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