DTMF Decoder Relay Driver

Posted on Feb 7, 2014

Following a power interruption, all relays assume their previous state before the power interruption. (relay states and PIN are stored in non-volatile memory) An Motorola MC145436 DTMF decoder is used, this IC is clocked with a common 3. 58Mhz NTSC colour burst crystal and supplies a divide by 8 clock output (about 450 Khz) which is used as the clo

DTMF Decoder Relay Driver
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ck for the Atmel AT90S2313 microcontroller. As this MCU does not have a "brown out detector" feature, a MC34064 is used as a reset generator to prevent possible EEPROM corruption at abnormally low supply voltages. Audio from the radio RX is fed to pin 7 of the DTMF decoder. When the DTMF decoder detects a valid DTMF tone, it outputs a nibble representing the tone on pins 1, 2, 13 & 14 and it drives pin 12, it`s Data Valid output, high, which drives pin 11 on the mcu high. This wakes the mcu up from sleep mode, and the mcu samples the nibble on it`s input pins 2, 3, 6 & 7. Depending on the sequence of DTMF tone data supplied to the mcu, the mcu uses it`s pins 12 - 19 as outputs to control the relays, these outputs are fed to a ULN2803 open collector driver IC, which drives the output lines. All DTMF commands sequences start with a [*] and end with a [#]. Decoded DTMF tones are stored in a tone buffer, the command in this buffer is executed when the DTMF tone for the [#] key is received. Reception of the DTMF tone for the [*] key will clear the buffer without executing it. When a relay is pulsed, it will change state (XOR operation), wait about one second, then change back to it`s original state and if more than one pulse is being generated, there will be an approximately one second delay between each pulse. During pulse output, the controller is deaf to incoming DTMF tones and you must wait for the end of the pulse sequence...

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