
With the permission of K. Lind (LA9ZO) I will now describe an
interface for LPT which he has designed.

Printed cicuit board layout and component locations are given
in reference (1).

This interface has some advantages
       - it has been tested on the air in HELL QSO's
       - it is inexpensive, most components found in junk box
       - it is small, it can be mounted on a small circuit
         board with a 25 pin connector to be plugged directly
         into the parallel port of your PC
       - no batteries, power taken from PC (Not applicable
         to all portable PC models)

Demodulator

A CD 4011 CMOS quad nand gate chip is used as the main
component. One of the gates is used as an operational
amplifier to amplify the audio signal from the receiver.
The signal is rectified and the resulting DC voltage is
fed to the transistor to key it on or off. In this way, pin
12 of the parallel port is pulled low when a MARK is received,
and left high on SPACE. See figure 1.

Fig. 1.                                                  LPTx
                  150k                                10k
           _______----______       AA119             _----_ 9
          |       ----      |      _|/|__Gnd        | ----
          |      ________   |     | |\|             |______12
          |   __|        |  |     |                 |
Rx   10k  |  | 8| 1/4    |  | 27nF| AA119   8.2k    | BC182B
__||_----_|__|__| CD4011 |__|__||_|_|\|_____----__|/ c
  || ----      9|        |10 | ||   |/|  |  ---- b|\ e
  1uF           |________|   |           |          |__18..25
                            ===8.2nF    === 22nF    |
                             |           |          |
____Gnd                     Gnd         Gnd        Gnd



Modulator

Two of the gates in the CD 4011 are used as an audio
oscillator, providing a continous square wave at about 2 kHz.
The fourth gate is used as a switch, keyed on and off from
parallel port pin 2 via the transistor.
When a MARK is present at pin 2, the gate opens and the 2 kHz
square wave is passed to the output circuit via a capacitor.
This capacitor, together with the resistor and the two
germanium diodes removes the DC component of the signal. The
two diodes are necessary in order to clamp the DC keying
transients from the gate switching.
Power is taken from the parallel port using the three diodes
parallelling pins 6, 7 and 8 on the parallel port. See fig. 5.
IF your portable PC is unable to power this circuit, use a 3V
battery instead.

Fig. 2.

LPTx

    AA119                      1.8k   560pF
_____|\|__              _______----____||___________
6    |/|  |            |       ----    ||           |
    AA119 |            |  560k    100nF     330k    |
_____|\|__|            |___----____||_______----____|
7    |/|  |            |   ---- |  || |   | ----    |
    AA119 |            |        |     |   |         |
_____|\|__|            |       3|    5|  6|         |
8    |/|  |            |     -----------------      |
          |            |____|                 |_____|
          |            |  1 |      3/4        | 4   |
          |            |____|    CD4011       |_____|
          |               2 |                 | 13
          |_________________|                 |______Gnd
          |          |   14 |                 | 7
          |          |      |                 |
         === 2.2uF  | |     |                 |
          |         | |10k  |_________________|
        Gnd          |      12 |   11 |
                 ____|_________|     | |
                |                    | |2.7k
       3.3k     |                     |
_______----___|/c  BC 182B           ===47nF
2      ----  b|\e                     |     1.5k
                |                     |_____----_____Gnd
                |                     |     ----
               Gnd                    |     AA119
________ Gnd                          |______|\|_____Gnd
18..25                                |      |/|
                                      |     AA119
                                      |______|/|_____Gnd
                                      |      |\|
                                      |     1uF
                                      |______||______Tx
                                             ||



Some remarks about component values.

In the receiving part of the interface the gain of the audio
amplifier can be increased by replacing the 10 k input
resistor with a lower value (3.3k or even lower). As shown,
the circuit is best suited for a high-level 600 ohm output
from the receiver. If you intend to use the headphone output,
the lower value of the resistor is recommended.

In the transmitting part of the interface the only critical
components are the 1.8 k resistor and the 560 pF capacitor
which determine the oscillation frequency. The frequency
should be around 2 kHz, and not lower than 1.6 kHz.
This is because the waveform is quite rough, and staying above
1.6 kHz causes the harmonics of the audio signal to be knocked
down by the crystal filter in the transmitter.

All diodes are small-signal germanium diodes. The actual type
is fairly uncritical, but they should be germanium because of
the low forward voltage drop, especially in the "power-
stealing" part of the circuit. Other diodes, i. e. hot-carrier
diodes, may be a possible substitute, but the forward voltage
drop is critical.

The transistors are high-beta NPN silicon audio transistors.
Beta should be at least 200 or higher. Other suitable types
are BC 109, BC 547, etc.

The quad NAND gate used originally was of the unbuffered type.
However, buffered types may also be used, as the unbuffered
types are more difficult to obtain.

If RFI from your transmitter is a problem, try filtering input
and output leads with ferrite beads and suitable capacitors.



REFERENCES:
===========
(1) Sigfus Jonsson LA0BX: PC-HELL, SARTG NEWS No. 77, December 1990

