Wishing to fulfill the demand for a simple, reliable and extremely economical alarm system, we designed a simple main alarm unit that can be built by anyone and that supports all types of sensors. The main alarm unit that we present here supports:
- 4 zones (4 inputs for sensors)
- Activation of a siren, a phone or anything else from a relay
- Adjustable alarm shut- off time after non-sustaining trigger
- Armed-alarm flashing LED indicator
- Power supply terminal output for sensors powering
- Remote control terminal input
There are many alarm systems on the market. Most of them support several zones (inputs) and they accept various types of sensors such as magnetic switches (traps) for doors and windows, motion detectors, vibration detectors, fire or smoke detectors etc. The majority of those systems support time delay (typically of a few seconds) for some zones, so that there is sufficient time for the authorized user to arm or disarm the alarm within the surveillance area by using a key or by typing a password via a keyboard. The time delay is usually programmable on commercial systems, and those systems have a display and a keyboard, for displaying status or various messages and for commands and password typing, respectively. Many alarms also support additional advanced features such as automated calls via telephone or other telecommunication channels to inform the owner or any security authority for any invasion on the surveillance area.
All advanced features increase the cost and the complexity of the system and sometimes they increase the probability of a failure due to wrong system installation, error due programming or handling. The commercial alarm systems tend to become more and more sophisticated and therefore more expensive and more complicated on handling. This trend creates a gap in the market for those looking for a simple alarm system which will be able to activate a siren, will call a telephone number or playback an audio message, and generally, will perform basic functions that can be accomplished by a simple relay. Moreover, a simple alarm system may not have screen nor keyboard and the activation or deactivation may performed wirelessly via a remote control or with a "smart key". A remote control may eliminate the need for time-delay and any programming operations related to that time delay. So, such a system may become very simple and economical.
Wishing to fulfill the demand for a simple, reliable and extremely economical alarm system, we designed a simple main alarm unit that can be built by anyone and that supports all types of sensors.
The electronic schematic of the basic alarm
The circuit is extremely simple and is based on the popular 555 timer (U3) which is wired for monostable operation (see Fig. 1). In its stable state, pin 3, which is the output of a monostable multivibrator, remains inactive (off). It is activated (goes on) only when a sensor that is connected to any zone (input) happens to be activated. Once a sensor is activated, pin 3 of U3 switches to its temporal active state (on) and activates the K1 relay. The relay powers up the P5 terminal, which may be used to activate a siren, a phone or any other device. The relay remains activated as long as there is a sensor that remains activated. After activation, the alarm may be shut-off again, only:
- Automatically, after a certain period of time (adjustable time interval), and only if all sensors are deactivated (due to absence of a threat)
- Upon user's command via a key or a remote control
The time period required to be elapsed for automatic shut-off may be adjusted via the R5 potentiometer. With the recommended component values on figure 1, the range of setting is from about 0 seconds to 2 minutes, and it is directly dependent on the RC time constant (product of [R5 + R6] times C1). The upper limit of the range can be increased or decreased by using a larger or smaller capacitor or a potentiometer with a higher or a lower value, respectively, and the lower limit is directly dependent on the product of C1 * R6.
The monostable multivibrator is triggered by a negative pulse on pin 2 of U3. Thus, on normal state (stable state) the alarm remains deactivated as long as the voltage on pin 2 of the 555 remains at logic 1 (maximum positive supply voltage). Once the voltage on pin 2 is switched to logic 0, the monostable multivibrator is triggered. After triggering, the monostable circuit remains activated for a certain period of time and it cannot be deactivated before this time elapsed, even if the voltage goes high on pin 2 within this time period. In the case of the voltage on pin 2 being locked at logical 0 (a sensor remains activated), the alarm does not turns off automatically.
All sensors contain an internal switch that is closed under normal conditions (normally on) and the output of each sensor (which is connected to each input of the alarm) is essentially that same switch. In photo 1, resistors of 120 ohm are connected into the inputs, for replacing the sensors. That is because any input that is not used (not connected to any sensor) must not in any circumstances be left open. It must always be shorted by a wire jumper or by a low resistance of less than 1K (as in photo 1).
The output of each sensor (which is coupled to each zone) is essentially a normally-on switch. While the switch is closed (on), each input of the OR gate (U1A) remains grounded through that switch (the sensors are connected to the connectors P1 to P4). While all sensors are in their idle state (their switches are closed), all the inputs of the OR gate are grounded and the output of the OR gate remains at logical 0. The NAND gate (U2A), which is used here as a NOT gate, inverts the logic level of the OR output, and drives the pin 2 of the U3 with logical 1. Having logic 1 in its input, U3 remains idle. Everything changes when a sensor is activated and its internal switch goes off. An activated sensor changes the voltage to an input of the OR gate, and the input which is related to the specific sensor goes at logic 1 (due to a pull-up resistor- R1 to R4). Thus the output of the OR and the NAND goes to 1 and 0, respectively, and the alarm is activated.
At this point, many who have no experience on security alarms, may wonder why the sensors have a switch that is normally on (normally closed - conductive) and not vice versa. It seems logical that the normal state of a switch would be the open state (off) and not the closed one (on). The answer is simple but not quite obvious: If sensor switches were normally off, the main unit of the alarm could not discern whether a sensor is switched off or if the cable of the sensor has been maliciously cut from an intruder. That is because in both cases the input would be floated (remained open - non-conductive). But while the switch of a sensor is normally on (as is the standard case) the central unit of the alarm can sense the case of a cable cut, due to the loss of conductivity.
At this point someone may ask: And what about if the intruder short-circuits the sensor cable and then cuts it? Yes, this may happen, but only if the sensor cables are accessible from places which are not supervised by motion sensors and also if the system uses time delay. Since the alarm that we present here uses no delay, the only chance for the intruder to reach sensor cables would be due to wrong installation, which may give access to cables at places that are not monitored by motion sensors. But, if the installation is performed with care, be sure that the basic alarm system will be secure and it will always triggered at any effort of a burglar who will try to approach the cables.
Let’s discuss furthermore for security aspects about security alarms. Safe sirens and the housings of many sensors are usually secured (trapped) by an additional switch which is called the “tamper switch”. Tampers are normally closed and open when someone tries to open the housing of a siren or a sensor. Generally, for the sensors, there is usually no way to be accessed without the movement being detected, as they are in a monitored area (when installation done with care) but usually a burglar may have access to the siren, especially if it is located outdoors in a non monitored area. This is the normal case because sirens are usually placed above public areas where people, pets or birds may passing below. For securing the siren and the sensors, you may act as follow:
- Use 3 sensors and connect them on 3 inputs of the basic alarm and
- connect always the tamper switch of the siren in the 4th input, or you may connect all tamper switches (including that of the siren) in series with each other, and then connect the end nodes of the series circuit, in the 4th input.
Furthermore, prefer to use an autonomous siren which has a built-in battery and will continue to sound even if someone cuts its power supply. In addition, use a backup battery or a UPS unit for the power supply of the main unit of the alarm and do not rely on power from the public grid because any burglar will try to cut the power supply in order to deactivate the alarm. The same they will try to do with telecommunication routes. Please note that nowadays not only wires but wireless telecommunication routes may also be subject of cut (e.g. by using interference).
Finally, it is worthy to say that you may extend our basic alarm board to support more than 4 zones. This may be done by simply using one OR gate which would have more than 4 Inputs, or by making your own multiple-input OR gates, by combining many two-input OR gates.
Description of a typical alarm system
A typical alarm system, based on our main alarm board, is presented in photo 2. This prototype was built for educational purposes. The main unit of the prototype was originally built in a perforated universal board.
At the left of the photo there is a simple siren (of 12V) which is connected to the output P5 of the main board. A motion detector is on the upper right corner and is connected to one input of the main circuit board (zone 1). In a second input terminal, a magnetic trap is connected (a magnetic switch). That switch is at the lower left of the photo. Above the main alarm unit, we have attached a wireless remote control receiver which we use for remote activation - deactivation of the alarm. The remote control transmitter is near the center-right position of the photo. Also, there is a 470uF capacitor, which is connected in parallel with the motion detector. The capacitor, in conjunction with internal resistors, causes a short time delay - about 15 sec- in the sensor’s response. In the center of the main board, you may see the LED indicator, which flashes as long as the alarm is armed.
Armed – alarm indicator
Our basic alarm has a LED indicator, which flashes when the alarm is armed. The LED flashes due to an astable multivibrator circuit based on an additional 555 IC (the U4). The duty cycle of the multivibrator is configured so that the LED flashes instantaneous every 1sec so that there is minimum power consumption. This is useful especially when the alarm is installed in places that are powered by solar panels or batteries. The duty circle of the multivibrator is set from the values of R8, R9 and C3.
The existence of the LED is useful for two reasons:
- It acts as a warning sign for the intruders. The warning becomes more effective when the LED indicator is placed in an obvious place or on a surveillance camera (true or fake).
- Since the alarm has no display, the LED is the only indicator of the alarm being armed or disarmed.
At this point it is worth to mention that the alarm will be armed while it is powered from 12V. It may be disarmed by disabling its power supply. This is a subject of further discussion.
The unit must be powered from 12V. You may use a power supply unit, a battery, or solar panels. The current consumption is about 20mA at rest (on idle state). The power supply unit should be connected to the P6 connector. In series with the power connector, there are two diodes (D3 and D4), used to protect the main board and the sensors from reverse polarity.
The sensors may be powered from P7. P7 provides uninterrupted power supply to the sensors while the power supply to the main board is normally switched off from P8.
For the safety of the system, some special care is essential so that the power supply unit always be monitored by a sensor or a tamper switch. Otherwise, it will be an easy target. In addition, it would be safer to use a backup battery or a UPS and not to rely on the public grid which would be the primary target of an intruder.
Activation and deactivation of the alarm
At this point you may wonder how the alarm is armed or disarmed despite it lacks of a keyboard? The answer is again simple but not obvious:
There is no remote control circuit, key or keypad on the main unit. The unit remains activated while it is powered from 12V. Thus, power supply cutting is the only way to deactivate the alarm. Therefore we have P8. P8 is actually an input for a remote controlled switch, a smart key or an electronic lock, which may be used to arm or disarm the alarm. Here, of course, a problem arises:
Since there is no time delay, a switch, a lock or any other control mechanism should be outside the surveillance area. Thus, would be an easy target for the average intruder. To solve this problem, we propose some solutions:
- Use a wireless remote control. That is the solution we used in our prototype. Using a wireless remote control, you’ll be able to turn on and off the system remotely just as happens in typical vehicle alarms. Be careful of course not to give the key to unauthorized persons because, wireless keys are always subject of copy. Simple remote controlled switches are extremely inexpensive and you can find them at any electronics or automations store. If you select this option, note that you must use a normally-on switch on P8, so that the alarm remains activated if someone manages to cut the power supply of the remote control receiver.
- Use the smart lock that we have published on our website. The lock remains safe since no one (unless the "secret" is revealed) can suspect how it works. But beware that there must be no access to the cable which connects the PCB of the lock with P8.
- You may use the infrared remote control that we have published on our website. Having in mind that our infrared remote controller is based on a public protocol (RC5), it is essential for you to program your own password by altering some code lines of the initial published code.
How to assembe the circuit
You may assemble the circuit on a perforated universal board or alternatively, you may etch a single-sided printed circuit board as presented below. Components must be soldered on the pcb according to the following guide:
Blue lines on figure 2, represent printed circuit lines. The red lines are connection bridges (jumpers) that must be soldered on the components side. Jumpers can be made from thin wires (usually available from what left from cut components terminals).
Components placement is denoted clearly in the guide. What is not so obvious in figure 2, is the polarity of the electrolytic capacitors and the polarity of the LED. However, those are clearly denoted on the schematic.
The printed circuit board of the Basic 4 Zone Alarm