Homemade bell controller

Homemade bell controller

The attached figure shows a special automatic bell circuit for schools. The school has eight classes a day (45 minutes each), and the bell automatically rings after each class. After the fourth class, there is a 30-minute lunch break, and then the bell automatically rings again for class.

As can be seen from the circuit diagram, the control circuit consists of an integrated time base circuit NE555 (IC1), two decade counters CD4017 (IC2 and IC3), an AND gate CD408l (IC4) and a bell drive circuit. IC1 is connected as an astable multivibrator, and its output pulse is sent to IC2. IC2 increases the pulse time interval (4.5 minutes and 3 minutes) generated by ICI by 10 times, and generates 45-minute and 30-minute clock pulses and sends them to IC30. The output of IC3 passes through diodes D4 to D12, making transistors T1 and T2 work in the switch state to ensure that the bell time for each class is carried out in the predetermined time sequence.

The series resistors R4 and R5 connected to the emitter of transistor T2 determine the time interval of IC1 output pulses (45 minutes). The output of ICI is sent to the ⑩th pin of IC2, which can generate the time of a class (45 minutes); similarly, the series resistors R2 and R3 connected to the emitter of transistor T1 determine the time interval of ICI output pulses (3 minutes), and the output of IC1 is sent to IC2 to generate a 30-minute lunch break.

When the +12V power supply is connected to the circuit, the circuit cannot work because it is not grounded. Once the switch S1 is pressed, the thyristor SCRI is triggered to conduct, thereby grounding the circuit, and the decimal counter IC2 and IC3 (after reset) circuits work normally. At the same time, the light-emitting diode LED1 lights up, indicating that the bell circuit starts to work. When the eight classes are over, Q9 of IC3 outputs a high level, the light-emitting diode LED2 lights up, transistors T1 and T2 do not get the output voltage of IC3, the collector and emitter of T1 and T2 are open, and the astable multivibrator stops working, indicating that all eight classes have ended. The manager presses switch S2 again to turn off the bell circuit, and LED1 and LED2 are not lit.

The bell at the end of each class lasts about 8 seconds. If you want to increase/decrease the duration of the ringing, you can increase/decrease the number of series diodes between the 6th and 7th pins of the ICI.

The administrator should press switch S1 on time at the beginning of the first class. When the last class is over, LED2 will light up and the administrator should press switch S2 to close the bell circuit.

If the manager is late, the time of pressing switch S1 will be delayed, causing the bell time to be delayed. The time can be adjusted through switch S3. Each time switch S3 is pressed, the time delay can be adjusted to 45 minutes (equivalent to reducing the resistance value between the 6th and 7th pins of ICI and reducing the time constant).

One end of the 220V AC bell is connected to the normally open contact (N/O) of relay RL1, and the entire circuit uses a 12V regulated power supply or a battery as power supply.