555 Timer With Audio Alarm Circuit

Here is a very simple circuit for electronic timer with audio alarm. With the help of this circuit, audio alarm (sound) can be heard for a few seconds regularly and automatically, after a preset time period. Such a circuit has a number of applications in dark rooms, kitchens etc. A wonderful application of this circuit is in debate competition, where a candidate is allowed to speak for, say, ten minutes. As the candidate starts, the reset button is pressed to give a sound for few seconds indicating your time begins. The circuit can be set to give alarm, say, after every five minutes to indicate five minutes  are over.

IC1 and IC2 are used in monostable and astable modes respectively. The output of IC1, is coupled to IC2 through  transistor T2, Which works as an inverter. The trigger input (pin 2) of IC1is generally high, and when it is momentarily low, monostable multivibrator changes the state, and the output is high. Now capacitor C2 starts charging through resistance R (i.e. Vr1+10kΩ) and as the capacitor voltage becomes 2/3 of supply voltage , the monostable multivibrator changes the state from high to low.

The time taken to change the state from high to low, depends upon time constant RC. The larger the time constant, longer it takes for the capacitor voltage to reach 2/3 of the supply voltage. The time duration t is given by t=1.1RC seconds. Maximum time duration obtained with the values given in the circuit is only about half an hour.

555 Timer IC PIN Configuration

Now when the output of IC1 changes state after the preset time period, transistor T2 which is used as inverter makes its collector high. The collector of T2 is connected to the reset point of IC2 which works as astable multivibrator. When reset point (pin4) of IC2 is high, astable multivibrator starts functioning and the alarm is heard.

The collector of T2 is also connected to variable resistance VR2 and capacitor C1 combination. As soon as the collector of T2 becomes high, capacitor C1 starts charging through resistance Vr2. Preset potentiometer VR2 is adjusted, such that the time taken to charge capacitor C1 to a voltage just sufficient to conduct transistor T1 is about 10 or 15 seconds (as required). As transistor T1 conducts, pin 2 of IC1 is low and its output becomes high, which makes pin 4 of IC2 low and disables IC2, stopping the alarm. At this point transistor T1 is cut-off and pin 2 of IC1 becomes high immediately. Thus it is set for next cycle.

In this way a regular sound can be heard for 10 or 15 seconds after the preset time period. The circuit can also be reset at any time by pressing the reset  button. By pressing it, alarm is heard for 10 to 15 seconds.

Note:

• S1 for Reset
• Without Using R11, you can get more sound

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