Another possible way to use the AF output is to rectify it (diode + capacitor) and use that as a plain on/off
keying signal to a CW transmitter. This would be suitable for CW and Hellschreiber modes, but not modes
where a frequency shift encoding is involved.
5.7 LCD backlight options
a) Full brightness:
For basic operation connect A0 to A1 using a wire jumper (made from a
capacitor lead offcut, for example). The blue LED backlight of the LCD module will be at full
brightness. This is shown as the Red line (left). In this case, do not fit any resistor at the R2
position.
b) Reduced brightness:
You may find the LED backlight of the LCD module too bright for
your liking, or if you wish to operate from battery power, perhaps you consider that it
consumes too much current. In that case, fit a wire jumper between A1 and A2, and use a
suitable valued resistor at the R2 position: experiment, e.g. starting at 100-ohms. 180-ohms
has been reported to work well.
c) Switched backlight:
If you do not wish the LCD module's LED backlight to be on all the time, then
instead of the wire jumper, you could connect wires to an on/off switch. This could be useful for battery-
powered operation, to reduce current consumption. For example, you could switch the display on to set up
the kit, then leave it off during operation.
5.8 DDS outputs
The connection points labelled Sqr and Sin provide direct connection to the DDS module's squarewave
and sinewave outputs. The squarewave connection is the output pin of the DDS module's comparator and
is a 5V squarewave. The sinewave connection is the analogue frequency output of the DDS module and
has a 1V p-p amplitude. In both cases, you should refer to the AD9850 datasheet and ensure that you
understand the specification, and do not draw excessive loads from these connections.
5.9 In-circuit programming of AVR
If you own an AVR programmer, you can make connections to the U3 to allow updating the firmware
without removing the AVR chip.
In this case, you should fit a 100K resistor as R3, not a jumper link. This is shown circled in RED on the
right side of the below diagram. The other connections required to your programmer, are MISO, MOSI,
SCK and Res (reset), these are indicated in the RED square on the diagram. The connection layout is
designed to fit a 2 x 5-pin header, to match common AVR programmer cables. Before use, you should
check that the connections match your programmer, because there are several standards in use. This
board is designed to be as flexible as possible. In some programmers the Gnd connection between MOSI
and Res is “NC” (no connection). These are also suitable for connecting to the U3.
Notes:
1) Power needs to be applied to the U3 during programming! When you start programming, whatever
the U3 is doing at that moment will be interrupted.
2) Some constructors have reported that if the relay-switched LPF board is fitted, the relays chatter
because the programming lines are shared with relays 3 and 4, and programming errors. If this
happens with YOUR programmer, you need to either remove the relay board during programming,
or cut the wires to relays 3 and 4 and put a DPST switch in, to disconnect them when required.
11
