In this auto shut off tone generator project, once the switch to the 9V power supply is connected, the alarm will trigger at a frequency of approximately 1.27 kHz. It will remain ON for a duration of approximately 170 seconds or 2.8 minutes before it stopped.
This is a typical home burglar alarm system of which once the alarm is triggered ON, it will not be shut OFF until the duration of time set has elapsed. This project is useful when built as one can carry it along wherever one goes or placed it in a vehicle. In times of emergency, one can easily switched ON the switch and the loud speaker will emit a loud sound that will frighten the uninvited guest.
The schematic of the tone generator is as shown below.
It is based on two 555 timer ICs or one single 556 timer IC(which contains two 555 timers). In this schematic, two 555 timers are used. U2 is configured as a timer in astable mode.
Once triggered, it will emit a frequency from its output at pin 3 that will drive a Q1 transistor. Q1 transistor will turn ON and OFF according to the frequency of the circuit. It will in turn used to drive a 8 ohm loud speaker to emit a loud audible sound.
The astable frequency of circuit U2 is given by the formula of 555 timer as shown below.
f = 1.44/[47K + 2(33K)][10nF]
= 1.27 kHz
The frequency of the sound can be adjusted by changing the values of R3= 47K, R4= 33K and capacitor C1=10nF. Change the values of these components and by using the formula for astable mode, the frequency of the sound can be obtained.
U1 circuit is used as a delay circuit which is configured as a monostable mode. It is a one shot multivibrator that will generate a pulse at its output at pin 3 which will disable the astable circuit U1. In this circuit, pin 2 of U1 will go to logic 0 when the power supply is connected via the capacitor E1 and hence circuit U2 is immediately triggered.
The pulse duration of the monostable circuit is given by the formula:
T = 1.1(330K)(470uF)
= 170 seconds
Once this timing is up, it the pulse output will disable the astable circuit of U2.