Basic design questions

I've always wondered what exactly is needed for a save and "professional" design when building even simple circuits.

I do understand the basic concepts behind "current limiting" and "pull up/down" resistors but I never know when to use them.

Usually when I look at schematic I tend to get pretty confused with all the resistors and capactiors becuase I don't know what they are used for. Is it just a matter of recognizing the basic building blocks of the circuit(such as "That part is an inverting amplifier", etc...)?

An example I have is a simple toggle circuit I'm using in something I'm trying to make. Right now I'm just simulating the stuff so I can get an idea of the parts I need and such.

it consists of something the like the following.

VCC --- Momentary Switch --- Toggle Flip flop --- resistor --- GND

the idea is simply that when I push the momentary switch the flip flop will toggle its state and hold it. The Switch is connected as:

Switch ----- T | --- CLK

The reason is that since the IC FF I'm using is rising edge triggered I have to set the clock so I can switch. This may not be the best way and I could just directly connect it to VCC directly but I'm not sure if it wastes more power there or not? (I have a feeling that maybe in the real world this method won't work because of the latency in switching... maybe the clock will turn off before the IC is able to toggle or something and hence cause some problem).

Another thing is that I'm wondering if I need to current limit the connection between VCC and the switch. Ohms law tells me that there would be infinite current going into that branch if there is no resistance but I'm not sure how "resistance" works when it goes through an IC. I surely could put a small resistor there to make sure it is limited but then I'm afraid of wasting power(maybe in this case its not much but) but I definately don't want to burn up the IC. I also don't want to waste any extra components(such as adding a current limiting resistor) when there is no need for one(i.e, if the T and CLK have high impedence themselfs). How do I go about figuring out what to do for IC's? do I treat it as an open connection such as

Switch -----o T | ---o CLK

I guess I have to also make sure the pins don't float either ;/ (and maybe debounce the switch ;/)

Is there any book/web site that goes through the pratical aspects of design?

Thanks, AD

There is no need for a resistor between the flip flop output and ground.

What is the part number of the flip flop that you are using - knowing that would enable us to give you useful suggestions, rather than random guesses.

With TTL logic, you normally have a resistor (3 - 5 K) from the input terminal to Vcc (+5V), and connect the switch between the input terminal and ground.

I'll make a wild guess that you're working with a 74LS74 or similar D flip flop. For that part, if you want it to toggle, you connect the "D" input to the "not Q" output, and wiggle the clock input.

If you have a CMOS part (74C74 (or HC or AC)), the set and reset inputs must be pulled high - connect them either directly or through a

5K resistor to Vcc.

Logic IC inputs are generally high impedance, so do not require a series resistor to limit current. As I stated above, you do need a resistor between the input and Vcc if you also have a switch from the input to ground - the resistor limits current through the switch, as well as ensuring that the input goes high when the switch is open.

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I was doing that so I could measure the voltage drop across it. Ofcourse I guess I don't need to do that but I wasn't thinking I could measure it directly off the pin for some reason at that time.

not sure. Its just T_FF in multisim. I'm using one of those virtual ones I guess.

i.e., a pull up/down on the pin so it doesn't float?

yeah, T can be set high and just flip the clock. basicaly what I was doing in a round about way(T was set high immediately when the switch was pulled)... I suppose its safer to set T high all the time though.

yeah. I read about that somewhere and tried it earlier. now I'm working on debouncing the switch.

Thank, AD

so are most resistors, say on any random PCB, used for pull up/down and current limiting? when I look at a pcb board I tend to kinda get scared when I see tons of resistors and capacitors and have no clue what they are used for. I can recognize transistors and ICs pretty easy ;) and generally think that transistors are used for amplifying things and the ICS are probably memory/cpu/etc... But when it comes to the resistors and capacitors I don't have a clue what they could be used for(Although I'm sure some are used for filtering and stuff but surely not that many?).

ok.

yeah ;/

ok.

Are these samples real? I mean are they all the stuff that would be used or is it like a "scaled" down version where they expect you to know that you need to add resistors here and there and stuff?

Thanks, AD

In a digital logic circuit, most resistors will be used for pull up/down or for time constants (in oscillators and timers), and most capacitors will be for AC bypass (that is, for making sure that the DC power supply lines actually behave like DC) or for time constants.

In analog circuits, they serve many more functions than that. Resistors, capacitors, diodes, and transistors are the basic building blocks of circuitry (not the only ones - for instance, inductors are important too, and there are other sorts of components as well), so asking what they are used for is sort of like asking what metal and concrete are used for in a skyscraper. Most of the metal in a skyscraper is used for structural support, but metal is also used for plumbing, wiring, elevator cables, and so on. The "integrated circuits" are just that: circuits, made of resistors, capacitors, diodes, and transistors, which have been shrunk down and implemented in a hunk of impure silicon.

But you should also understand that a circuit works because these components interact. It's not really right to think of, say, a transistor as amplifying something and a resistor as current limiting: rather, you should think of a particular combination of resistors and transistors as forming a functional block, like a flip-flop or a switch or an audio amplifier or a clock oscillator.

They are scaled down. But most of them show at least most of the "extra" stuff you need to make it work in the real world. Most of the sample circuits will work if you build them as is, they just might not work as well or reliably as you'd hope.

It is very educational to find the schematic for an actual product, and try to figure out what all the parts are used for. "Art of Electronics" (which I mentioned earlier) includes some complete schematics, with discussions of all the details. Some manufacturers put their products' schematics online, though of course without any discussion. It is easy to find full schematics and service manuals for older Tektronix oscilloscopes - these are often extremely complex, but contain a lot of discussion of how the circuits work. Of course a lot of this circuitry is analog, but scopes such as the 2245 also include plenty of digital circuitry.

This is presuming the left wire to "Toggle Flip flop" is its input, and the right wire is its output. You may have correctly interpreted it, but it's unclear WHAT the wiring is.

OP (this means you, Mr. Dissonance), get one of those ASCII circuit schematic drawing programs, and draw us a "real" schematic.

This will

In this case the button will follow the D input on the appropriate edge of the clock. A more interesting circuit is to tie the D input to the not-Q output, the 3k resistor between VCC and the clock input, and the pushbutton between the clock input and ground. The output (if driving an indicator such as an LED, with a 470 ohm resistor in series, of course), will appear to randomly change about half the time. Can the OP determine why, before connecting a two-channel sampling scope to the input and output?

You don't need "resistance", just the current to make the input "logic 1" or "logic 0". It's sometimes different for each logic level. Data sheets will have this. For TTL it's a current, for CMOS it's a capacitance. Perhaps you should get one of those old TI hardback data books, they have the actual circuitry in the chip with transistors and resistors and such. It was Good Stuff, except when they started using those silly IEEE logic diagrams.

Look on the data sheet for the max and min input currents for input-high and input-low. For CMOS (4XXX, 74C, 74HC, 74HCT, etc.) inputs it's virtually nothing, so an input can be 'pulled up' with a

100k or 1 Meg resistor to VCC, and likewise 'pulled down' with a similar value between the input and ground. TTL (74, 74S, 74H, 74LS, 74ALS, ...) logic inputs take a little bit of current, so such high resistor values won't work. As an exercise, look at the data sheet for max input current for an input held at logic low, and max voltage at the input pin to be considered a logic low, and from these calculate the max resistor value between the input and ground guaranteed to make the input 'low'.

Dear AD,

Instead of searching through catalogues for things which aren't in it, a lot of people have fun looking at a small pile of miscellaneous components and asking themselves "Now what can I do with this."

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Reg.