ELECTRONICS - [alarm system] - [page 3/8]

Home alarm system - Schematic Design

The complete diagram

You can view the complete diagram. It contains all parts that are described on this page.

Heart of the System: PIC16F84

The core of the alarm system is the PIC 16F84 microcontroller. This chip is set to run at a frequency of 8.86 MHz. It has a reset switch,which can be used to reset the controller in case of a failure. It operates at a voltage of 5 V. The Ra a & Rb ports are used for all I/O (sensors, siren, code lock, code lock LED, code lock buzzer, zone1 & zone2 & zone3 activation).

The description of the software inside the PIC is placed on a further page.

Alarm system circuit: the PIC16F84 microcontroller

Stable Power Supply

The alarm system requires a stable power supply of 5VDC. This is done by a 7805 regulator which is mounted on a heat sink. The capacitors and diodes around it are making sure the 7805 is working correctly.

Alarm system circuit: the power supply

Rectifier and backup circuit

The transformer gives an AC output signal of 12V AC (Alternating Current). This signal must be converted to a DC (Direct current) signal: This is done using a bridge rectifier, consisting of the diodes D5 to D8. The AC input is applied to pin 5 & 6 of connector J1.

The backup battery is applied to pin 7 & 8 of connector J1: If AC power fails the diode D4 will start conducting. Doing so the backup battery can power the system.

Alarm system circuit: the rectifier

Siren Activation

The siren is switched by the relay S2. For this the microcontroller has to set a logical "1" to the input of resistor R17 to make the transistor conducting. Then the relay will close and the Kojak is activated.

Alarm system circuit: the sirene activation

Code lock Interface

All wires for the code lock are connected to J7:

Pen #1 & #2 are used for the +12V power supply, the voltage that is required by the Velleman K2607 (stabilization is not required).
Pen nr #3 & #4 are used to control the buzzer which is operated through a 5 VDC voltage. The buzzer can be activated by applying a logical "1" (5 V) to the input of the resistor R16.
Pen nr #5 & #6 are two inputs that must be connected to confirm the correct code was entered on the keyboard: those inputs are connected together when the relay in the Velleman keyboard gives a pulse. R14 is a pull-down resistor to make sure the inputs of the PIC are never floating.
Pen nr #7 & #8 are used to control the LED on the keyboard. The needed current for the LED is directly coming from the PIC.

Alarm system circuit: the code lock

Sensor/Zone Activation Input

Connectors J2,J3,J4 are used for the sensor inputs. When a complete zone is closed correctly pin #1 & #2 of these connectors are connected: sensors uses a NC contact. If someone cuts a wire, the alarm goes off.

Each input has a capacitor to prevent noice. The information is passed to the PIC through an optocoupler. This was done to protect the microcontroller from sabotage through the sensors. A second reason I used optocouplers was to overcome the voltage levels: the sensors are powered with 12 VDC while the PIC operates at 5 VDC.

The connector J5 is used to connect the zone activation switches. To activate a zone you must connect +12 VDC to its corresponding connection..

Alarm system circuit: the zone inputs