Audio Generator of Bells Ringing

Figure 1 

Parts List

The circuit was designed similarly with door bell units as it generates an audio of bells ringing with dual tones.

• MC14106 – a hex Schmitt trigger constructed with MOS P-channel and N-channel enhancement mode devices in a single monolithic structure primarily used where low power dissipation and/or high noise immunity is desired due to its features of capability of driving two low-power TTL loads or one low-power Schottky TTL load, 3V to 18Vdc supply voltage range, and increased hysteresis voltage
• BC337 – a small signal NPN Silicon AF medium power transistor used for general purpose switching and amplifying applications with features such as TO-18 manufactured package, suited for AF driver stages and low power output stages, and divided into three group types
• Photoresistor – a resistor made up of high resistance semiconductor and whose resistance decreases with the increasing incident light intensity

There are several applications by which this circuit can be used other than being a door bell. The common thing as compared with an ordinary door bell is that it produces a “ding” tone when the SPST push button switch P1 is pressed while a “dong” tone is created when P1 is released. The first tone is being generated by IC1D while the second tone is generated by IC1F. The shaping and fading of two tones are being controlled by Q2, Q5, and other related components while trying to replicate the natural sound of bells as close as possible.

The outputs are being mixed using resistors R7 & R13 and while being filtered by capacitor C5. In order to drive the loudspeaker, the outputs are being amplified by a simple Class A audio amplifier which is made up of Q3 & Q4. As the switch P1 is pressed, Q1 switches ON the amplifier then switches OFF after some seconds of releasing P1. The time delay is made possible by the parallel RC network from R2 & C1. A negligible current will be drawn at this stage when held in standby mode.

To create a “ding-dong” sound regardless of the release of P1, the circuit should be modified by removing D4 while the time delay between the first tone and second tone is arranged using C10 & R15. There is also a possibility of producing a single tone generator by doing more modifications on the circuit. The switch S1 can be excluded since the current being drawn at standby mode is 200 uA at 3 V, when in standby mode. The frequency of the tones is better set at approximately 2 KHz and 1650 Hz.

One good application of this ringing bells generator is during Christmas where a soft bell sound can be heard at the switching ON and OFF of the chosen bulbs. This can only happen if a photoresistor is used in the place of P1 then placing the unit near the flashing lamps of the Christmas tree. A better way is to synchronize the circuit on the lights of the Christmas tree.

Source:redcircuits.com/Page37.htm