Basically, I have a big project going on in my campus which we were told to make digital counter. So when the timer counts reach at 10 minutes (by this case, the timer counts is shown by 7-segment display), it will produce alarm sound or buzzer sound, and when it continues and reach to 15 minutes, exactly doing the same by how the timer goes to 10 minutes. Any ideas guys on how to make the circuit? I'm kinda stuck here. (We aren't allowed using an IC NE555, but we are told to use 7-segment display, logic gates IC like IC 7408, any basic IC. And I'm on Digital Electronics course, I'm still learning about basic logic gates, boolean algebra, XOR and XNOR gates, multiplexer and duplexer. My reference book is Digital Electronics, a practical approach with VHDL by William Keiltz)
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\$\begingroup\$ What are you allowed to use? (microcontroller? just a crystal? fpga? ...?) \$\endgroup\$– SwedginCommented Oct 23, 2019 at 9:54
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\$\begingroup\$ no microcontroller, just a simple breadboard and some IC circuit, but not NE555 (Thanks for the review) \$\endgroup\$– Fahad NajedCommented Oct 23, 2019 at 9:56
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\$\begingroup\$ You say "digital" and "exactly 10 minutes". How precise does it need to be? And you need to a lot more specific about the ground rules. "Some IC, but not 555" leaves open a lot of possibilities. \$\endgroup\$– Dave TweedCommented Oct 23, 2019 at 12:42
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\$\begingroup\$ I'm sorry, it doesn't need to be so precise, but it's like when the circuit reaching 10 minutes, and it exactly turns to 10 minutes and showing it's already 10 minutes (by this case it's 600 seconds because I'm gonna use 7-segment display), it will rings an alarm or a buzzer which alarms a sign that we already passed 10 minutes. Same by this case when it continues to 15 minutes, and when the circuit turns to 15 minutes, it will ring an alarm or a buzzer. I'm really sorry guys for my confusing question, I'm really a new in this site. But thanks for your contribution \$\endgroup\$– Fahad NajedCommented Oct 23, 2019 at 23:31
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\$\begingroup\$ What's the common denominator for 10-min, 15-min? You really only need a two-bit counter, clocked by a 5-min timer. That makes decoding the 10-min mark and the 15-min mark very easy, because the counter has only two outputs. The counter outputs change every 5 minutes. The 2nd part of the project involves driving LED, buzzer. The 3rd part of the project involves constructing a digital clock that increments every 5 minutes. Divide your complex project into smaller parts like this, each of which can be individually tested. \$\endgroup\$– glen_geekCommented Oct 26, 2019 at 14:39
2 Answers
You could use a crystal (lets use 20kHz in this example). Connect a 32bit counter to the crystal and check when the output is equal to 12000000 (=60*10*20000; 10 minutes) or 18000000 (15 minutes).
So for the 10 minute buzzer:
The output of the counter should be 1011 0111 0001 1011 0000 0000
Add logic to the output of the counter to see if the output equals to this binary number. As in: add a inverter on the line where the corresponding bit is 0 and a buffer if the corresponding bit is 1. Add those lines together to an AND-port, if this output is 1, then the 10 minute mark is passed. Be aware that this 1 is only high for 1 period of the crystal. Use it to start some other logic to let the buzzer buzz.
Similar for the 15 minute buzzer.
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\$\begingroup\$ so what's a 32bit counter again? is it a breadboard or another thing? In this case, maybe we aren't going to use an Arduino or laptops which can can code it \$\endgroup\$ Commented Oct 23, 2019 at 10:18
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\$\begingroup\$ what's the name of the IC should I use? I searched it on google and still confused of which crystal IC it is \$\endgroup\$ Commented Oct 23, 2019 at 10:20
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\$\begingroup\$ first link googling
32 bit counter icti.com/lit/ds/symlink/sn74lv8154.pdf, for the crystal look on your suppliers website. (an example mouser.be/ProductDetail/ABRACON/… note that this one is 4MHz) \$\endgroup\$– SwedginCommented Oct 23, 2019 at 10:39 -
\$\begingroup\$ what if I use a 20 MHz? How should I connect it in breadboard? \$\endgroup\$ Commented Oct 24, 2019 at 4:06
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\$\begingroup\$ You'd need a 32 bit counter and a 8 bit counter. Same thing. A 20MHz crystal oscillates 20e6 times a second. So for 10 minutes:
10*60*20e6is the value, and alas this is bigger than a 32bit number can hold. So you need a carry counter or a lower frequency crystal. \$\endgroup\$– SwedginCommented Oct 24, 2019 at 7:21
You may not be allowed to use an IC 555 timer, but you could just breadboard one yourself.
See: https://www.electronics-tutorials.ws/waveforms/555_timer.html
As you can see, the main components you would need are a couple of comparators, an NPN transistor, and an SR flip-flop. The output driver would basically be another transistor which you would use to drive your buzzer circuit.
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