This is really a very simple home telephone ring tone generator circuit which is built with applying only several electronic components / parts. It generates simulated telephone ring tone and requires only DC supply with 4.5V DC to 12V DC voltage. One may possibly use this circuit in ordinary intercom or phone-type intercom. The sound is pretty loud when this circuit is operated on +12V DC power supply. Even so, the volume of ring sound can be adjusted.
The widely available 14-stage binary ripple counter with built-in oscillator (CMOS IC CD4060B) is chosen to produce three kinds of pulses, that are obtainable from pin 1 (O11), pin 3 (O13), and pin 14 (O7), respectively. Preset VR1 is fine-tuned to get 0.3125Hz pulses (1.6 second “low” followed by 1.6-second “high”) at pin 3 of IC1. At the same time, pulses obtainable from pin 1 will be of 1.25 Hz (0.4-second “low”, 0.4-second “high”) and 20 Hz at pin 14. The three output pins of IC1 are connected to base terminals of transistors T1, T2, and T3 through resistors R1, R2, and R3, respectively.
Transistors T1 through T3 are cascaded in this kind of a way that the positive voltage obtainable at the emitter of transistor T1 is extended to the collector of transistor T3 once the outputs of all of the three stages are low. As a result, transistors T1 through T3 are forward biased for 0.4, 1.6, and 0.025 seconds, respectively and reverse biased for similar durations.
Working with a built-in oscillator-type piezobuzzer generates about 1kHz tone. In this particular circuit, the piezo-buzzer is turned “on” and “off” at 20 Hz for ring tone sound by transistor T3. 20Hz pulses are obtainable at the collector of transistor T3 for 0.4-second duration. Just after a time interval of 0.4 second, 20Hz pulses become again obtainable for another 0.4-second duration. This is followed by two seconds of nosound interval. Thereafter the pulse pattern repeats by itself.
Refer the figure that signifies waveforms obtainable at several points including the collector of transistor T3. Preset VR2 could be utilized for adjusting the amplitude of the ring tone.
Telephone ringtone generator circuit taken from EFY magazine edition July 2001