Burglar alarm using IC555 is not new who already worked on IC555. But the project presented here is the simplest of all projects. It uses the property of RESET pin-4 of the IC.
Consider the following circuit. Here IC555 is used as simple astable multivibrator circuit whose output frequency is decided by the combination of R1, R2 and timing capacitor C. You can refer to the IC555 calculator to decide your required frequency for outputting a shrill whistle like sound in the speaker.
Now as shown in the above circuit, when there is a thin wire like loop connected between pin-4 and ground, the IC is disabled and so the output will remain in LOW state.
But whenever, the loop is broken by a burglar, the pin-4 of the IC is held HIGH and the timing capacitor starts charging through R1&R2 and discharging through R2 only. Due to this, a high frequency signal is produced at the output, which is decoupled by the electrolytic capacitor, to the speaker to produce a whistle like sound.
- The wire loop should not be quite long. The tested length is 3m maximum for a 40 SWG wire.
- Do not change the value of resistor connected between pin-4 and positive of the supply.
- Do not change the value of output capacitor connected in series with the speaker.
- Also do not remove this capacitor and do not connect the speaker directly between pin-3 and ground.
- Using the polyester capacitor as timing capacitor is preferably better for good quality at variable surrounding temperature.
Know complete details about IC555 on this link.