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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.