6 Zone Alarm System

Circuit : Andy Collinson
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This alarm system has 6 independent zones, 1 timed entry/exit zone, a 7 segment LED display and a test or walkthrough facility. Suitable for a small office or home environment, it can also be adapted to use a combination lock or keypad to set and reset the alarm.

6 zone alarm Please Note: All orange +V terminals connect to the point marked +5V at the side of the set switch, S1. IC's U1,U2,U3,U4,U5 and U7 are drawn without power connections for clarity. The power connections need to go +5V and ground. See Practical Section for IC pinouts and relay contacts.

Circuit Notes:
All zones Z1 to Z6 use normally closed alarm contacts. Zone 1 is a timed zone which must be used as the entry and exit point of the building. Zones 2 to 6 are immediate zones, which will trigger the alarm with no delay. Some RF immunity is provided for long wiring runs by the input capacitors, C1 - C6. The key switch, S1 acts as the Set and Reset/Unset switch. For best security this should be the metal type switch with a key.

All IC's except IC6 are CMOS types with buffered outputs, these are denoted by the suffix "B". Unbuffered CMOS IC's have a suffix starting "U" and will not work in this circuit. IC6 is a 5 Volt regulator providing power to the main CMOS IC's, the alarm power supply can be any suitable 12 to 15V power supply.

In operation the DPDT switch S2 is set to the "run" position. When keyswitch S1 is turned reset, this is the unset (off) state of the alarm. In this condition capacitor C8 will discharge via D9, R1 and Z1 and capacitor C7 will have discharged via D8, R17 and S1. Relay RLY1 will not be energized and all CMOS IC's and the display will have no power.

When S1 is turned to set all CMOS IC's receive 5 Volt power. C11 will briefly charge and apply a low input signal to one half of U7A a CMOS4001B, a dual input OR gate. The output of U5A will also be low (make sure all windows and doors zones 2 to 5 are shut) and the output of U7A is high. The output of U7A is then inverted by U7B and fed back via R18 to U7A's input keeping the circuit latched. The output of U7B is low and so Q1 and the relay RLY1 is off and no alarm will sound.

Also when S1 is set, C8 slowly charges via R13. C8 and R13 form the exit timer and allow time to vacate the building. The delay is approximately 1.1 x the value of C8 (in uF) or about 52 seconds with values shown. During the exit delay zone switch Z1 can be opened and closed without triggering the alarm. After the exit time ends, C8 will be charged and one half of the 2 input AND gate, U4A will now be high. Any opening of zones 2 through 6 will cause the alarm to trigger and relay RLY1 will energize. If an intruder attempts a break-in via zone 4 for example, the output of U1D will change state from low to high. When this happens, the high signal is forwarded by U2C a triple input OR gate CMOS4075 and is sent to input C on the CMOS4511 BCD to Decimal display driver. The binary code for four is 100 and input C is high, A abd B are low, and the LED display will illuminate digit 4. The high output from U2C is also forwarded to U5A, again a triple input OR gate. The output of U5A is now sent via S2 to the input of U7A. U7A and U7B form a bistable latch, the change in state causing the output of U7A to change to low, the output of U7B to become high and fed back via R18 to the input of U7A again. The circuit is now latched in the high state. The high output of U7B does two things. First it switches on Q1 and relay RLY1 sounding the alarm. Secondly the high output at U7B is applied to the blanking input of the CMOS4511B via S2 and also to the enable latching pin. The display will now continually show the number of the triggered zone, even if the zone switch is opened or closed again. It is a similar process for any of the other immediate zones.

As this alarm uses 6 zones, the CMOS 4511 BCD to decimal decoder must count to 6 which is 110 in binary. Therefore only inputs A,B and C are required, D is simply tied to ground. The pinout for the CMOS 4511 is shown below.
CMOS4511 pinout
CMOS 4511 Pinout

If the building is entered via Zone 1 then the entry timer starts. The output of U1A (in the set condition ) is low. Entry via Z1 triggers U1A to become momentarily high as the door is opened. U4A then produces a high output, as does U4B. The high signal is now passed via D7 to the input of U1A latching it in the high state. C7 then charges via R15. This is the entry delay and is approximately 1.1 x 0.47 x C7 or about 24 seconds with values shown. Once charged U4C will become high, trigger the alarm and cause the LED display to be latched, as per the preceding paragraph.

Switch S2 is normally used in the run position. However in the test position, this allows a useful "walkthrough" test of the alarm. In the test position the input of U7A is always low and will not trigger the alarm, also the blanking input is also high, meaning that the 7 segment display is always illuminated. With all zones closed, open any zone, the corresponding number will be shown on the display. Note that if two zones are opened the diplay will not necessarily indicate the correct zone, this is not a fault, just the way the circuit is designed. When in run mode, the first zone to trigger the alarm is "caught" and latched and will be displayed until the alarm is reset.

This alarm has a 7 segment LED display to indicate which zone triggered the alarm.
Each zone Z1 to 6 has its own indicator.
Switch S1 is a single pole, double throw switch. One position is set, the other is reset/unset.
Switch S2 enables a "walkthrough" test to test all zones and the display.
Zone 1 has independent exit and entry times.

Turn S2 to run. The alarm is now set using S1, exit delay is determined by C8 and R13. Any entry via zones Z2 to Z6 will now immediately trigger the alarm. If alarm is triggered, reset using S1. Entering via Z1 starts the entry delay. Switch S1 to reset before the entry timer expires. If S2 is in test mode, then a walkthrough test of zones can be made.

R13 and R15 may be changed for preset resistors to create variable entry and exit delays. Using a 2M2 preset will allow larger entry and exit delays when set to maximum resistance. To use with a keypad e.g. Ron J's 5 digit enhanced keypad use a DPDT relay in the keypad. The unused DPDT relay contact will replace the switch S1 in this alarm circuit, see diagram below.

addon expansion
Parts List:
Designation Value Quantity
BZ1 12V Buzzer 1
C1, C2, C3, C4 100n ceramic 4
C5, C6, C7, C9 100n ceramic 4
C7, C8 47u electrolytic 2
C10 100u electrolytic 1
LED1 to LED6 5mm Red LED 6
D7, D8, D9 1N4148 diode 3
D7, D8, D9 1N4148 diode 3
Q1 2N3904 transistor 1
R1,R4,R6,R7,R10,R11 100k 1/4 watt resistor 6
R2,R3,R5,R8,R9,R12 270R 1/4 watt resistor 6
R13 1M 1/4 watt resistor 1
R14 4k7 1/4 watt resistor 1
R15,R18 470k 1/4 watt resistor 1
R16 100R 1/4 watt resistor 1
R17 1k 1/4 watt resistor 1
R19 10k 1/4 watt resistor 1
RLY1 12V Relay SPDT Coil 500R 1
Z1,Z2,Z3,Z4,Z5,Z6 NC Contact switches 6
S1 SPDT Switch 1
S2 DPDT Switch 1
U1 CMOS 4050B 1
U2, U5 CMOS 4075B 1
U3 CMOS 4511B 1
U4 CMOS 4081B 1
U6 7805 5V Regulator 1
U7 CMOS 4001B 1

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