# Construct a 9V Door Bell Chime

This door bell chime project provides the schematic and the parts list needed to construct a very simple wired door bell alarm that you can place on the door of your house. It is a very low cost and affordable project that every beginners to electronic design can hands on.

Constructing this project will help the beginners to electronics to understand one of the applications of 555 timer that is configured in an astable mode. This project uses a 555 timer integrated circuit, a speaker, 5 resistors, 4 electrolytic capacitors, 1 ceramic capacitor, 3 diodes, 1 push-button switch and 9 V battery as power supply.

555 Timer Configuration

An astable timer operation is achieved by configuring the circuit in such a way that its output will be triggered continuously. The result of the output is a stream of clock pulses with a fixed pulse width and duty cycle determined by the resistors and capacitor connected to the IC.

In the astable operation, the trigger terminal and the threshold terminal are connected so that a self-trigger is formed, operating as a multivibrator. When the timer output is high, its internal discharging Tr. turns off and the VC1 increases by exponential function with the time constant (RA+RB)*C. When the VC1, or the threshold voltage, reaches 2Vcc/3, the comparator output on the trigger terminal becomes high, resetting the Flip/Flop and causing the timer output to become low. This in turn turns on the discharging Tr. and the C1 discharges through the discharging channel formed by RB and the discharging Tr. When the VC1 falls below Vcc/3, the comparator output on the trigger terminal becomes high and the timer output becomes high again. The discharging Tr. turns off and the VC1 rises again.

Door Bell Chime Circuit Description

The frequency of the oscillation is given by the formula:

f = 1.44/(RA + 2RB)C where RB= 68K, RA= 68K+39K = 107K , C = 15nF

= 1.44/(175K*15nF)

= 395 Hz

The duty cycle is given by:

DHI = (RA + RB)/(RA + 2RB) = 0.72

DLO = 1 - 0.72 = 0.28

The duration of the pulse is given by:

T1(HIGH) = 0.693(RA+RB)C = 1.82 ms

T2(LOW) = 0.693RB*C = 0.71 ms

Parts List