Technical Aid to the Disabled

Project 98-553

REV B 30-Oct-1998

Construction of light box communication device for clients with severely limited communication skills.

The box has a translucent cover with two lamps underneath, such that one side or the other of the box may light up. A control circuit provides switch input detection, lamp switching, lamp pre-warming, and lamp voltage reduction.

(Click on images for higher resolution view)


The box consists of a wooden frame, carrying handle, and adjustable tilt-back legs. The inside of the box is divided into two halves, left and right.


The control circuit uses a microcontroller to detect closure of the remote switch and light the lamps. The software in the microcontroller commences operation by running the lamps at 5% duty cycle at about 500Hz, via the two power MOSFET switches. When the remote switch is closed, the software cycles to the next lighting state. After the last state, the software returns to the initial state.


Left Lamp

Right Lamp

One Warming (5%) Warming (5%)
Two On (80%) Warming (5%)
Three Warming (5%) On (80%)

The software also implements a three minute timeout, such that it will automatically revert to the initial state (both lamps at 5%) if no input is detected for three minutes. This is to prevent heat build-up within the case, which might otherwise deform the perspex cover.

A pair of fans are glued to the base angled such that they disturb and redistribute the heat plume from each lamp.

See also the assembly language source file that was compiled using gpasm and written into the 12C509 chip.

Theory of Operation

Protection & Regulation

The unit uses a 12VDC 2A unregulated supply from Jaycar. This generates some 20 volts DC when unloaded, and drops to about 15 volts under single lamp load. A two amp PCB mounted glass fuse provides fault protection, followed by a 1N5404 diode as reverse polarity protection.

The supply after the diode is distributed to the cooling fans, the logic regulator, and the lamps. A second diode, 1N4148, separates this supply from the logic regulator's 1000uF capacitor, to prevent lamp switching power being supplied by the capacitor. The LM7805 voltage regulator provides a stable 5V supply for the microcontroller. A pair of capacitors on the 5V bus quench switching transients from the microcontroller.


The unregulated supply is applied to two 12V fans, in series, with a 100 ohm resistor and two 1N4004 diodes. The fans are operated at about 6V each, running slow and quietly by comparison to their top speed.

Input Conditioning

The R/C network attached to the trigger switch acts to (a) reduce contact bounce, and (b) increase resistance to induced noise. Without R3 (100 ohm), the switch had been found to short the capacitor, inducing a reset on the microcontroller.

Lamp Drive

Two outputs from the microcontroller connect directly to the gates of two 2SK2175 logic level power MOSFETs. When +5V is applied to the gate, this type of MOSFET will conduct via source and drain with a resistance below 0.1 ohm, lighting the lamp.

A prototype circuit used MTP3055E MOSFETs, but they required a gate drive around 12V to switch fully, and hence ran particularly warm in this configuration.

Parts List

Technical Aid to the Disabled                        
Project 98-553         
Revision AX3           
10th October 1998      

Supplier   Part        Description                     Cost   Qty  Price

Stock      N/A         PCB 4x2 Veroboard               3.25  0.50   1.62
Jaycar     WH-5543     Heatshrink Tubing               1.95  0.08   0.16
Jaycar     YX-2502     Fan 12V DC 0.2A 40mm USED      10.00  2.00  20.00
MicroZed   IC 12C509-4 PIC12C509/04/P 8 pin RISC CPU   3.00  1.00   3.00
Jaycar     PI-6452     Socket IC 8 pin DIP             1.05  1.00   1.05
Oatley     2SK2175     2SK2175 TO-220 Logic MOSFET     2.60  2.00   5.20
J V Spares 47382       Globe 12V 21W BC                1.50  2.00   3.00
Stock      N/A         Resistor 100 0.25W              0.04  1.00   0.04
Stock      N/A         Resistor 100 0.50W              0.15  1.00   0.15
Stock      N/A         Resistor 10K 0.25W              0.04  1.00   0.04
Stock      N/A         Capacitor 0.01uF Ceramic        0.40  2.00   0.80
Stock      N/A         Capacitor 10uF Tantalum         1.00  1.00   1.00
Jaycar     RE-6230     Capacitor 1000uF Electrolytic   0.95  1.00   0.95
MicroZed   IC 7805     LM7805 5V 1A Regulator          0.45  1.00   0.45
Jaycar     ZR-1014     1N5404 3A 400V Diode            0.30  1.00   0.30
Jaycar     ZR-1100     1N4148 Signal Diode             0.06  1.00   0.06
Jaycar     ZR-1004     1N4004 1A 400V Diode            0.08  2.00   0.15
Jaycar     HM-3180     10 Way PCB Terminal Strip       1.50  1.00   1.50
Jaycar     PP-0524     2.5mm Bulkhead Socket           2.25  1.00   2.25
Jaycar     PP-0512     2.5mm ID 5.5mm OD DC Plug       1.30  1.00   1.30
Jaycar     PS-0123     3.5mm Insulated Socket          1.60  1.00   1.60
Jaycar     MP-3018     13.8V DC 2A Non-reg Supply     42.95  1.00  42.95
Jaycar     SF-2160     M205 2A Fuse                    0.35  1.00   0.35
Jaycar     SZ-2018     M205 Fuse Clips, PCB            1.40  0.25   0.35
                       Total                                       87.58