# Resister value help

Discussion in 'The Projects Forum' started by Gadersd, Dec 8, 2012.

Dec 8, 2012
98
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Hi. I am very new to electronics so please have your answers simple. As of now my ultimate goal is to build a full adder. I first want to create a not or inverter logic gate. The picture of it is included in the post. In case you cannot figure out what the resister values are the one above the collector of the transistor is 290 ohms, the one right of that is 100 ohms, and the last one is 1k. The voltage is 9v. The transistor is a 2n2222 transistor so the ic max is 800 ma and the vceo is 40v. Is this the right way to do it and are the resistor values correct?

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Oct 29, 2009
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3. ### WBahn Moderator

Mar 31, 2012
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Whether your resistor values are good depends on your performance specifications. Assuming that you are going to build the basic gates, such as a NAND, and hook those up to create larger logic blocks, then you need to consider the I/O constraints. What this entails is looking at the inputs of your gates and determining how much current you need to sink or source in order to get the gate to do its job and also what voltage at the input that happens at. Be sure to give yourself some margin. Then you have to decide what your fanout is going to be, meaning how many inputs is a given output going to need to be able to drive? With that, you then size your output resistors to allow that to happen.

Another big factor would normally be sizing the resistors to meet speed and power considerations, but I'm guessing that you can tolerate pretty slow gates and can therefore choose pretty large resistors to keep the power consumption tame.

Unless you really, really want to make a full adder from transistors and resistors (and if that's the goal, then more power to you and keep us informed of how well it goes), I would recommend taking a bootstrap approach in which you demonstrate that you can make, and understand, how a functional NAND gate can be built from transistors and resistors (realizing that such RTL -- resistor-transistor logic -- is never used for logic families anymore, though it is sometimes still used for tiny "glue logic" here and there) then use some NAND gates from one of the logic families, such as 74LS00 or 74HC00, and learn how to make, and understand, how you can make all of the other basic logic gates from just NAND (and also just from NOR) gates. Then learn how you can make small combinatorial functions, such as half-adders and full adders and multiplexers and encoders and decoders, from the basic logic gates. Also learn how you can make simple latches and flip flops from basic gates but, after that, use commercial parts, such as the 74xx74, from that point on.

The goal is to understand how each step of complexity can be built from lower levels and, once built, it can then become a "block box" building element for future steps. That way, at each step, you are focused on the logic appropriate to understanding and implementing that step.