Home Blog  

27 Mar 2014

We are always vulnerable to thieves but we don’t know when will they strike. When they do, it will be too late for us to notice that our property was already taken. This project helps alert the owner if a thief is picking a lock. It can be used on doors, luggage or anything else with a lock.

The concept is simple, if the lock is opened without our consent, an alarm will be turned on indicating an intruder. This circuit is based on a NAND gates configuration that will turn the switching side on or off.

In this project we used the HEF4011B, a quad 2-input NAND gate. The outputs are fully buffered for the highest noise immunity and pattern insensitivity to output impedance. The configuration of the IC HEF4011B produces a HIGH output if the input is HIGH. Based on the figure, when the wire loop is closed, the IC HEF4011B will have a LOW input, and when the wire loop is triggered, the IC HEF4011B will have a HIGH input which will make its output also HIGH. The HIGH output of the IC HEF4011B will then be inputted to the base of the BC547 transistor allowing the current to flow on the relay switching the indicator or the buzzer ON.


  • HEF4011B quad 2-input NAND gate
  • BC547 transistor
  • 1N4007 diode
  • 1kΩ resistor
  • 4.7kΩ resistor
  • 0.1µF capacitor
  • Relay
  • 5V dc power source

Anti-theft Security Alarm Circuit – [Link]

8 Dec 2013


An IR detector that sounds a buzzer when an IR beam is broken, meaning the IR signal is lost.  A pulsed IR signal generator is necessary, but not included in this post.  This project would be ideal for doorwaLM311ys or hallways to alert when someone enters or exits an area.


The IR sensor responds to pulsed IR, not ambient or continuous IR.  This means that another transmitter project is necessary in order to complete this one!  Note though that some forms of lighting like fluorescent lighting may interfere with the sensor.  For convenience,  the the buzzer is internally driven so that a only Vdc is needed to make a sound.  In this case, the IR sensor senses 38kHz pulsed infrared light.

Pin 3 of the IR sensor is actually low (0V) while receiving a signal.  When the sensor is blocked from receiving the IR signal, the sensor outputs a high signal to the comparator, which then allows current through the LED/Buzzer circuit, and alerting you that the beam is broken. In the Scheme-It drawing the LM311 IC is a grouping of three components, in a functional block diagram style, to show how it functions in the circuit beyond what the pinouts would show normally.

IR Beam Breaker Alarm Circuit - [Link]

17 Jul 2013


T.K. Hareendran writes:

All items stored in a deep freezer will thaw out if, for some reason, the temperature inside the freezer rises to the thaw point. However, a freezer monitor alarm can warn you of the rising temperature before the thaw point is reached.

This simple circuit is powered by regulated 9V and built around a few discrete components that are readily available in the market. Current consumption of the circuit is only a few milliamperes in idle state. So you can also use a 9V 6F22/PP3 type compact battery for powering the circuit.

Freezer Monitor Alarm - [Link]

12 Jul 2013

Markus Gritsch shows off his Dot-Matrix MSP430 alarm clock. [via]

Dot-Matrix MSP430 alarm clock - [Link]

11 Jul 2013

At the heart of this circuit is a precision integrated temperature sensor, LM35 (IC1), which provides an accurate linear and directly proportional output in mV, from 0 to +155 degrees C.  Designed to draw a minimal current of its own, the LM35 has very low self heating in still air. Here the output of the LM35 is applied to the non-inverting input of a comparator wired around a CA3130 opamp (IC2). A voltage divider network R3-P1 sets the threshold voltage, at the inverting input of the opamp. The threshold voltage determines the adjustable temperature trip level at which the circuit is activated.

When the measured temperature exceeds the user-defined level, the comparator pulls its output High to approx. 2.2 V causing transistor T1 to be forward biased instantly. T2 is also switched on, supplying the oscillator circuit around IC3 with sufficient voltage to start working. The 555 set up in astable mode directly drives active piezoelectric buzzer Bz1 to raise a loud alert. Components R7, R8 and C4 determine the on/off rhythm of the buzzer.

Overheat Detector Alarm - [Link]

20 Jun 2013


Boris Landoni writes:

It’s small but packed with features. The GSM alarm we present today, sports a PIR motion sensor, can be battery operated and it’s capable to communicate via GSM. It can transmit alarm conditions and receive commands from remote. It’s also capable to indicate problems such as is insufficient voltage supply or tampering.

It’s not so conventional for antitheft system: to integrate a movement sensor, a PIR motion sensor and a temperature probe in a single appliance: all this accompanied by an SMS sending GSM / GPRS module. This circuit was born as a very versatile, ready to use, built-in alarm system: no installation is required, you can just drop it and it’s ready to work. It’s not by chance that is designed to be battery operated: it also features a battery state control to check the power.

GSM Multifuction Alarm with SIM900 - [Link]

9 May 2012


An Arduino based burglar alarm that sends an SMS when motion is detected. It will send an SMS to you every 30 minutes while motion is detected (configurable).

Arduino burglar alarm sends SMS - [Link]

31 Jan 2012

The STD32 offers the user the possibility to remotely switch ON or OFF electronic devices and to receive alarm messages via (SMS). You can switch devices either with an SMS or using a simple voicecall. Alarm messages (SMS) can be received with any mobile phone supporting SMS functionality.

With the new generation of the STD32 you now also have the possibility to receive alarm messages via e-mail. With the help of the digital camera which is available as an accessory, pictures can be taken and sent via e-mail triggered by an alarm.

The STD32 has an integrated webserver which allows direct access to the device via the internet and a standard webbrowser (e.g. Internet Explorer or Firefox) from a computer or a mobile phone with web functionality. Thus it is very simple to switch electrical devices remotely and to change the configuration of the STD32 from anywhere.

Simple GSM control

  • New! Fixed IP address (in-built server)
  • New! Camera interface (sends still images via email)
  • Two alarm inputs, e.g. for movement, vibration, temperature or moisture sensors
  • Two relay outputs, potential-free max. 6A/250V
  • Alarms via text message
  • Remote setting of parameters (on times, etc.) possible
  • Five alarm addresses
  • Basic functions can be activated by telephone with no call charge
  • Online configuration via the internet
  • Camera and built-in box available as accessories
  • Supplemented with SIM cards

STD32IP Remote Controlling /Alarm for GSM Network - [Link]

22 Nov 2011

jumperone.com writes:

Well, basically break beam sensor consists of two parts: transmitter and receiver. Transmitter emits light (it could be, for example, an LED or a laser) and that light goes to receiver. If that light beam between transmitter and receiver is broken by some obstacle, and receiver detects no incoming light even for a brief moment – it triggers an alarm (or any load you want). For example you can count people that entering some room, or you can use it as an alarm system, yeah, just like in movies!

First thing that comes to mind is to connect LED or laser to DC power supply on transmitter side, and use phototransistor with an amplifier on receiver side. But that is not gonna work with changing ambient light level. For example you calibrated your amplifier to work on a cloudy day, and then bright sun shined at it and in this case it won’t trigger an alarm because it already has sufficient level of incoming light.

Break-Beam Sensor - [Link]

18 Oct 2011

Professor Mark Csele writes:

Presented is a simple temperature alarm which uses a PIC16C84 microcontroller and a 2-line LCD display. The alarm displays current temperature in both Celsius and Fahrenheit degrees and features a 3-key keypad which allows the user to set independent high and low temperature alarm points. The alarm also records the minimum and maximum temperatures encountered.

A PIC-Based Temperature Alarm - [Link]





Search Site | Advertising | Contact Us
Elektrotekno.com | Free Schematics Search Engine | Electronic Kits