The project uses a PIC 16C84 to monitor 4 NC alarm inputs that when triggered, activates an output relay. The first zone triggered is illuminated with a solid lit LED, while the second and subsequent zones are indicated with a flashing LED.
System Details
When the alarm system is disarmed, no LED’s are activated, so there is only the IC devices activated,
and so the current reading for the entire program is only 10.0 – 20.0mA. There is some slight variation
because the PIC controller’s current consumption is not constant and fluctuates with the program section
being executed.
If the user then presses the Arm/Disarm button, the armed LED (Green) lights to indicate the change in
state. The current consumption at this point increases to 25-30mA, which is predictable as the only
increase in load is that of the LED and protective resistor. As before, there are current fluctuations
because of the nature of the PIC.
In this armed state, if any one of the Zone Simulator switches is pressed, then the PIC triggers the Alarm
output pin. This is connected to the second status LED, the Alarm LED, and also to the base of the
transistor. This fully saturates the transistor and allows enough current to flow to activate the relay,
which switches the heavy load to the external strobe light and siren. Within another millisecond or so
the LED corresponding to the Zone Switch triggered is illuminated as a steady, continuously on, supply.
The current compunction at the moment is found by disconnecting the siren and strobe so that only the
project’s readings are shown. In the state of only one device triggered, the current has risen to 130-
140mA, which when you remember the current though the relay coil alone is 100-120mA, is a sensible
figure.
If any additional zone switches are depressed while the PIC is in this alarm state, there is no change in
the Alarm output, but again the corresponding LED is illuminated. However this time, the trigger is
known to be less important, and is indicated with a flashing LED output, as are any further triggers,
(until all four zones are activated). In this way it is clear for the user to see which zone/sensor was
activated first by the steady pulse.
As the LED’s are continually changing there on off status, the current readings change a lot, but peak at
about 210mA. When connected to the strobe and siren, each which draw an additional 200-300mA, the
total current is about 700mA
Assuming a current of 700mA, and a voltage of 12V, the total amount of power used by the project and
the external alarm components can be calculated.
Power =Voltage×Current
=12V×0.7A
=8.4W