This IC contains the developed code required to take the DTMF tone data from U3 and process while reading and writing to FLASH memory, generating the feedback tones, monitoring the programming switch, and turning the relays on and off as needed. Quite a bit for one little part! Since the four relay driver circuits are identical, we’ll walk through the operation of one section and leave it to you to trace out the other three. Take a look at the K1 relay driver circuit on the schematic. It consist of these 5 components: R1, R14, D5, Q1 and K1.
When the relay driver output from the micro-controller (RELAY_1) switches ‘High’, Q1 is forward biased pulling its normally high impedance Collector down to ground potential. This completes the circuit path for relay K1 to turn on causing its internal contacts to close providing you with a ‘switch closure’ for your custom applications. This all seems fairly simple, but what is the function of R1, R14, and D5? Resistor R1 acts as a current limiter so that the Base-Emitter junction of the transistor does not pull the ‘High’ output (+5 VDC) of the micro-controller down to 0.7 VDC. In this way, the loading on the micro-controller is well within the needed parameters for proper operation.
Resistor R14 is also used as a current limiting resistor just like R1. Since the relays used in the DCI1 are all 9 VDC models and our power supply puts out upwards of 12 VDC, R14 allows us to limit the current to the needed 50 mA or so needed to fully activate the relay and set its terminal voltage to the desired 9 VDC. Now why is Diode D5 placed across the input terminals of the relay? It looks like it is always reverse biased (the Cathode is more positive than the Anode) so it can never turn on! If you look on a schematic for any quality product that uses a relay, you should see a diode set in the same configuration (we aren’t just trying to dump off a few extra parts here!).
Relays such as K1 are very inductive due to there construction. Since inductors have the property V = L di/ dt, they resist changes in current over time (di/dt). As the magnetic field begins to collapse when Q1 turns off, a huge ’kickback voltage’ is generated with reverse polarity on the terminals of the inductor (upwards of a 1000 V is easily possible!). This generated ’kickback voltage’ is more then enough to destroy Q1 and other parts on the board! To avoid this, D5 is inserted to provide a safe discharge path when the reverse ’kickback voltage’ starts to rise above 0.7 VDC. Neat huh!
When the micro-controller receives valid data from the user, it flashes LED D4 (the activity indicator) and sends a feedback tone back over the phone line to let you know the command was received. Components R19, R21, Q6, C5, D10 and D11 work together with the micro-controller to generate the tone pulse. The pulsed tone output from U1:pin 11 is amplified from 0 to 5 VDC up to 0 to 12 VDC to ensure that enough signal will couple through to the phone DCI1 6