12 Volt DC Electronic Door Chime

3 Tone Gong: Suitable for use as a door bell

555 Tone Generator 8 Ohm Speaker:

Audio MilliVoltmeter

Battery Low Voltage Beeper

Bells Ring Generator: 3-Circuit options Can be synchronized to Christmas tree flashing lights

Inexpensive LED Drives Flash & Beep Circuit

Personal Alarm: Small portable, anti-bag-snatching unit.  Also suitable for Doors and windows control

Piezo Device Generates Buzz, Beep, or Chime

Plane Locator Beeper #2

Plane Locator Beeper

Police Siren: This circuit produces a sound similar to the police siren. It makes use of two 555 timer ICs used as astable multivibrators.

Speaking Doorbell: SpeDoB was designed and built to gain experience with the PIC microcontroller. This doorbell uses a speech recorder integrated circuit (ISD series) to play back3 pre-recorded "theres someone knocking on the door" messages. Its a very easy and simple project to build, ideal for beginners!

Timed Beeper: Beeps 7.5 seconds after a preset Time Adjustable Time settings:15 sec.30 sec.1 min.2 min. & others

Turn Signal Alarm

Two Tone Generator: This two-tone generator includes 3 ICs - NE555 Astable Multivibrators. You can vary the duration of each tone by changing the 10k resistor or 100MF capacitor at IC1 or changing resistors and capacitors at IC1/2 for higher or lower tone.

You've got Mail:
Many e-mail programs provide a "beep" or a pop-up message box signaling the user that a new e-mail message has arrived. If you are too far from the computer to hear the audible signal or if the monitor is turned off, then you miss the new-mail audible and visual signals. The simple circuit in Figure1 latches on an LED and an audio sounder when an appropriate new audible e-mail signal occurs. The method replaces the normal e-mail sound.wave file with a .wav file of any valid recorded dual-tone multifrequency (DTMF) sound.

12C508 Smart Lock: The base sends a data sequence (the password) which is recognized by the key. The key checks the sequence and -if it recognizes it -sends to the base another data sequence, as an answer to the password. At the Time when the base gets a correct answer (which means that the key has been introduced), the relay is put into action, and it can command a number of consumers. 

Auto Burglar Alarm: This alarm circuit is based on two555 Timers. The alarm will sound your car horn if anyone opens the car door while the circuit is armed. The Timers will allow you to leave the car without sounding the horn. To turn the circuit on S1 must be closed. To set the alarm, open S2 ( it is normally closed ) this will give you about5 seconds to get out and close the door. The exit delay Time is set by R1 and C1. If anyone opens the doors for more then two seconds the horn will sound until power is removed from the circuit. The2 second Time is set by R2 and C2. If you open the door, you must deactivate the alarm by closing S2. This very basic circuit could be used for a home also.

Combination Lock: This circuit is very basic to build. To open a the lock which is connected to the K1 Load. you must press each momentary switch in the correct sequence. The sequence used in this circuit is S1,S2,S3,S4. If any of the other switches are pressed the circuit will reset and you will need to start over. Depending on how you wire the switches, you can use any4 switch combination 

Cut Phone Line Detector #1: Can be used to alarm security system 

Door/window sensor resists tampering
PDF contains multiple circuits - scroll to find this circuit

Oscillating output improves system security: 02/06/03  EDN-Design Ideas / Many electronic-control systems have digital outputs that use transistors. One method of improving the security in these outputs is to use an oscillating signal to represent a logic-high state instead of a fixed voltage level (Figure 1). This type of signal, a dynamic variable, can drive the circuit shown in Figure 2...

Passive Infrared Pyroelectric Infrared Motion Sensor:

Perimeter Monitor: Using a single cable such as speaker wire or doorbell cable, this circuit can be remotely positioned, for example, at the bottom of a garden or garage, and used to detect all sound in that area.  The cable can be buried in a hosepipe or duct and is concealed out of sight. The mic is an ordinary dynamic mic insert and should be housed in a waterproof enclosure with the rest of the circuit.

Simple Electronic Lock Uses Single-Transistor Circuit:
Electronic Design - Ideas for Design / (added 8/03) Building on the recent Design Brief theme of one-pin keypads, this design uses only one analog microcontroller (MCU) input to scan numerous keys, plus one resistor per key and another resistor and capacitor. The circuit is simply a crude... 

Single Button Lock Provides High Security:
PDF contains multiple circuits - scroll to find this circuit.