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Hybrid Security Alarm

NE555 is a versatile timer available as 8 pin DIP and has a wide operating voltage. The main function of 555 is to switch its state and is used as monostable, astable and bistable multivibrators. It can produce oscillations up to 500KHz and can be used as a time delay generator. It has both active high and active low triggers and a dedicated reset pin. Therefore a 555 can be configured as many useful circuits with minimal external parts.

Security Alarm

This is a simple security alarm system based on NE555 using the astable and bistable operations. I made this circuit for detecting intrusions inside my lab. The circuit produces a loud alarm sound when triggered and stays latched until it is manually reset. The circuit can be triggered in a number of ways including mechanical switches fixed to doors, laser beam - LDR combination, reed switches, proximity sensors, PIR motion detectors etc. All these sensors can be connected serial or parallel to make a hybrid security alarm. The alarm sound delay and frequency is user configurable.

Circuit Explanation

Security Alarm

The heart of the circuit is three 555 ICs; IC1, IC2 and IC3. The IC3 functions as an astable multivibrator which generates a square wave of frequency in the voice range. The frequency is set by C4 ceramic and VR2. The frequency can be controlled by setting the VR2 to desired points. The output is taken from the pin 3 through an electrolytic capacitor C5. The IC2 functions as a low frequency astable multivibrator (free running multivibrator). It controls the power to the IC3 through pin 3. It generates a delay for the alarm sound. The frequency is controlled by setting the VR1. These two forms the sound generator. You can use the NE556 dual timer instead of using two 555s. But I didn't have one then.

The trigger circuit consists of IC1 and a NOT gate IC4. The IC1 functions as bistable multivibrator or simply a latch. The internal flip-flop of the 555 is set by an active low trigger pulse applied to the pin 2 and reset by a low pulse applied to pin 4. R1 and R2 are pull-up resistors which set the triggers to idle mode. The pin 2 of IC1 is activated by a NOT gate, 74LS04. Only one gate of six used here. When the input of the NOT gate is HIGH, the output is LOW and vise versa. The two terminals of SW2 are where we connect an activating sensor.

For example, think that a reed switch is connected between SW2 and a magnet is placed near of it. When the reed switch is in closed position, it connects the input of the NOT gate to the GND making its output HIGH. As long as this output is HIGH, the IC1 remains in idle state. When we remove the magnet from the reed switch, the output of the NOT gate goes LOW and triggers the IC1. The output of the IC1, pin 3 goes HIGH. This HIGH is applied to the base of an NPN switching transistor Q1. Now the transistor connects the alarm sound generator circuit to GND and completes the circuit and setting off the alarm. The alarm system remains active or triggered even if you place the magnet near the reed switch. That means any further action has no effect on the circuit until it is manually reset by SW1. The momentary switch SW1 resets the IC1 by connecting the pin 4 to the GND. The alarm only turns off if the reed switch is closed and the reset button is pressed.

The power for the IC1 and IC4 is +5V and should be supplied by a regulator such as 7805. This is because the 74LS04 is a TTL IC whose working voltage is 5V. Only the 74LS04 can be used but not the other 74HCT04 versions. The power for the alarm sound generator can be 5-12V and should be supplied by a battery or regulator for clear and smooth sound. The two power supplies, if used, must share a common GND. The pin 5 of the 555 ICs may be connected to GND using a 0.01uF ceramic cap, when not used. The idle current drawing of the circuit is very little and is ideal for battery operation.


  • Variable alarm sound frequency and delay.
  • High noise immunity.
  • Multi level triggering using multiple sensors.
  • Suitable for battery operation.


  • An intruder can short the trigger terminal to disable the alarm.
  • Other possible ways of hacking.

Security Alarm
Security Alarm

The circuit can be built on a matrix perforated PCB. You can see above how I did it. When using a PIR motion detector, it may need a power on delay generator and a 'push/pull-pull' network. I don't recommend building the circuit as it is given. Try to improve it by modifying it. You can have a wailing siren instead of the beep sound and it can be done by an another 555. Using NE556 instead of a number of 555s, will reduce the board size. You can build the trigger circuit entirely on transistors or make latch only using a 74LS04.


Date published : 1:14 AM 2-12-2014-Tuesday
Last updated : 10:12 PM 01-03-2016, Tuesday
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  Dec 1, 2014, 11:49 AM Vishnu M Aiea

Security Alarm  Dec 1, 2014, 11:49 AM Vishnu M Aiea