NHRC-
Intelligent DTMF Remote Control
Programming and Operations Guide
Software Version 3.00, January 4, 2012
1 Introduction
The NHRC was designed to meet requests for a simple way to remotely control things around your shack
or at your repeater site. Unlike other "dumb" remote controllers, the NHRC can provide Morse code
telemetry in response to all valid commands. It is a very inexpensive but elegant way to implement control of any
device or to help you comply with FCC rules by having a remote control channel for your repeater. The NHRC
can be connected to any audio source, including a radio receiver or transceiver, without the need to
modify the radio for any signals. All connections are similar to connecting a external speaker and microphone to
your transceiver. In many cases, the NHRC- can be wired directly to a “data” or “accessory” jack on your
radio. Consult your transceiver’s operating manual for data or accessory connection instructions.
The NHRC allows you to control up to 8 loads by sending DTMF (“touch-tone”) commands over any
audio circuit. The heart of the NHRC is a Microchip PIC16F648A microcontroller and the Teltone M8870
DTMF decoder IC.
The NHRC will send command confirmation messages in Morse code. The confirmation can be sent over
any audio circuit. This makes the device useful for radio or wireline applications. For radio applications, the NHRC
can send a Morse code ID message a user defined amount of time after first transmitting.
2 Theory of operation
When audio arrives at the DTMF decoder, valid DTMF tones decoded by the M8870 cause the STD data valid
signal to go high, at which point the microprocessor starts "listening" to the data from the DTMF decoder. Since it
is only listening to the audio it does not know when you have stopped transmitting. The internal program starts a
timer (the “inter-digit timer”) that is restarted with each valid DTMF received from the DTMF decoder. All
commands are evaluated and executed after the inter-digit timer expires. (The inter-digit timer is hard coded at 1.5
seconds) If DTMF tones have been detected, and there is a 1.5 second gap, the tones received up to the gap are
evaluated and if a command code is matched, then the command is executed. This allows the NHRC- to
operate without a unsquelched signal from the receiver.
There are 16 unique command codes that control the outputs. Each port has its own “on” and “off” command. In
addition to the 16 output control commands, there are four additional programming commands. All 20 of the
commands can be re-programmed by the user. The user commands are stored in the microcontroller’s internal
EEPROM and are retained indefinitely. Each command can be up to 8 digits in length, and all 16 DTMF digits can
be used (0-9, A, B, C, D, *, and #) allowing over 250,000,000 combinations.
The microcontroller processes the received digits, and if the digits match a command, sends the appropriate logic
signals to the IRF510 power MOSFETs to control the loads. A high on the gate of the MOSFET causes the drain of
the MOSFET to close to ground. (We chose open drain MOSFET outputs instead of relay outputs because relays
can bounce making connection to digital circuitry a problem in some cases.) The NHRC ’s outputs can
drive well in excess of 1 amp allowing control of huge relays if required. Valid commands are acknowledged by
asserting the PTT (push-to-talk) line (pulling it to ground) and sending a Morse code message of “ON”, “OFF”,
“OK, or “ERR”.
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