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The following information is provided 'as-is' with absolutely no warranty, expressed or implied. I (David Stafford) shall not be held responsible for ANY consequences of the use or misuse of this information. In particular, I am not responsible for errors, or damage resulting from errors in this information, even if I am aware of such errors. In other words, I if you build this and if fries your computer, I'm not responsible. Period. By using this information you agree to this. If you cannot agree to this, you may not use the information in this document and must leave now.

The at the core of the board is a MC4040 12 bit binary counter, a 74LS253 demultiplexor, a 74LS564 register and a timer. These are wired to count the number of pulses from a external source in a given time quantum (set by the timer) and present the latest datum on the parallel bus. Each time the timer "ticks", the current output of the counter is loaded into the register and the counter is cleared. Therefore, the register always presents the latest valid data to the parallel port.

The temperature is sensed by connecting a thermistor to a 555 timer and using its output as input to the counter. If the correct thermistor is used, the output to the computer will be directly proportional to the absolute temperature.

The fan speed is monitored by placing an IR LED/phototransistor pair across the blades of the fan. The IR beam will then be periodically broken by the movement of the fan. The output of the phototransistor circuit can be used as input to the counter, forming a tachometer.

Two of the control lines from the printer #AF, and #SEL are used to select one pair of inputs from the demultiplexor. #STROBE is used to reset the timers, reset the counters and disable output. #INIT disables the outputs but allows the circuit to continue running. The truth table for #STROBE and #INIT is:

(RESET) #STROBE	(#OEN) #INIT	ACTION
LOW	LOW	Run out output data
LOW	HIGH	Run but keep outputs at hi-Z
HIGH	LOW	reset and hold outputs at hi-Z
HIGH	HIGH	reset and hold outputs at hi-Z

PINOUTS:

My name	Parallel port name	Pin number
DATA0	DATA0	2
DATA1	DATA1	3
DATA2	DATA2	4
DATA3	DATA3	5
DATA4	DATA4	6
DATA5	DATA5	7
DATA6	DATA6	8
DATA7	DATA7	9
Sel0	#AF	14
Sel1	#SEL	17
#OEN	#INIT	16
RESET	#STROBE	1

The 74LS564 inverts the outputs so software needs to invert them once more.

To read one of the 4 input channels, execute the following procedure:

1. Put the printer port in INPUT mode
2. Assert #OEN and RESET and the desired input channel number on the control pins.
3. wait a few ms for the card to respond
4. De-assert RESET (don't change the input channel or #OEN)
5. Wait a LONG TIME (500 or so ms)
6. Read and invert the bits from the 74LS564 register.

char get_register(int reg) {
   int output;
   unsigned char data;
   switch(reg):
      case(0): output=8;break;
      case(1): output=10;break;
      case(2): output=0;break;
     case(3): output=2;break;
   }
   outb(32|output,0x27A);
  usleep(100000);
   outb(32|1|output,0x27A);
   usleep(600000);
   data=inb(0x278);
   return(~data);
}

A PC with a free bi-directional parallel port.

Some basic electronics tools and equipment.

Linux 2.0 or higher if you plan to use my code.

The parts for the board itself, specifically:
(1): 74LS564 (octal register w/ 3-state outputs)
(1): mc4040 (12 bit binary counter)
(1): 74HC14 (hex inverter w/ Schmitt triggers)
(1): 7404 (hex inverter)
(1): 74LS253 (dual 1-of-4 demultiplexor)
(2): 555 (timer)
(2): signal diodes.
(1): 7805 (power converter)
(1): IR LED/Phototransistor pair
(1): 10K thermistor (or similar temperature sensor)
Resistors: (1): 100ohm, (1): 220 ohm, (4): 470 ohm, (10): 1,000 ohm, (2): 3,300 ohm, (1): 10K ohm, (1): 1M ohm
Capacitors: (2): 100uF, (6): 0.1uF, (1): 0.2uF, (1): 0.5uF
Wire, connectors etc..

Here, you can find the schematic for a circuit.

Notes:

I recommend building the circuit on wire-wrap perfboard. Disassemble an unneeded printer cable and connect the wires to your circuit. Mount the whole thing in an unused 5 1/4" bay with the LEDs sticking out through holes in the cover.

Heavily decouple the circuit. Preferably a .1uF capacitor across every chip with big capacitors at the input and output of the power converter. The 74LS64 chip and the timers must be heavily decoupled because of their capacitive switching.

The temperature sensor should be as close as is practical to the 555.

D3 and Q1 form the tachometer. They should be aligned so that the fan blades periodically break the IR beam as the fan rotates.

If you add more devices which still use the main timer (on the right) connect pin two from the 74HC14 to the corresponding input pins on the 74LS253. If all the inputs use the same master timer, then the inverted timer output can connect directly to the counter and register.

You may need to add an RC delay of a few uS if your MC4040 chip resets faster than the 74LS564 can load the data.

This isn't exactly supposed to be good EE design. I designed it to work with the parts I happened to have on hand.

The circuit takes about 65mA of current without the LEDs