automatic school bell

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ulot

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hi, i'm working with some high school students, we're trying to build an automatic school bell circuit that i got online.
The circuit used a 555 to generate a 4.5min interval pulse, and 2 decade counter to set the time for 9 periods.
HOwever, the school has 12 periods, meaning a single decade counter that work for it.
Any suggestions that could help?
 

hi,
Post the circuit you have for the 9 periods.
 
schematic

this is the schematic
 

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ulot said:
this is the schematic
Click to expand...

hi,
I see that there are 4 periods of ~45min and 1 shorter period followed by 4 periods of 45min.
It should be possible to add another 4017 to give the extra 3 , 45min periods, by connecting IC2/12 to the clock pin 14 of the extra 4017, also using diodes back to the 555 timer control.
If there any spare gates on the AND gate N1 these would have to be used to decode the extra 4017 outputs to the T3 transistor.

Do you follow OK.?
 
hi,
Look at this option for 12 periods.
Changes to wiring marked in RED
 

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I've gone through the circuit, its amazing. i thought once the enable pin goes HIGH, the counter stops operating and state values are lost, i didn't realise it retains the state in which it was in until the pin goes LOW again or until it is reset. Thanks alot.
 
hi i need the circuit description completely i have 7 periods and two breaks of 10min and 40min respectively and cant the 555 ic give the 50min delay?
 
hi how we would set the time delay to 50min,30min and 10min by varying the resistance values plz help me on this.............
 
elegantashok said:
hi how we would set the time delay to 50min,30min and 10min by varying the resistance values plz help me on this.............
Click to expand...
hi,
What is the exact sequence of time delays you require.
eg: period1/period2/............etc. include the 10min break times in the sequence..
 
i have 7 periods of 50min time. first two periods is 50min then 10 min break, then two periods 50min, then 40 min lunch break and after that three periods are 50min how we would set the time at T1 and T2 ?
 
elegantashok said:
i have 7 periods of 50min time. first two periods is 50min then 10 min break, then two periods 50min, then 40 min lunch break and after that three periods are 50min how we would set the time at T1 and T2 ?
Click to expand...
hi,
I make that, in minutes.
50,50,10,50,50,40,50,50,50

I'll look at the calculations, let you know.

EDIT:
In the original circuit in post #5, the time interval is set for 4.5mins, this will be changed to 5minutes.
Also the original circuit [4017's] had only two period types, your circuit would require 3.
This would mean that as well as T1 and T2, we would have to add T3, this just means another transistor and the timing resistors.

If you want to use this method, let me know and I will modify and post a new circuit.
 
Last edited:
yes sir you are right i also thought the same but how the resistor values will be set according to the time requried
?????
 
elegantashok said:
ok thank you sir but can u tell me that how we would calculate the resistor values plz sir.................
Click to expand...

hi,
Download and install this linked program
https://www.electro-tech-online.com/blogs/ericgibbs/125-simple-7555-cmos-timer-calculator-v1-2.html

Look at this clip showing the timing components also the modified 4017 output wiring.
Any questions please ask.

NOTE: a 1000uF capacitor can have a very wide value tolerance band, so you will need preset resistors to get the accurate timings.

EDIT.
Added timer plot
 

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Last edited:
hi sir. we need to build a school bell with these periods:
10 periods of 30minutes then 1hour break, then 9 periods of 30 minutes again.
 
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