Connectors and external circuitry
The following two figures present all details of connecting power supply, radio, keyboard, squeeze paddle and D/A converter.
Paddle keyer
Connect the DIT/DAH paddle switches from keyer pins to ground level as shown in the figure above. Do not apply external voltage to these pins because no precautions were taken to prevent damages of the processor inputs. For more security one might insert a weak resistor into these lines (22 ...47 Ohms).
| DIT paddle |
| DAH paddle |
| JP3-2 (P3.0) |
| JP3-1 (P3.1) |
DFCW / FSKCW pin
When a dash is sent in DFCW mode the "DFCW/FSKCW pin" JP2-3 outputs logical "HIGH". In FSKCW-Mode this pin is set to "High" during dots and dashes but cleared in pauses. To convert the digital information into a frequency shift an external circuit had to be connected to the keyer. As shown in the picture a simple voltage divider, which controls the varicap of a crystal oscillator solves the problem. R2/R3 is in parallel with R4 when the DFCW/FSKCW pin outputs "Low" level. In this case the combined resistance of R2,R3,R4 is a minimum and so is the voltage at the varicap. When the output is switching to "High" the influence of R2/R3 is decreasing, the rising voltage on R4 lowers the capacity of the varicap and the frequency jumps up. You could use here as well a transistor stage which switches a capacitor in parallel to the frequency determining circuit.
D/A converter
To use SLOW HELL and the graphical modes pins 2exp0 to 2exp4 of the PCB headers JP1, JP2 shown in the picture below have been slated for wiring an external D/A converter.
These five pins are the outputs of a binary 5-bit counter, which can be controlled by the ^ - commands. In dependence on the counter state the output of the attached DAC will step up or down in equal voltage steps finally converted by the varicap into frequency steps. The resistor values given in the schematic diagram are in fact serial combinations of various single resistors (with 20% tolerance). For a triangle voltage of 32 steps with randomly chosen resistors the following result was achieved :
There is still a slight unlinearity at step 8, 16 and 24 as you see it in the picture which might be corrected by adjusting the values of R1 - R5.
The R-2R network as shown below maybe is a better solution. A 4-bit DAC is sufficient for HELL and most of the graphical modes and also easy extendable to the possible maximum of 5 bits when required. The right part of the picture shows a proven 10,140 MHz XTAL oscillator circuit which is quite stable. To avoid frequency drift caused by air circulation all osc. components should be at least encased but an additional heating is recommended.
Output control
The digital keyer outputs are also applicable for switching between different power levels, antennas or frequencies. How to influence outputs was already said in chapter "Output Control Commands". In addition to that an example circuit for transistor stages attached to the CWKtiny outputs. When in LOW state each output can sink approximately 10 mA. Detailed informations in the description of the AT89S52 processor on the ATMEL website.
