Basic Logic Circuit

Okay, so this is my basic basic logic circuit. It contains one logic gate and all it does is....

If two batterys are plugged in (in the proper places) then it makes the beeper beep... If only one or no batterys are plugged in then the beeper doesnt beep.

Now, my question is does this design (picture) look right? I think im missing a transitter somewhere or something like that...

design pic:
[IMG](URL address blocked: See forum rules)[/IMG][/URL]

Thanks, daniel

Dude. No homework please. I would love to help, had it not been for your homework. And this problem is so basic, you don't even need us. Read your book, and that should be sufficient.

Honestly, This isn't homework (note all the other questions about Microproessors and such)...

I want to learn to make a basic microprocessor and i thought a basic logic gate would be the way to start. now, what parts would be required to build a logic gate that just checks to see if 2 batteries are plugged in and then makes the beeper go off? also, how would I do it? (look, if i can get one logic gate going then 2 shouldn't be hard and so on)...

And I know, I hate it when people post homework for others to do on forums, i see people all the time since I'm a C++ programmer and visit C++ forums regulary.

- cheers, Daniel

Further readings:

link, link, link, link.

Also, link - Lessons in Electric Circuits - Volume 3 - DIGI.

link - Automating Manufacturing Systems with PLCs - read 2nd and 15th chapter.

link - Complete Digital Design: A Comprehensive Guide to Digital Electronics and Computer System Architecture - read 1st chapter.

And, of course, be sure to check this, this and this out.

A really comprehensive image: here.

You really shouldn't miss this, this to practice and experiment, therefore deepening and enhancing your knowledge in the area of 'digital logic'.

That would be all in a nutshell. Reading the above links will guarantee you to succeed in your endeavors.

First of all, why didn't we helped? Because it sounds like a homework, even if it is not, it may be for someone else and answering questions similar like yours are violating our rules. No homeworks, please.

Second, you posted your diagram but it didn't appeared since you are a newcomer. You need at least 5 (five) posts to be able to post url's (and your screenshot contains http:// in it).

Third, try to reformulate your question, ask us specific terms and such so that we can help out easily and give you an indirect but helpful advice. Also, fix your diagram issues-- either post more until you reach 5 posts, or edit your posts and remove "http://" prefix, we know that we need to add that tag.

Good luck!

PS: This book looks very awesome, unfortunately it's very expensive. Anyway, you might want to look into...

You need an AND gate. Period. For the AND gate, from a CMOS perspective you would need 6 transistors. Read more here.

Cheers, Ive been a C++ programmer for awhile now, and lately ive been thinking... hmmm wats inside that silicon chip on my motherboard... well im slowly learning.

- Cheers, Daniel

It all begins with Transistors. They work as voltage controlled switches. Can be programmed to behave almost in any pattern. We covered the discussion here. this thread.

Oh why not! A NAND gate coupled with an inverter - 6 transistors circuit. CMOS works on the fundamental of providing inverted outputs. It is easy or more practical to create a NAND gate and then invert it. The same can be done using NOR gate too. I am sure that link on Play-hookey will be very informative.

Edit: Yes. I just re-read my previous post and realized it was a mistake. 4 transistors is not possible. Sorry about that.

I honestly dont get this logic gate thing... I have been looking thro digital logic on play hookey and it shows the picture of an AND gate. But I dont get how the AND gate works. Where are all the transistors?

I've got a basic idea but not perfectly sure. Also if I was going to use 2 1.5volt batteries as my 2 switches (as the A and B) would i be using 1.5v or 3v transistors?

- Thanks, Daniel

There are a myriad of ways to build a logic gate. The AND gate you are looking at (that is, if this is what you are looking at) is composed of two diodes, instead of transistors. It is as effective as a transistor-based AND, but cheaper to produce and easier to understand.

The gate requires both inputs to be HI (in this case, +5vdc) for the output to be HI. If there is a LO on either or both inputs, then there will be a voltage drop across the resistor and the output will be LO.

If you look over here, you will find some transistor-based logic circuit diagrams, such as this one-- a three-input AND



In microcircuits, gate-type transistors are used, such as FETs, instead of the traditional bipolar models demonstrated here.

I would seem like a bigger pain to implement such a small circuit in CMOS and then have to worry about packaging and everything, and actually finding some place to fab you one of these


Or did you mean do it CMOS style, but non-integrated, using BJT's or something? Don't know if there even are complementary BJT's... I guess discrete FETS could work??

TBH, I don't have an answer to your question right now, arch. I will look up some more, but I am sure in the mean time D1 will come up with some sure-shot reply. He has worked in electronics far deeper and for far longer than I could even imagine.

My feeling is that NAND as a CMOS devise is just an idea to get your started. And that's a basic block for more complex logic circuits. So, as D1 put it, and as in all circuits at Play-hookey, it is constructed using complimentary transistors. But as a stand-alone component, it is made as shown here. Here's another example of it as IC 7400.

I hope it helps answer some of your questions. I will definitely look up more..
Thanks for bringing that up. I never thought of it..

This site should also be insightful to you.

By the way, Drezard, we still haven't seen your prior diagrams. You should have enough posts by now to 'post' again without cutting down the 'http://' prefix, otherwise do so and post it. We need to know what we're dealing with to provide you with better advices.

OK..just to make sure..I thought you were going to have a circuit where if both the buttons were pressed the beeper would turn on. That's a different circuit.

Transistors came into the picture because we thought you wanted to learn how microprocessors work. To learn about any digital logic circuit you need to learn about gates, which are made of transistors [for the purpose of understanding at least].

Now: Let's put gates on a side for the circuit you are talking about. You can have two buttons in parallel, and connect beeper to both of them. So, when you press either of them, the beeper turns on. To have a circuit where both buttons need to be pressed to turn the beeper on, these buttons will be connected in a series fashion.

If you read more, you will realize that gates run on the same basic principle of series and parallel connections. AND gate for that matter represents a series circuit, while OR represents a parallel circuit.

The basic equation for an AND gate would be
Output = X*Y [so Output is 1 if both the X and Y are 1] ------------ (1)
The same for an OR gate:
Output = X + Y [ Output is 1 if either X or Y is 1] ------------ (2)

Back to your question about transistors: Transistors work as a voltage controlled switch. When a negative voltage triggered* transistor is coupled with a positive voltage triggered* transistor, you achieve a circuit that works for both 0 and 1 cycle of an alternating input, or clock. But in this design, better known as Complimentary MOS, the output is negative logic. So, you can basically build NAND and NOR gates, which are negative-AND and negative-OR.

The equation for NAND gate:
Output = ~(X*Y) ----- (3)
The equation for NOR gate:
Output = ~(X+Y) ----- (4)
The equation for simple Inverter:
Output = ~Input ----- (5)

By now if you have looked at those circuit in those links, you would have realized that in a CMOS circuit, you have 2 transistors per input.

Following simple logic, if you have NAND and Inverter available, the way to create AND gate is to feed output of NAND to an inverter so that your output is complimented. Therefore from equations, AND = ~(NAND)
For 2 input circuit, you would be using 4 transistors in your NAND gate, and 2 in Inverter.

The discussion so far was for a digital logic circuit. If you are implementing the same, you would get ready-made AND gate, or NAND gate. Inside those gates, probably CMOS technology will not be used. So for your purpose, this much discussion should be more than sufficient. What you need to know is the circuit at an abstract level, and how it can be used for your design.

Feel free to shoot questions.

Edit: Why did you remove the question after I made painstaking efforts to type so much!!!

Yea, but I was using the buttons and the beeper as an example to learn how logic gates work. I know that a circuit like I explained would be 100% useless but its the knowledge principal behind it.

I still don't understand what kind of transistors i would be using to create my logic gate.

- cheers, Daniel

Dude. Tell me for once what you are using for creating gates.

If you are creating a software such as Pspice, you will be using a library component. It is necessary to state here that it is impossible to create physical gates at home. I am not sure if it's just me, but I am still unclear about your actual purpose. If you only want to learn, those links and explanations are the best way to start and after that lot of things are self-explanatory. But you CANNOT create logic gates at home. You can buy couple of ICs and understand their working, but you cannot fabricate them.