Instruction manual
DCI1 • 6
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