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EE Design > Wireless Home Alarm System
Features Exit/Entry delay selection pre-setable by user. ARM/DISARM alarm system using keyfob unit. Trigger 'Panic' mode using keyfob. Comply to Europe standards. Timed and instant trigger (anti-tamper) mode (Wired + Wireless). Siren output driver. Wireless Remote Siren/Strobe Unit. LED and Buzzer status indicator at Main Console unit. Unlimited key fobs, wireless sensors, RF keypads, remote wireless siren units can be added. Backup-battery and auto-charging circuit. Optional phone dialer unit. Design Objectives To design a high security, low cost and easy to install home alarm system. The system must be simple to install at existing home without much need of breaking walls to run and hide wirings. The batteries powering the devices must be able last long enough so that they need not to be changed at least a few month once. Design Concept & Illustration  The whole system consist of four (4) building blocks. They are the:- 1) Wireless Sensor Transmitter Unit 2) Key Fob Unit 3) Wireless Remote Siren Unit 4) Main Console The alarm system includes a sensor interface connected to a wireless sensor transmitter unit, to link up communication with the main console unit. Example of sensors used in security systems are Passive Infra-Red or PIR sensor, shock (glass breaking) sensor, ultrasonic sensor & door sensor (reed switch). There are two (2) inputs to the wireless sensor transmitter unit which is user configurable. They are the "Timed input" and "Instant-trigger" input. Timed input allows some timing for the user to disarm the alarm before the siren sounds, if it is triggered. In Instant-trigger input triggers the siren immediately after triggering. They are also usually used for anti-tamper purposes. In all sensors, a typical microswitch is fitted on the casing action as the anti-tamper detector. The siren will sound instantly if a thief tries to open the sensor casing which has this anti-tamper switch inside connected to the instant trigger input of the wireless sensor transmitter unit. The second building block of the alarm system is the keyfob unit. This is a handy, key-chain size transmitter, used to arm and disarm the alarm unit. When the "ARM" key is being pushed, the alarm will go into "EXIT/ENTRY" mode. This mode gives user enough time to go into or come out from the protected zone before the alarm is activated or armed. The "DISARM" button will disable this alarm, and thus when any of the sensors are activated, the siren will not trigger on. Besides the arm and disarm functions, it also includes a "PANIC" feature. The "PANIC" function will trigger the siren instantly when it is being pushed. The "PANIC" key share the same button as the "DISARM" key. The "DISARM" key will only function when the alarm is engaged or armed. Another push on the DISARM/PANIC button will disable this panic mode and the whole alarm system will be disarmed. The third building block of the alarm system is the wireless remote siren unit. This device waits for signals from the main console unit, then determine whether to trigger it's siren output on or off. When the correct data has been received from the main console unit, this remote sensor unit will turn on or off its siren output. The siren output will then drive the 110dB sounder and strobe lights. Additional signaling devices can also connect to this unit like the telephone dialer or additonal remote siren units. An auto-rechargeable SLA (Seal-Lead-Acid) backup battery unit is built-in into the unit, just incase the mains power to this unit is being interrupted for some reason. The last, but most interesting part of the system is the main console unit. This device waits for sensor and key fob signals and decides whether to trigger the wireless remote siren unit or not. The status of the alarm ARM/DISARM, triggered inputs, siren activation, EXIT/ENTRY delay, all are indicated on this unit. A battery backup unit (NiCd) with an auto-charging circuits is included to maintain the power to this unit when the mains power is disconnected. It will also indicate user if the back-up battery supply is low. An optional phone dialer unit can be added into this unit. The main cosole unit must then be connected to a phone line. When the alarm triggers, the siren output will enable the alarm to dial a series of preset numbers to acknowledge the status of the alarm system.  View of all the working wire-wrap PCBs Circuit Description Below shows the schematic diagram of the wireless switch transmitter unit. A PP3 9V battery will power the entire unit. U2 is used to regulate the battery voltage to 5V DC supply for the MCU. It only consume as little as 8 to 15uA quiescent current making it ideal for this low power application. The MCU ideally "Sleep" and consumes not more than 20uA and wait for in interrupt wakeup if the Instant-triggered input or the Timed input are activated. If any of the inputs are activated, the MCU will wake-up and then start sending a packet of data containing the switch status to the main console unit. D1 will also turn on to indicate data transmission. Upon completion, the MCU will prepare to go back to "Sleep" mode, to conserve power.  Schematic Diagram of the Switch Transmitter unit If you observe at the switch input, there are R and C configuration (R4-C2, R5-C3). When any of the input is shorted to ground, the RC circuit (time constant) will give a pulse to trigger the MCU's input. This prevents re-firing or repeat activation of the switch, upon activation. The switch must be disconnect from the 0V, waits for the Cap to charge before the next firing can be produced. CN3 provides 9V supply to external sensor devices like the PIR sensor which requires power to function. More switch transmitter unit can be added easily, as the MCU recognised a switch transmitter type with a unique "Device Type". The transmitter also sends it's "Device ID", making the system to be able to determine which sensor unit is transmitting. Due to that,up to 255 of the same transmitter can be installed and monitored (with different "Device ID"). The key fob unit basically works the same way as the switch transmitter unit. It has it's unique "Device Type", which is different from that switch transmitter unit. It is powered by a CR2032, coin cell, 3V 150mAh lithium battery. At ideal, it consumes less than 10uA. The keyfob consist of an "ARM" and "DISARM/PANIC" button. When user pushes any of the buttons, the MCU will wake up and send a packet of data. D1 will indicate a transmission.  Schematic Diagram of the Key Fob unit The Remote Siren unit basically waits for an RF signal, decodes and check if the packet is transmitted from the main console unit. The main console unit will send a packet of data repetitively (2 times in-between delays), to either turn on or turn off the siren output. This is increases chances of triggering the siren, if say, there are distrubtion in the signal lines, which prevents the first transmission of data fail to be accepted by the remote siren unit. D1 is used to indicate if there are any sensor transmitter unit or keyfob unit transmitting on same RF band. D2 is used to indicate that a valid packet has been sucessfully received! The whole unit is DC powed by an external DC adapter. The unit is battery backup by a 12V >1.5mAH lead acid battery, with a constant battery charging circuit.  Schematic Diagram of the Remote Siren unit The main console unit, wait for an RF signal, decodes them and see if the packet contains a valid "Device code". If the "Device Code" is valid, it then checks for the "Device Type" to see what device transmitted the signal. It then response to that decoded data in the packet. The status are indicated using 4 LEDs (D0-D4). The buzzer will be activated if the timed input or Exit/Entry input are triggered. This tells the authorised user to disarm the alarm using their keyfob unit, and if fail to do so, after the it has reached its timeout (10 sec), the MCU will transmit an RF signal to turn on the remote siren unit.  Schematic Diagram of the Main Console unit According to europe standards, the siren needs to be turned off automatically after 30 seconds. This features also includes in the main console unit's firmware. An ICSP connected is included into the design for future on-site program updating. The whole unit is DC powed by an external DC adapter. The unit is battery backup by a Ni-Cd 180mAH 9V PP3 battery, with a constant battery charging circuit.  View of all the working wire-wrap PCBs  Angle view of all the working wire-wrap PCBs  The switch transmitter (right) and the keyfob unit  The main console (top) and the key fob unit  The switch transmitter (top) and remote siren unit Upgrades & Comments The overall system is effective, but for longer distance, a longer antenna is required. The RF transmitter modules may be powered by a 12 volts supply for higher emitted power and therefore a longer transmission range. However, for the microcontroller, a regulator is required to regulate the 12V battery to 5 volts supply for the microcontroller. An ultra low-quiescent current LDO regulator such as the LM2936 from National Semiconductor should be used to conserve battery power. The main console unit should add a keypad for user to arm or disarm the alarm, besides using the key fob unit. The sensor transmitter unit which carries a 9V PP3 battery would not last as long as expected if it is required to power the PIR sensor modules as well. Each PIR sensor module consumes about 15mA at quiescent, therefore it is advisible to have an external higher capacity power supply to power up these PIR sensor modules. Instead of PIR sensor modules, you could use a simpler magnet contact sensor, which its circuitry may draw only about 1mA quiescent current which is more appropriate to be powed by the sensor transmitter unit's 9V battery. Related Design Files & Materials PIC12F629 Datasheet PIC16F627A Datasheet |