iMac G4 Video - Open Source Hardware

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Controls
An Open Source Hardware/Firmware/Software Project

Overview
The controls, video interconnect and temperature sensors are built on one PCB to save costs. The controls comprise the upper two thirds of the PCB.

Many components are optional and not necessary if only a subset of functions is required. These are detailed in Construction.

The main purpose of the control circuitry is to read CPU and enclosure temperatures and drive fans. In addition, the controls board also drives the LCD backlight according to analog or USB commands. Plus one or two other useful functions (and still we have spare pins).

Circuit Description
The latest schematic can be downloaded here.

The Controls Board is the heart of the project and contains the following functionality:

a) Remote CPU temperature sensor on J11.
b) CPU fan PWM output. (For PWM fans.)
c) CPU fan PWM/FET output. (For regular fans.)
d) CPU fan pulse input.
e) Remote enclosure temperature sensor on J13.
f)   Enclosure fan PWM output. (For PWM fans.)
g) Enclosure fan PWM/FET output. (For regular fans.)
h) Enclosure fan pulse input.
i)   TTL PWM drive for LED backlight on J7.
j)   USB interface on J14.
k) TTL serial debug output (J3 for displaying on terminal).
l)   Small character LCD display (J2).
m) Regulators (5V and 3V3).
n) Programming header (J9).
o) Support iMac LCD backlight header (speaker, fan, brightness on J7).

*Sheet 1
The brain of the controls is a Microchip 18F27J53, chosen because it has a USB cell and three PWM outputs. The PIC can be reprogrammed over connector J9. Be careful as this is a 3.3 V micro and can be blown up by some programmers.

(Click thumbnail)

iMac Video Controls PCBThe controls board can be loaded with an optional character LCD display (J2). R5 is the contrast pot. Alternatively, the fixed values of R1 and R2 will work in most cases.

FET Q4 drives the LED backlight of the character LCD. This has primarily been done for testing purposes as the same signal is used to drive the iMac LCD backlight.

Connector J15 is for analog or PWM brightness input. If there is no signal on here the brain will read USB commands.

*Sheet 2
Connectors J11 and J13 read the remote temperature sensors and drive the fans. If you are using PWM fans leave the Q2 and Q3 FETs off and fit the zerohms R11 and R12.
Two regulators U1 and U5 take the system 12V down to 3.3 V and 5V.

*Sheet 3
Video interconnect, see circuit description here.

*Sheet 4
Connector J7 drives the existing iMac CFL backlight and speaker, as well as the enclosure fan.
Assembly of the temperature sensors connectors (J16, 19, 20, 21) is detailed here.
Connector J3 provides TTL level serial data for debugging.
Connector J14 is a mini USB for connection to a PC.

Construction
Description here as the prototype is built.

(stay tuned)

Documentation
BOM (Bill of Materials) is available as a Google Docs Spreadsheet here.