The post explains a very simple 10 stage long duration programmable timer circuit which can be used as a school bell timer circuit. All the 10 stages can be individually programmed from zero to 5 hours. The circuit can be modified in many different ways to suit other specific related applications.
The Circuit Concept
Normally in most of the schools even today the period bells are rung manually by the concerned staff or the peon. Although the job is quite traditional and is carried out without much difficulty and reasonably accurately, the concerned person has to be always on a stand by position for implementing the actions.
However with the help of a simple electronic circuit the above implementations could be made fully automatic, eliminating human intervention, thus saving a lot of inconvenience and time.
The functioning of the proposed automatic school bell period timer circuit may be understood with the following points:
At the first glance the circuit might look quite complex, but seeing it closely shows that actually it's very simple, just a repetition of identical stages for the required number of times.
We'll try to understand the top left stage and that will clarify the entire circuit within no time.
The circuit is based on the timer/oscillator chip 4060. It's wired in its usual timer/counter mode with the help of the resistors and capacitors fixed over its pin#9, 10,11.
Rx determines the time period for which the IC counts until its pin#3 becomes high.
The value of this resistor may be determined through trial and error for acquiring the required time intervals for all the relevant subsequent stages.
The design is repeated for all the stages.
However the top left stage being the first power switch ON stage is rigged with additional components.
When the push button P is pressed, the SCR latches, grounding pin#12 of the IC.
This initiates the counting procedure within the IC. After the predetermined time lapses, pin#3 of the IC goes high and also the stage gets latched via the diode connected to pin#11.
With pin#3 high, the associated transistor pulls pin12 of the next stage to ground, which in turn initiates the counting of the second stage.
The procedure is repeated exactly the same way for the second stage also, and consequently all the corresponding stages get activated serially one after the other as per the set time for the individual stages.
When the the time period for the last stage is elapsed (bottom left), the transistor at pin#3 momentarily grounds the anode of the SCR via the 1uF capacitor switching off the SCR and the entire circuit.
The situation resets the whole circuit to its original state, until the push button is pressed the next morning for initiating yet another cycle.
The second circuit shows the driver stage which is responsible for ringing the AC bell in response to the triggering of each subsequent stages in the given order.
The diode ends are connected to pin#12 of the different stages.
The moment these pins are pulled to ground by the BC547 transistors, sends a momentary biasing to the BC557 transistor which in turn activates the connected relay and the load for a brief period of time depending upon the values of the transistor base resistor and the capacitor (chosen arbitrarily here)