5
Installation
Power Connection
:
Many modern radios p5v or +8v or +9v at the
microphone connector – this will power the HF3 directly from the mic socket
with a single cable to suit your radio (e.g. RJ45 to RJ45).
Radios that do not pDC power at the mic socket can still be used -
in these cases 12 volts can be applied separately to the HF3’s “12V DC”
external power connector (note that 12v DC power filtering and suitable
fusing is recommended). Note that the 12V DC connector is NOT protected
against reverse polarity.
IMPORTANT:
The external 12V DC connector is only for radios that DO
NOT provide DC power via the microphone. If you use this external power
connector, then you must not apply power via the microphone cable as well.
How It Works
The HF3 incorporates the following circuit building-blocks:
Voltage regulator:
The electronics inside the HF3 runs at 3.3 volts and a
LP2950 low-dropout regulator provides 3.3v internally and accommodates a
wide input supply-voltage range of up to 30 Volts DC. Normally, the
microphone socket on most radios will provide from 5 to 10 volts.
Mic preamp and audio equalisation:
An MCP602 dual low-noise
Operational Amplifier provides an audio mic gain adjustment, a low-
frequency boost/cut and high-frequency boost/cut using a Baxandall bass
and treble tone control circuit.
Microcontroller:
The low-power microcontroller provides all the timing and
switching functions associated with:
•
Detecting a momentary push on the TX switch to start transmission
•
Illumination of the LED indicator
•
Activation of the radio’s transmit line (PTT)
•
Operation of the built-in piezo buzzer to warn impending timeout
