0.25W	D1	OA91
R8	15K	0.25W	LED1	RED

Derek Walker, EUG #17

Derek Walker