Introducing the “Programmable Automatic Bell” system, which revolutionizes the conventional approach followed by schools and colleges. Typically, educational institutions rely on manually operated electric bells to adhere to their class timetables, necessitating the assignment of a person to activate and deactivate the bell at specified intervals. However, our innovative solution eliminates the requirement for human intervention. We have devised a straightforward digital electronics-based automatic bell system that is both user-friendly and straightforward to assemble. This system employs a programmable time switch (Frontier TM-619) for clock and digital timer programming, ensuring seamless adherence to schedules.
Circuit and working of Programmable Automatic Bell System
Figure 1 displays the schematic representation of the programmable automatic bell system. This system is constructed using several key components, including a step-down transformer (X1), a 5V voltage regulator IC 7805 (IC1), three 74121 monostable ICs (IC2, IC3, and IC5), a quad AND gate IC 7408 (IC4), six 1N4007 diodes (D1 through D6), a 230V AC bell, a digital timer programmable time switch (TM-619), and several other essential elements.
Understanding the circuit’s operation becomes straightforward when referring to the truth table provided in the IC 74121 datasheet, which is readily accessible on the internet.
In Figure 1, you can observe how to configure the inputs of IC2 and IC3 to achieve the desired modes of operation, whether it be falling-edge-triggered or rising-edge-triggered monostable multivibrator. In this configuration, we employ input A2 for the falling-edge trigger of IC3 and input B for the rising-edge trigger of IC2.
When utilizing input B for a rising-edge trigger, it’s necessary for either A1, A2, or both of these inputs of IC2 to be in a low state, effectively connected to ground. To facilitate this, we connect both A1 and A2 to the ground. Similarly, when using A2 as a falling-edge trigger for IC3, pins A1 and B of IC3 should be set to a high state, connected to 5V. In this setup, IC2 operates as a rising-edge-triggered, one-shot monostable multivibrator, and its time period is determined by the combination of resistor R1 and capacitor C4, as outlined below:
In this setup, we’ve chosen a 10-kilo-ohm resistor (R1) and a 2.2µF capacitor (C4) to achieve a pulse width of 1.5 milliseconds for IC2. Similarly, for IC3, configured as a falling-edge-triggered one-shot monostable multivibrator, we’ve selected resistor R3 and capacitor C7 to produce the same pulse duration of 1.5 milliseconds.
The Q outputs of both IC2 and IC3 are connected to input pins 2 and 1 of IC4, respectively. In the absence of a trigger for IC2 and IC3, the Q outputs at pin 1 of both IC2 and IC3 are in a high state. Consequently, the output at pin 3 of IC4 (corresponding to AND gate N1) is also high. This signal is linked to trigger input A2 of IC5, which functions as a falling-edge-triggered one-shot monostable multivibrator. We’ve set its pulse width to 3.2 seconds by selecting resistor R5 and capacitor C9 with values of 10-kilo-ohm and 470µF, respectively. These values are tailored to the desired bell ringing duration. If needed, you can substitute the fixed-value resistor R5 with a potentiometer to adjust the ringing duration to match each class’s time slot.
Under normal conditions, when the Q outputs of IC2 and IC3 are high, the N1 gate output of IC4 remains high as well. In this scenario, the Q output (pin 6) of IC5 is low, preventing transistor T1 from conducting. Consequently, relay RL1 remains unenergized, and the bell connected to CON2 remains inactive. The bell only rings when the Q output of IC5 goes high, activating it for the predetermined time span of 3.2 seconds. During this interval, LED1 illuminates.
For programming digital timer switch TM-619, refer its user manual on the Internet. Working of the circuit with timer switch TM-619 is given below:
1. Program the digital timer switch (TM-619) as per the required timetable. As an example, typical timetable of a college/school is shown in the table. Note that at the end time of the lecture, the timer should be in ‘off’ condition as shown in the table. If the sequence ends with ‘on’ time, put an additional ‘off’ time by adding 5-minute time interval. This ensures that timer TM-619 output is in off condition so that it begins the next cycle with ‘on’ condition. Note that whenever the timer output toggles between ‘on’ and ‘off’ conditions, output pin 6 of IC5 goes high for 3.2 seconds. This means, the AC bell rings for 3.2 seconds.
2. After programming, put the timer in auto mode.
3. When the timer clock reaches preset time, its status changes from ‘on’ to ‘off’ and vice versa.
4. Accordingly, the timer clock generates a falling-edge or rising-edge trigger signal for monostable IC2 and IC3. Q output of the respective IC generates a low-going pulse with pulse width of 1.5 milliseconds, which, in turn, triggers IC5. Transistor T1 conducts to energise relay RL1 and the bell rings for 3.2 seconds.
5. The process repeats as per the timetable.