The G8TMV CTCSS encoder board Rev 5
The Rev 5 board is 31mm by 28mm
and 5mm high, it is a minor update to the Rev 4 board with
the addition of a 'Down' step button.
The board comprises a voltage regulator, an MSP430 micro controller to generate the tone output
and control the functionality of the system, an active filter to turn the voltage
ladder from the 3-bit DAC into a sine wave and a trim pot to allow setting of the output level,
plus a few ancillary parts to make it all work.
There is provision for 'Up' and 'Down' buttons, an led and a PTT input.
The encoder is designed to run from 6 - 15v and draws approx 9mA. It makes sense to have
the supply line switched so that tones are not generated during simplex operation.
The tone output is only active when the PTT input is grounded.
The tone encoder will generate any of the standard CTCSS tones used
by the U.K. repeater system and some of the ones used in Europe.
Other tones are possible, please contact me for details.
Each time the 'Up' push button is pressed the encoder moves to the next tone
in the sequence. The 'Down' button steps in the opposite direction. Each time a new
tone is selected, and at power up, the LED annunciates
the code letter for the tone in morse code.
A tone can be made the default by pressing and holding the 'Up' push button until the LED
flashes once. This stores the currently selected value in the micro controller
non-volatile memory so that it will be used the next time the encoder is powered up.
The tones and their code letters are:
BT || None |
A || 67.0Hz |
B || 71.9Hz |
C || 77.0Hz |
D || 82.5Hz |
E || 88.5Hz |
F || 94.8Hz |
G || 103.5Hz |
H || 110.9Hz |
J || 118.8Hz |
K || 79.7Hz |
L || 123.0Hz |
Tones A through J are the standard UK repeater system tones. The 'None' entry in the table effectively
turns off the output.
The CTCSS board can not drive a Piezo sounder for audible Morse code output directly due to
microprocessor drive current limitations, however if audio output is prefered then the simple
circuit shown overcomes this.
The transistor can be any NPN type that is rated for the current required by the sounder.
The value of 'R' determines the volume and will depend on the sounder type and the supply voltage,
for a 5V sounder and a 12v supply 560 ohms is a good starting point.