The relay stays activated only until the incoming cell phone remains connected, on disconnecting the call the relay also disconnects. T3 boosts the ringtone level to much higher levels, however this signal is still not powerful enough for driving a relay, therefore it is yet further amplified by T4, until finally the drive becomes just suitable for activating the relay. The preamplifier consisting of T1 and T2 amplifies the signal to reasonable levels and feeds it to the T3 via C2 for further amplification. The ringtone passes through the headphone connection to the base of T1 via C1, R13. As can be seen the input signal needs to be derived from a modem cell phone's head phone socket.Ī suitably selected ringtone starts playing on the cell phone when called from the users personal cell phone. Transistors T1 and T2 form a high gain looped audio amplifier. Looking at the circuit diagram, we see a pretty straightforward configuration using only transistors. The peon would be able to do his other scheduled work and ring the bell from any position inside the school premise or even from outside, in fact from any corner of the world. Thus the use of this type of cell phone controlled remote bell can eliminate the need of running for the bell switch on every class period intervals. The explained circuit can be used in schools for ringing bells using a cell phone from anywhere irrespective of the distance.
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