TOUCH SWITCH
A simple circuit to show the operation of
a NAND gate.
Before
reading the article, go to the two animations below and see how the
circuit operates.
HOW THE CIRCUIT WORKS
The chip used in this project is a Quad 2-input NAND gate. It
normally goes under the part-number
CD 4001 or HCF 4001 or simply "the 4000 family."
Only
two
of the gates are used. Read
pages 11 & 12 of this course to see how a NAND gate operates.
The second gate in the circuit has both inputs tied
together and this changes it from a NAND operation to an inverter (NOT
gate). So, there is only one NAND gate and one INVERTER in
the circuit.
When
power is applied, pin 1 of the first NAND gate will be pulled HIGH by the
action of the 10M resistor. The output of the gate will be worked out
in a minute, we now go to the input of the second gate and see it has a
TIME DELAY CIRCUIT connected to it. A capacitor and resistor
connected in series is called a "Time Delay" circuit. It does
not matter if we consider the output of U1a is HIGH or low. With any time-delay circuit, the capacitor will be charged after a
period of time and so the voltage on the input of the INVERTER will be
LOW.
This makes the output HIGH and the high is transferred back
(called a FEEDBACK LINE) to the first gate. Thus U1a has a
HIGH on both inputs and the output is LOW.
This is how the circuit "SITS" with the output LOW.
If a finger is briefly placed across the two touch pads, the voltage on
pin 1 will go LOW.
The output of U1a goes HIGH and since the capacitor is
uncharged, it takes the input of the inverter HIGH. The output of the
inverter goes LOW and this is passed to input pin 2 of U1a. In effect, the action
of the circuit is taking the place of your finger as far as U1a is
concerned and you can
now remove it from the Touch Pads.
The 10u electrolytic starts to charge and after a short period of time the
voltage on the input of U1b goes LOW. This makes the output
of U1b HIGH and this is transferred to pin 2 of the first gate.
If your finger has been removed before this "time-out," U1a will see two HIGHs and the output will go LOW. The fully
charged capacitor will be discharged through a diode on the input
line of the second gate and the circuit will sit in this state until it
is activated again.
A LITTLE KNOWN FACT
A set of diodes is present on each input line to prevent the input
voltage rising above or falling below the rail voltages.
Under normal conditions these diodes are "reverse biased" and
do not have any effect on the operation of the gate.
But if the input voltage rises above rail voltage, the top diode
becomes forward biased and "clips" the input voltage to rail
voltage + 0.65v above rail voltage. The same applies to the negative excursion. The input
voltage is clipped to -.65v
This is how the electrolytic gets discharged. It discharged
through the lower diode. When the output of the first gate goes low, the charged electrolytic
will try to take the input(s) of the second gate to about -9v (if the
rail voltage is 10v). But the lower diode on the input prevents it going more
than -.65v and thus the electrolytic is discharged through it.
See the action of the capacitor by referring to the multivibrator animations
on the previous pages of this
course.
CIRCUIT
OPERATION
When a finger is placed on the "Touch Pads" (and removed very quickly) the output
of the circuit goes
HIGH, remains HIGH for a short period of time, then goes LOW. In other
words the circuit produces a brief pulse when a finger touches the Touch
Pads.
There are two animations for this circuit:
A: The first animation shows the
operation of the circuit if a finger is kept on the touch pads for a long period
of time. The circuit "times-out"
but does not change state. This is not how the circuit is
intended to be used. It only requires a brief touch of the pads for the circuit
to operate correctly. Keep the mouse on the Touch Pads to see how the
circuit reacts.
B: The second animation shows the correct operation of the circuit. Move the mouse
quickly over the Touch Pads to see it operate. In the
first animation, the things to observe are:
1. Pin 1 goes LOW when a finger is placed on the Touch
Pads.
2. Output of U1a goes HIGH.
3. Electrolytic is uncharged and takes input of U1b HIGH.
4. Output of U1b goes LOW.
5. Pin 2 of U1a goes LOW.
6. Electrolytic begins to charge.
7. After a period of time, input of U1b sees a LOW from the "timing circuit"
8. Output of U1b goes HIGH
9. Pin 2 goes goes HIGH but NAND gate does not change because a finger is on the
Touch Pad is keeping pin 1 LOW. In the second animation, the things to observe
are:
1. Pin 1 goes LOW when a finger is placed on the Touch Pads.
2. Output of U1a goes HIGH.
3. Electrolytic is uncharged and takes input of U1b HIGH.
4. Output of U1b goes LOW.
5. Pin 2 of U1a goes LOW.
6. Pin 1 goes HIGH when finger is removed..
7. Electrolytic begins to charge.
8. After a period of time, input of U1b sees a LOW from the "timing circuit"
9. Output of U1b goes HIGH
10. Pin 2 goes goes HIGH.
11. Output of NAND gate goes LOW. THE
"TIME DELAY" CIRCUIT
The capacitor and resistor make up a circuit known as a TIME DELAY
CIRCUIT. When power is applied to the combination, the capacitor
charges via the resistor and the voltage at the join can be monitored.
The capacitor can be placed above or below the resistor and the voltage
at the join will either rise or fall during the charging process.
It does not matter if the voltage rises or falls, the end result is the
same.
We are waiting for a CHANGE from LOW to HIGH or HIGH to LOW and
the time for this to occur is the feature of the circuit.
The join of the time delay components (the point on the circuit where
the capacitor and resistor meet) is monitored by one of the input
lines of the NAND gate and in the Touch Switch circuit, the capacitor
is above the resistor. This means the voltage will fall when the
capacitor begins to charge. A point is reached where the gate sees a LOW and the output goes
HIGH.
Keep your mouse on the Touch Pads to see the operation
of the circuit if a
finger is kept on the pads too long.
Flick over the TOUCH PADS very quickly to
see the circuit operate
FACTS TO REMEMBER :
Basically there are two different types of DIGITAL GATES: TTL and CMOS.
TTL gates have a very low input impedance
(resistance) and CMOS has a very high input impedance. The end
result is they both work the same i.e. the circuit has the
came outcome, but the value of resistors and capacitors for the
biasing and timing components is
completely different. The difference can be a factor of 10 to
1,000 or more so you must design around "TTL" or "CMOS"
and you cannot replace a CMOS NAND gate, for instance, with a TTL
NAND gate without completely re-designing the surrounding
components.
TTL chips require a small amount of current to drive the
internal circuitry. CMOS chips require almost NO POWER to drive
the internal circuitry. CMOS gates require
less than a fraction of a micro-amp. TTL requires a milliamp or
more for each gate.
When designing with CMOS gates, you can consider the input
impedance of a gate to be infinity. In other words the gate
does not put any load on the surrounding circuit.
In general, a gate changes state when the input voltage rises above about 55 - 65%
of rail voltage. And when the voltage is falling, the gate changes at
approximately 45 - 35%.
If the voltage is rising or falling SLOWLY, the output of a gate can
flutter HIGH-LOW-HIGH-LOW during the time when the voltage is between
34 - 54%. This is called the INDETERMINATE zone and the
voltage should not be kept in this region.
It takes a short period of
time before a gate starts to flutter and during normal operation the voltage changes from LOW to HIGH very
quickly and the gate does not have time to start to flutter.
In the Touch Switch circuit the voltage changes from one level to the
other quite slowly. When a finger is applied to the touch pads, the
voltage on the input of the first gate rises slowly in digital terms.
And the time-delay circuit raises the voltage on the second gate very
slowly. This may causes the gate
to flutter and if this circuit is used to activate a device such as
counter module, it may produce false triggering.
Unless you know the quirks of digital chips, you will wonder where
the false triggering is coming from!
This is only a demonstration circuit and may produce multiple pulses.
WHY?
Why is it important to know how a circuit works?
Why do we go to
so much trouble to explain the operation of a circuit?
The answer is simple. You may need to modify it, adapt it or interface
it to another circuit. If you don't know how
everything operates, it will be almost impossible to connect the
two circuits together.
For instance, you may want to increase the time-delay of the Touch
Switch. For this you need to know the components that create the
delay (the 10u electrolytic and 100k resistor). Once you
know the components, you can experiment with increasing
or decreasing the values. You don't have to know the mathematics
to arrive at the time-interval as five minutes of experimenting
will produce the values but it is handy to know that
increasing the value of the capacitor or resistor will increase
the time.
Why is the layout of a circuit so important?
All the circuits we present in this course (and all our
publications) follow a very strict code of layout
so their operation can be quickly worked out. That's why we
include all component values on the circuit. Anyone who draws a
circuit without including the component values has absolutely no
electronics appreciation at all.
Quite often the operation of a circuit is entirely dependent on
the value of the components and if they are missing, or
contained in a list of parts, it will take time to work out how the circuit works.
The whole idea of a circuit diagram is to be able to quickly work out if
it is doing what you want it to do. And to do this it must
contain as much detail as possible.
As an example, this Touch Switch circuit is very hard to follow. The component values
are not on the
diagram and the layout makes it difficult to see how the two
gates are connected together. If a circuit is laid out properly, you can
"see" it working - just like the animation we have
included above.
Keep this in mind when drawing diagrams. Keep them simple, clear
and easy to follow. This will help others to follow your
circuits
- especially when you have designed something new.
The main fault with the diagram on the left is the feedback line (from pin
4 to pin 2). It should be much clearer as it is extremely
important in the operation of the circuit. In our diagram above, we
have placed the two gates apart, with one gate feeding
the other. Then the feedback line goes from the output of the
second gate to the input of the first. The line is also marked
with a "backward arrow" to emphasise the fact that signals on
this line are travelling backwards.
WHERE ARE WE GOING?
We may have said it before, but the main aim of this course is to get
you familiar with electronic components and "building
blocks."
As far as gates are concerned, it is not necessary to go past the
elementary knowledge as any complex gating situations are
best handled with a microcontroller design.
This is the way we are heading with this course as electronics is
changing rapidly and for less than $2.00 (in bulk quantities) you can
get a microcontroller chip that will take the place of dozens (if not
hundreds) of gates and do it all in an 8-pin chip!
It's all done by programming the chip and you can keep modifying the
program until it works perfectly, without having to connect lots of gates
together or even take up a soldering iron.
There are other advantages of a microcontroller. It is
much easier to interface a
microcontroller to the outside world than a digital chip.
Everything on the outside runs too slowly for digital IC's and
they create the effect known as FALSE TRIGGERING.
On the other hand, microcontrollers can be turned on for a very
narrow "window" of observation and the information gathered
in this way is very accurate.
Some chips do not have the problem of "flutter" when
the voltage is rising and falling because they do not have an
INDETERMINATE ZONE. They have a circuit on each input line
that prevents the gate from changing
state until the input voltage reaches exactly 66% of rail voltage. The input voltage must then drop to
exactly 33% for the output to change back to its original
state.
If the voltage swing is between 34% and 65%, the chip does not change
state. This circuit is called a SCHMITT TRIGGER
and the gap between the high and low trigger points is called
the HYSTERESIS GAP.
This gap is not a problem when the chips are used in DIGITAL SITUATIONS
as a digital signal is required to rise to at least 85% of
rail voltage and drop to 15%. Finally, a point to remember.
Many digital IC's are no longer manufactured, so keep this in mind when
designing a new project.
The NAND gate is called the "universal gate" as it can
be converted into all the other gates by simply wiring 1, 2, 3
or 4 NANDs together.
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