How can I build a digital circuit with a row of LED lights that light in sequence?

Question

I am planning to build a clock with a row of 60 green LEDs and another row of 12 blue LEDs. One of the green ones will light to show the minute and one of the blue ones will light to show the hour. On the final project they will be placed in a circle, but on a breadboard I will have to place them in rows. How can I build this without having a massive number of logic gates?

Answer 1

Use Flip-Flops.  D is easiest to use, R-S, J-K, and T flops are also not too bad.  Make a state machine, Mealy or Moore, whichever you prefer.  You can build a 4-bit counter, that counts how many times the 60th LED is lit (4 bits = 16 possible states, only 12 are used).  Also, a 5-bit (5 bits = 64 possible states.  You need only 60) counter that increments after x-number of clock pulses(In order to build your clock, you'll need a clock signal that is a set frequency.  i.e. if you had a 10 Hz clock signal, your 5 bit counter would increment after 600 clock pulses.)  Now that I think of it, doing this with logic gates kinda sucks.  If I were you, I'd invest in a microcontroller that can store stuff in memory for you, and can do I/O.  That'd be your best bet.

Answer 2

Here's a 72 LED clock circuit. (scroll down a ways).

http://ourworld.compuserve.com/homepages...

I would use a PIC nanoWatt 2-5V microcontroller. Their outputs can drive 25mA, enough for most reasonable diodes. Arrange the LED's in banks: for hours, 3 banks of 4, for 7 I/O's total. For minutes, 6 banks of 10, for 16 lines total. Pick one with an A/D converter to measure the input voltage, use it to shut down outputs when power lost. Use 3V lithium button cell on with a diode for backup. They have onboard oscillator (use 32kHz) and prescaller for real time clock, and with a 40 pin device for $5, you'll have enough extra inputs for switches to change minutes/hours/seconds.

Only problem, you need a programmer. The PicKit 2 for $50 would do nicely. You may be able to pick something used on ebay for less.

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Answer 3

The microcontroller is a good idea.  Also, how about a CPLD?  You can get a kit from Xilinx for under 100 bucks.  if you are lucky you might be able to dig up a free demo board from companys like xilinx, or possibly try ebay.

Answer 4

You need at least as many logic gates as LEDs.

The logic for the green LEDs can be as simple as a logical AND of the previous LED latched ouptut and a timer that counts off a minute.  The first green LED is just a latched output of the minute timer.  The green latches get reset a minute after the last green LED is lit.  The first blue LED is latched at the same time.  Etc, etc, etc.