I'm new to Logic gates and digital electronics and could use some help please!

[JustAddZero]

Hey! I'm really new to using logic gates but I have recently started playing with a logic simulator and thought it would be a good problem to start with to make an LCD output count to 8 using logic gates.

I have attached a picture of all of the logic gates available on the app and a (hopefully) clearer explination of how the LCD component works.

All constructive comments will be appreciated. Thank you in advance!

 

[hevans1944]

May we assume that you can "use" the logic elements more than once to create a digital "circuit" for your emulations?

Regardless, what you need is a four-stage binary ripple counter to count from 0000, 0001, 0010, 0011, 0100, 0101, 0110, 0111, 1000 which when applied to the binary coded inputs of the simulated "LCD" display will count 0, 1, 2, 3, 4, 5, 6, 7, 8. It will count higher after reaching 1000: 1001, 1010, 1011, 1100, 1101, 1110, 1111 which represents (in hexadecimal notation) numbers 9, A, B, C, D, E, F. Counts after F "roll over" to 0. Your "LCD" may not display counts greater than 9 correctly. The biggest problem I see is your available logic elements are combinatorial logic only. You need four flip-flops that you can "clock" to serve as a four-stage binary counter. Depending on how the simulator evaluates the states of the logic elements, you may be able to construct a D-type flip-flop using combinatorial logic and the "Memory Cell", but nothing is guaranteed.

If you really want to learn digital logic, I would give up this simulator and go play with real components. CMOS logic is fairly forgiving and readily available. I would start with a solderless breadboard, a few CMOS logic components, a handful of LEDs (so you see what the outputs and/or inputs are), and a handful of 1/4 watt film resistors in various resistances from 10 ohms to 10 Megohms. You might want to throw in a mixed bag of electrolytic, ceramic, and film capacitors of various values, and a few signal and power diodes. All this "stuff" is available in form of electronics tutorial kits, most of which you can purchase online for considerably less than a hundred dollars or a few dozen quid. If you do decide to putter around with digital logic, purchase a CMOS logic probe. This makes it easier to poke around to find the binary state of inputs and outputs. A really good logic probe will also have a pulse capability that can force a logic zero state or a logic one state for a few nanoseconds, which is really handy for testing counting circuits, flip-flops, etc.

Welcome to Electronics Point and the never-ending wonder of digital electronics.

 

[Geonovast]

You could also download Multisim Blue from Mouser (which is free). It's basically Multisim 13, but with much fewer components and other limitations. However, it has a 7 segment LCD display, a BCD to 7 segment converter (74LS47N), and a whole mess of logic devices.

I just found out about Multisim Blue a few days ago, since I've long since lost my Multisim Student key and was looking for a free alternative.

That being said, once you get the basic functions down and see how they work together, definitely go play with the real stuff. Doing this on a breadboard is much more rewarding than on a screen.

 

[Merlin3189]

I would applaud your intention here, but say that it is a bit misguided. To build a 4 bit counter needs 4 flip flops, as Heavans has said. Now learning to build a flip flop from gates is a good idea, but once you've done it (and played around with it and understood how it works, which is a whole lesson or more in itself) then there is no value in building 3 more exactly the same. You may as well start using the flip flop as a basic unit.
Since you don't have that (I think), then you either need a more advanced simulator or to go on to the real components as Heavans said.
Similarrly, once you have used flip flops to build a counter (and played etc.) then that becomes a basic unit which you can use for more complex circuits.
In fact there are several type of flip flop and types of counters, so sorting them out should keep you busy for a while, without getting much more complex. ( SR, T, D & JK flip flops, simple encoder/decoder and half adder are all worth making from gates; ripple counter, synchronous counter, shift register from flip flops and gates.)
And once you are interested in more complex logic circuits, often just ( buying it on a chip or using it in a simulator) checking out all its operations is quite enough work, without building it from simpler bits.

One warning I'd give if you are going to follow Heavans advice and use real cmos, is don't leave inputs unconnected. It's the biggest problem I've found in students building logic circuits. It leads not just to circuits that don't work, but much worse to circuits that behave erratically, which is even more frustrating.

 

[hevans1944]

... One warning I'd give if you are going to follow Heavans advice and use real cmos, is don't leave inputs unconnected. It's the biggest problem I've found in students building logic circuits. It leads not just to circuits that don't work, but much worse to circuits that behave erratically, which is even more frustrating.

I learned on TTL but didn't think this was used much today. If you can get your hands on this logic family, it is a little more forgiving than CMOS. With any logic family, it is always a good idea to either connect unused inputs to ground, or pull them to logic 1 level using a resistor between the input and +Vcc. This is especially important with CMOS as @Merlin3189 pointed out. With TTL, an unused input left unconnected will generally behave as if connected to a logic 1 level, but this is not guaranteed and unconnected inputs are more vulnerable to noise.