Technical Aid to the Disabled

Project 99-11707

REV A 5-Jul-1999

Control circuit for hearing test response panel, Cochlear Implant Centre, Sydney.

The final device was to be a large desk mounted box with three cavities visible to the patient, but covered with perspex or glass that allows for little light entry. The cavities were to be randomly lit when triggered, to cause the patient to look at the cavity. Some nice stuffed toy was to be visible in each cavity; when the lights turned on. This was to be used as a part of the threshold determination procedure following implantation.

(There are no finished photographs of this project, as it was handed over to another volunteer at the time I left Sydney. My task was the design, construction and programming of the control circuit.)

Requirements

three driving outputs, each with
- 100mA motor,
- 20W halogen lamp (1.8A),
two switch inputs,
- big mac trigger for children on 2.6m cable,
- foot switch for operator on 1.6m cable,
disable switch on back panel,
beeper, should beep for a short time after activating an output,
output selected should be randomly chosen,
avoid 3MHz frequency spectrum,
output on for five seconds only,
output off for five seconds before "listening" again.

Functional Specification

on power up,
- disable all driving outputs,
- enable beeper,
- wait one second,
- disable beeper,
- select next output,
- commence listen.
on listen,
- wait for either switch.
(Why did we need two switches? Is there a requirement I'm missing? Guess: operator switch just triggers for duration of it's closure, but child switch triggers for five seconds.)
on either switch closure
- enable beeper,
- enable next output,
- wait half second,
- disable beeper,
- if operator switch is down
  - wait for release
  - disable outputs
- else
  - wait for five seconds, (but if operator trigger, go top)
  - disable outputs
  - wait for five seconds, (but if operator trigger, go top)
- select next output.

Detailed Design

Hardware Design

PIC 16F84 Microcontroller
- chosen to avoid stuffing around with UV because I might lose the borrowed (thanks Tim!) eraser as I left Sydney,
- cost not much of a consideration, customer is paying $500 for whole unit,
ex TAD/GALE PC power supply,
- chosen for small footprint,
- remove excess DC distribution cables,
- rated at 2.5A on 12V bus,
- separate cables for 12V and 5V bus may help noise reduction,
power supply input using molex connector to PCB,
inputs via 3.5mm sockets,
three logic level power MOSFETs for output drive,
direct drive to piezo buzzer,
drive three additional status leds,
input status leds hard wired,
protection
- via 2.4A polyswitch on 12V bus,
- via some lower value polyswitch on 5V bus,
PCB, veroboard.

Software Design

forth to pic, or assembly,
no cyclic requirement like previous TAD units,
minor timing requirement; five seconds, one second, half second,
timing is not critical,
RC oscillator to reduce component count,
vary RC oscillator for trimming timing,
use functional specification above as psuedo code.

Schematic

(Prepared using xcircuit on Linux.)

Layout

(This shows the broad layout of the veroboard PCB that was constructed, along with the names and cable colours for the cables attached to the PCB via headers and screw terminals. Prepared using xcircuit on Linux.)

Switches

(Showing the wiring of the child and operator trigger switches with integrated LED. Note that the Jaycar footswitch unit was modified by drilling a hole and inserting the LED in the side of the top cover. Prepared using xcircuit on Linux.)

Source Code

The file main.asm was assembled using gpasm and programmed into the 16F84 chip using picprg 2.2. Source editing was done using emacs with a modified font lock mode for PIC assembly code. Since the editing and assembly machine platform was a different machine to the chip burning machine, the Makefile took care of copying the assembled HEX file to the other machine.

Parts List

Technical Aid to the Disabled                        
Project 99-11707       
Parts Used (Electronics Only)                        
Revision C 
12th April 1999        

Supplier   Part        Description                     Cost   QtyPrice

Jaycar     HP-9540     PCB 95mm x 76mm                 3.25  1.00   3.25
Jaycar     ZZ-8500     PIC 16F84-10/P 18 pin CPU      14.95  1.00  14.95
Jaycar     PI-6458     Socket IC 18 pin DIP            1.75  1.00   1.75
Stock      BUK455      BUK455 TO-220 Logic MOSFET      2.60  3.00   7.80
Jaycar     SL-2793     20W 35mm Halogen Lamp           3.16  3.00   9.48
Jaycar     SL-2735     Base for Halogen Lamps          3.25  3.00   9.75
Jaycar     RR-0572     Resistor 1k 0.25W               0.04  6.00   0.24
Jaycar     RR-0596     Resistor 10K 0.25W              0.04  3.00   0.12
Jaycar     RR-0562     Resistor 470 0.25W              0.04  1.00   0.04
Jaycar     ZD-1740     LED 3mm Red (Diagnostic)        0.30  3.00   0.90
Jaycar     ZD-1710     LED 5mm Red (Child Switch)      0.30  1.00   0.30
Jaycar     ZD-1720     LED 5mm Yellow (Operator)       0.30  1.00   0.30
Jaycar     ZL-1715     LED 5mm Green (Power)           0.30  1.00   0.30
Jaycar     SL-2612     LED 5mm Green (Front Panel)     1.75  1.00   1.75
Jaycar     RC-5324     Capacitor 100pF Ceramic (Osc)   0.15  1.00   0.15
Jaycar     RC-5348     Capacitor 0.01uF Ceramic        0.15  1.00   0.15
Jaycar     RZ-6648     Capacitor 10uF Tantalum         1.00  1.00   1.00
Jaycar     RE-6194     Capacitor 470uF Electrolytic    0.60  1.00   0.60
Jaycar     AB-3458     Piezo Beeper                    3.95  1.00   3.95
MicroZed   IC 7805     LM7805 5V 1A Regulator          0.45  1.00   0.45
Jaycar     ZR-1014     1N5404 3A 400V Diode            0.30  3.00   0.90
Jaycar     ZR-1004     1N4004 1A Diode                 0.06  1.00   0.06
Jaycar     HM-3217     PCB Header, 2 pins              0.50  1.00   0.50
Jaycar     HM-3218     PCB Header, 4 pins              0.65  1.00   0.65
Jaycar     HM-3219     PCB Header, 8 pins              0.80  1.00   0.80
Jaycar     HM-3211     PCB Launcher, 14 pins           0.70  1.00   0.70
Jaycar     HM-3202     Plug/Socket Terminal Strip      8.50  1.00   8.50
Jaycar     PS-1060     XLR Female Panel Socket         6.00  2.00  12.00
Jaycar     PP-1052     XLR Male Line Plug              5.95  2.00  11.90
Jaycar     WB-1575     Cable, 6 core, Shielded         1.95  4.00   7.80
TAD/Gale               12V Regulated Supply            0.00  1.00   0.00
Jaycar     SK-0984     Rocker Switch (Override)        1.95  1.00   1.95
Jaycar     SP-0760     Foot Switch (Operator)         17.95  1.00  17.95
TAD                    Big Mac Switch (Child)                       0.00
Jaycar     HM-3106     PCB Socket Header, 6 pins       2.15  2.00   4.30
Jaycar     HM-3126     PCB Terminal Plug, 6 pins       6.00  2.00  12.00
Jaycar     HM-3102     PCB Socket Header, 2 pins       0.70  1.00   0.70
Jaycar     HM-3122     PCB Terminal Plug, 2 pins       2.00  1.00   2.00
                                                                  ------
                       Total                                      139.94

5-Jul-1999