Timed Burglar Alarm

This is a simple but effective alarm circuit which can reset its self after a time that you select. it has normally open and normally closed triggers which make this circuit very practical. This alarm has normally open and normally closed triggers. It's on a 555 timer so the alarm will reset it's self after a certain amount of time. The time is adjustable with the variable resistor in the circuit. The alarm has a reset switch which you can replace with a key switch to make it more secure, and you can change the triggers to other types of door or window switched too. The alarm uses a relay which is connected to a siren but you can replace the siren with whatever you want. The circuit is running off 9VOLTS but can range from 4V - 16V.

Infrared beam barrier/ proximity sensor

This circuit can be used as an Infrared beam barrier as well as a proximity detector.
The circuit uses the very popular Sharp IR module (Vishay module can also be used). The pin nos. shown in the circuit are for the Sharp & VIshay modules. For other modules please refer to their respective datasheets.
The receiver consists of a 555 timer IC working as an oscillator at about 38Khz (also works from 36kHz to 40kHz) which has to be adjusted using the 10K preset. The duty cycle of the IR beam is about 10%. This allows us to pass more current through the LEDS thus achieving a longer range.
The receiver uses a sharp IR module. When the IR beam from the transmitter falls on the IR module, the output is activated which activates the relay and de-activated when the beam is obstructed. The relay contacts can be used to turn ON/OFF alarms, lights etc. The 10K preset should be adjusted until the receiver detects the IR beam.

The circuit can also be used as a proximity sensor, i.e to detect objects in front of the device without obstructing a IR beam. For this the LEDs should be pointed in the same direction as the IR module and at the same level. The suggested arrangement is shown in the circuit diagram. The LEDs should be properly covered with a reflective material like glass or aluminum foils on the sides to avoid the spreading of the IR beam and to get a sharp focus of the beam.
When there is nothing in front of them, the IR beam is not reflected onto the module and hence the circuit is not activated. When an object comes near the device, the IR light from the LEDs is reflected by the object onto the module and hence the circuit gets activated.

If there still a lot of mis-triggering, use a 1uF or higher capacitor instead of the 0.47uF.

Beeper

This circuit produces the sound of a beeper like the one in pagers which produces a "beep-beep" sound. Basically the circuit consists of a 555 timer oscillator which is turned ON and OFF periodically.
The first IC(left) oscillates at about 1Hz. The second IC is turned ON and OFF by the first IC.
The first IC determines how fast the second IC is turned ON/OFF and second IC determines the tone of the final output.
By varying the VR1, the changeover rate can be adjusted. By varying VR2 the tone can be adjusted.

If you know something about electronics, you can try replacing the 2nd 555 IC circuit with a piezoelectric buzzer. This saves one IC and associated components but the buzzer cannot give a loud sound as the speaker and also its tone cannot be varied.

Big Ben Sound

This circuit produces the famous Big Ben sound. It produces the "ding dong" sound when switched ON.
Basically the circuit alternates between two frequencies which are adjustable. This produces the "ding-dong" sound.
The first IC(left) oscillates at about 1Hz. The second IC's tone is modulated by the changing voltage at the output of the first IC.

The first IC determines how fast the changeover from one frequency to the other takes place and second IC determines the tone of the final output.
By varying the VR1, the changeover rate can be adjusted. By varying VR2 the tone can be adjusted.