# Basic design questions

Discussion in 'Electronic Design' started by Abstract Dissonance, Dec 27, 2005.

1. ### Abstract DissonanceGuest

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

2. ### Walter HarleyGuest

That's part of it; and then the rest of the components are due to real-world
issues, such as the current limiting you mention.

For instance, if you were to look at a schematic for an audio power
amplifier, you'd find that about a third of the circuitry (or less!) is
related to amplifying an audio signal, and the rest is related to making
sure that there isn't a "pop" when you turn the amp on, that the speakers
don't get fried if something inside the amp dies, that the amp doesn't get
fried if the speaker terminals are shorted, that the amp does something
graceful if it overheats, and so on. Practical experience, and looking at a
lot of schematics, is about the only way to find out about all those
"real-world" factors.

Logic inputs are high impedance - that is, they behave as if they have big
resistors in series already. You do not need to add any more resistance.

Logic outputs are low impedance. If for some reason you tied a logic output
directly to ground, and the output was trying to be high, a lot of current
would flow and the IC would be destroyed.

Start with "Art of Electronics", 2nd Ed., by Horowitz and Hill. It is a
very good blend of basic theory and real-world practical issues.

Also, read the application notes that the chip makers provide. Many times
if you read the datasheet for a part, it will give you sample circuits.

3. ### Peter BennettGuest

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.

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
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4. ### Abstract DissonanceGuest

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

5. ### Abstract DissonanceGuest

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?).
yeah ;/

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

6. ### Ben BradleyGuest

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'.

7. ### Reg EdwardsGuest

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."
<smiley>

8. ### Walter HarleyGuest

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.

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