# How do you make a digital counter circuit which counts to 10 minutes and 15 minutes, it will give a signs buzzer? [duplicate]

I asked in this site before, but apperantly I don't quite understand. I want to make a digital counter circuit which it counts to 15 minutes for my big project in my campus . When the circuit counts to 10 minutes, it will give a signs a buzzer. Same when it continues to 15 minutes, it will give a signs a buzzer again. (the circuit is for public speaking, so the speaker knows that he or she already went speak for 10 minutes and 15 minutes). We weren't allowed to use an Arduino, Microcontroller, PLCs, and IC NE555. The requirements of this project is to use breadboard or PCB, LED (this LED acts as a sign buzzer), each IC Decoder and each IC counter (not a built-in IC Counter and Decoder, so it's a seperate Counter and Decoder), any type of flip-flop (SR/D/JK) wether it's counter UP or DOWN, 7 segments display and 12 V battery. I'm kinda stuck here and I'm thinking it's impossible to make such a thing. Do you have any idea guys? I had an idea to use crystal oscillator, but I still don't understand

• It's not at all impossible. You can even do all this with nothing but relays and wiring. Or even cams, gears, motors, wheels, and shafts. Let your imagination float!! – jonk Oct 26 '19 at 5:36
• Thanks man, I know, but it gets hard for me to think for making this circuit. And also I'm still in third year of college in Engineering Physics. I'm still a beginner – Fahad Najed Oct 26 '19 at 5:55
• Are you barred from using the 74121? Just curious. Outside of that, are you allowed to use discrete parts? And what kind of precision (or, better, accuracy) are you required to achieve with the value of $10\:\text{s}$ and $15\:\text{s}$? – jonk Oct 26 '19 at 6:01
• My mistake. I meant minutes, not seconds. Sorry about that. You might also look at an answer I provided here. It provides a method that would easily provide minutes worth of delay. Combined, I think you could achieve your goals with discrete components. – jonk Oct 26 '19 at 7:03
• If you have additional information related to your original question, use the "edit" button to add the information there. It was put "on hold" in order to allow you to improve it before people try to answer it. Do not create a duplicate question. – Dave Tweed Oct 26 '19 at 12:10

I have a idea I don't know if it's works or not but I will test it on my own, use 1mF capacitor and 600k ohm resistor to charge capacitor technically it will take 10 minutes to charge capacitor and connect a not circuit into capacitor's output. So when capacitor charged not circuit will output high signal and connect that signal into MOSFET's gate and use MOSFET for opening flip flop circuit. So basically capacitor trigger flip flop circuit and flip flop circuit trigger buzzer.

I used this circuit for 10-60 second but I never tried for 10 minutes or longer. If you think you can use this circuit I can add schematics

• That's a great idea by the way, I would like to try that in my simulation software, so what I understand from you is that I build all those components in a schematic way (or possibly in breadboard). And when I apply 12 V into the circuit, it will charge capacitor for about 10 minutes, when it went 10 minutes, it will trigger the buzzer, isn't it? – Fahad Najed Oct 26 '19 at 7:05
• Yes but I made some tests and it's not reliable, +-%10 tolerance. Also capacitor triggers MOSFET not buzzer. I chose MOSFET because once MOSFET gate is triggered MOSFET stay opened. MOSFET will work as switch to flip flop circuit. When MOSFET opened flip flop circuit trigger buzzer constantly and you will get buzzing noise. But circuit designed for one time use only , you have to discharge MOSFET and capacitor for reset – Mordecai Oct 26 '19 at 8:18
• You have discovered that calibration of a long time-constant RC circuit is painfully slow. That's why anything sensible will use a fast clock and a counter to get the required delay. – Transistor Oct 26 '19 at 9:54
• Is it possible to make simple counter circuit then ? It will really change everything for me – Mordecai Oct 26 '19 at 18:29

You said you are thinking to use a crystal oscillator, so you seem to be allowed to use such. This is a good starting point for a task in a digital course.

You add enough counter stages to divide the clock down to a period at least of 15 minutes. The number of stages depend on your crystal's frequency. In the way you might like to have intermediate periods of 1 second and 1 minute to keep the design simple.

### Let's do an example:

If you choose a typical clock crystal it will have 32768 Hz. So you need 15 binary dividers to get down to 1 Hz because each stage divides by 2: 2^15 = 32768. There are some ICs that contain 4 stages while others have more. Look through the 40xxx series, some of them are also available as 74HC40xxx. A few even include an oscillator which might be usable with a crystal!

Now we are at a 1-second period. This will be divided by another suitable counter by 60 to get a period of 1 minute. There are counters to divide by 10, and IIRC another IC divides by 6. But presumably it is part of your task to design such a divider from flip-flops and logic.

Having reached a clock of 1 per minute you now need another counter that is able to count 15 clocks. This needs 4 stages, one IC with 4 flip-flops suffices. The next part of your task is now to design a decoder for the value of 10, and another decoder for the value of 15. I would add another logic to stop the counter after that.

The outputs of these decoders will most probably be gated by one of the earlier stages' output to let the buzzer sound for a shorter duration.

That's it. Good luck, and have a lot of fun!

### Note

The IC series to look into exclude the most common 74HCTxxx because those have a limited supply voltage range. The 74HCxxx (without the "T") may support 12V. You have to consult their data sheets!