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Very basic circuits (74HC04)

2017-07-18 20:36  
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This column is intended to be a place to find really basic
information on subjects related to robotics and electronics. If
you have questions about the basics on these subjects, this
column is for you. I invite you to submit questions or
information to this column. Send me some mail.

The Pull-up Resistor

This time in Very Basic Circuits, I would like to talk about
pull-up resistors. The basic function of a pull-up resistor is to
insure that given no other input, a circuit assumes a default
value. Actually, there are tw1o flavors of this circuit. A
pull-up and a pull-down. Their function is the same, to create a
default value for a circuit, but one pulls the line high, the
other pulls it low.

basics0 Very basic circuits

A floating input gate. Not Good!

Consider this schematic. The gate U1A has an input (pin 1) and
an output (pin 2). The input state of most logic gates is called
a high impedance. This means it provides no real power of its
own. Therefore, if nothing is connected to pin 1, the value
of the input is considered to be floating. Most gates will
float towards a high state. This is a very weak condition, and
any electrical noise could cause the input to go low.

When switch S1 is closed (on), the input state at pin1 goes
low. Since there is a definite connection to an electrical
potential (in this case ground), the state of the pin is stable.

When switch S1 is open (off), then input pin 1 is susceptible
to a wide array of electrical problems. The traces or wires
connected to pin 1 may very well allow enough electrical noise in
(by acting as little antennas) to cause pin 1 to incorrectly
switch states. What is needed here is a way to connect pin 1 to
an electrical potential that can be removed when the switch
is closed. This electrical potential will allow the pin to keep a
steady state.

basics3 Very basic circuits

A very bad idea!

One thought is to tie the pin to Vcc ( 5 volts) to insure that
pin 1 doesn`t float. The circuit to the right certainly does
that. With pin 1 tied directly to Vcc, the line does not float,
and has an ON state.
The problem with this circuit is
what happens when switch S1 is closed. This creates a direct
electrical connection betw1een Vcc and GND. In other words, it
will short out the circuit. If you are lucky, it will just
stop your entire system from working. If you are unlucky, it
will burn up the wires!

The problem with short circuits is they allow too much current
to flow from Vcc to GND. This causes heat to be generated, which
can sometimes burn parts, wires, or even start fires. In
addition, most circuits fail to function correctly because the
voltage at the power supply drops to zero. In general, this is a
bad situation!

basics1 Very basic circuits

Pull-up resistor limits the current

Now consider the next schematic, which is similar to the first
but has added a pull-up resistor. This resistors function is to
limit the amount of current that can flow through the circuit.

When switch S1 is open (off), pin 1 is tied to Vcc through the
resistor. Since pin1 is a high impedance input, a voltage meter
or logic probe placed on pin 1 will show Vcc ( 5v) if connected
to pin 1.

When switch S1 is closed (on), pin 1 has a direct connection
to GND, which takes it to the low state. The pin1 side of R1 also
has a direct connection to ground. Current will flow from Vcc,
through R1, and to ground. It isn`t considered a short, however,
because R1 will limit the amount of current that can flow to a
very small amount. In fact, you can compute this using Ohms
law.

I = V / R

I = 5v / 10,000ohms

I = .0005A (.5mA)

A variation on this them is a pull-down resistor. Just like
the pull-up resistor, it is used to limit the current that can
flow betw1een Vcc and ground. Though less often used, it is still
a valid thing to do.

Most digital circuits use a 10k or a 47k resistor for pullups.
The exact value doesn`t actually matter, as long as it is high
enough to prevent too much current from flowing. 10k seems to be
the most common, but if you are hoping to save as much power as
possible, the a 47k resistor may be right for your application.
In some cases, you can go higher, but then you are
depending on characteristics of the pins on the chip.

In Summary

You will find that pull-up resistors are extremely common is
most digital circuits. The key function for a pull-up is to
prevent input lines from floating. The key function for the
resistor itself is to prevent too much current from flowing
through the pull-up circuit.

basics4 Very basic circuits

The less common pull-down

The Current Limiting Resistor

If you have just read the above article, you hopefully
understand how a resistor limits the amount of current that can
flow through a circuit. In the above example, we were dealing
with input pins. Expanding on that idea, I would like to present a
simple circuit that use limiting resistors with output pins and
also with an LED.

basics5 Very basic circuits

Unlike an input pin, which has only high impedance, an output
pin is designed to have tw1o states: Drive (on, or high logic)
and Sink (off, or low logic).

Let us look at a simple digital circuit, the 74HC04 Inverter.
The picture on the right shows the symbol for the 74HC04.
Internally, this chip is constructed using transistors. I have
taken great liberty by reducing the circuit to a single
transistor version, T1. Digital circuits use transistors as
switches. When a current is supplied at pin 1, the transistor
allows current to flow from pin 2 to GND.

I have included a picture of a switch as well. Closing the
switch allows current to flow from pin 2 to GND. These are
functionally equivalent circuits. The actual implementation of
the 74HC04 is much more complex, but the basic ideas presented
are still valid.

In digital circuits, output gates are switches.

In the above drawing, you can see that the equivalent circuit
using the switch demonstrates that the internals of an output
gate use a pull-up resistor just like I described in the first
article. By replacing the original push button switch by the
equivalent transistor circuit, it looks like the schematic on
the right.

basics8 Very basic circuits

A quick analysis shows that when T1 is off, R2 pulls the
output pin high, which is good for U2A`s input pin. When T1 is
on, T1 ties U2A`s input pin to GND, which brings the input pin
low. R2 allows a little bit of current to flow.

Note that on the real 74HC04, the pull-up resistor R2 is
internal to the chip. Therefore, you can directly connect the
output of most integrated circuits directly to the input of
others without an external pull-up resistor.

There are times when you connect the output of a gate to a
device that isn`t the input of another device. For example,
driving an LED, is an example.

An important consideration when connecting anything to an
output gate is what will happen when the gate drops to logic
zero. This basically creates a direct line to GND, just like the
switch example. Before you connect anything to an output gate,
you should consider how much current will flow through the
gate in the zero state. Quite often, you will need to add a
current limiting resistor to insure that you don`t burn up the
part.

Most logic parts are capable of handling around 20mA of
current per pin. This means that you need to consider carefully
the device being attached.

How to make an LED work

basics9 Very basic circuits

An LED (Light Emitting Diode) is a semiconductor that emits
light energy when a current flows through it. Current will only
flow one direction, just like a regular diode. There are a few
things you need to know about an LED before you use one. First,
and most importantly, is that an LED has very low internal
resistance. This means that left to itself, an LED will pass so
much current that it will burn up. They require an external
resistor to limit the current.

Most LED`s have a current rating, which determines the size of
the resistor you will need. The current rating tells you what
the maximum allowable current for the part is. In general, the
higher the current, the brighter the LED.

Most LED`s seem to handle at least 15mA. If you are using a 5
volt circuit, then Ohms law tells you what resistor value to use.
R = V / I, so R = 5v / .015A = 333 ohms.

Now let us consider what happens when using the output of a
chip, such as the 74HC04, to operate an external device. For
example, the circuit on the right drives an LED. When the gate is
HIGH, then there is no path to GND for cathode of the LED L1.
When the gate is LOW, then output pin 2 is connected to
ground, and current flows.

basics10 Very basic circuits

Since R3 only allows 15mA of current to pass, the gate is safe
from being overloaded. Remember that most gates can handle 20mA
of current. The same holds true for most microcontrollers.

In Summary

The output from gates acts very much like a switch. When the
gate is logic low, it goes to ground. Most gates can only handle
about 20mA of current without burning up. You should always
understand how much current will flow when the device is
connected to a logic low gate.