This is another version of a simple electronic door buzzer project using the readily available 555 timer integrated circuit. Both bipolar or CMOS type of integrated circuit can be used. The timer is configured as an Astable mode in that the output signal will consistently output a steady period of waveform once the input has been triggered.
Pin 2 is a trigger pin and is connected together with pin 6 which is a threshold pin. See the internal structure of the timer diagram. By this configuration, a multivibrator is formed. The freqency of the multivibrator that is seen at pin 3 is given by the equation below:
frequency = 1.44/[(R1+2VR1)C1]
The variable resistor VR1 is part of the equation hence by changing its value, you will be able to change the frequency of the waveform being generated. If you have an oscilloscope, you can connect one end of the probe to GND and the other end to pin 3 of 555 timer integrated circuit. Notice the change of waveform shape as you vary potentiometer VR1.
As you will use a 9V dry cell battery to power up this circuit, power consumption is a critical element that has been considered in this circuit. You will notice that when switch SW1 is not been pressed, the entire circuit is power off.
There is no current being drawn by the circuit during standby hence the quiescent current is zero. This will help to ensure that the battery has a longer life span compared to many other circuits which actually draw a quiescent current when not in used.
Get the following parts list and a breadboard to test out the circuit. Try changing the values of C1, R1 and VR1 if you do not have the exact part. You will notice that the equation that determines the frequency of the output signal is still valid.
|R1||1.5K ohm 5% 1/4W Carbon Film Resistor|
|VR1||100K ohm Potentiometer|
|SW1||Normally OFF pushbutton switch|
|C1||0.022uF/25V Ceramic Capacitor|
|C2||0.01uF/25V Ceramic Capacitor|
|E1||47uF/25V Electrolytic Capacitor|
|E2||100uF/25V Electrolytic Capacitor|
|555 IC||555 Timer IC|