touch switch

All the circuits and projects we describe in these articles consist of very important "buildingblocks" that you can add to other designs.This time we describe the concept of touching a plate (or two plates separated by a small gap)and turning a circuit ON or OFF.A TOUCH PLATE is classified as a high impedance device (or high impedance circuit) as theeffect of a finger will be detected by the circuit connected to the plate.To learn more about the concept of high impedance circuits, see Page 77 of our BasicElectronics course.If only a single plate is present, the circuit will actually be picking up "mains hum" from the finger.To prove this, take the project into an open space such as a large park and try the circuit. It willnot work.If the plate has a signal on it (from an oscillator), the effect of your finger will be to remove thesignal (or reduce its amplitude considerably) and a detecting circuit will be activated.If the circuit has two plates, it will be registering the resistance of your finger. If the circuit has 4plates, it will use two to turn the circuit ON and two to turn the circuit OFF. There are a number of different types of TOUCH PLATES and different effects can be createdby the circuit.

1. Touch a set of pads and the project turns on. When the finger is removed, the circuit turns off.The finger can touch the pads for any length of time. We also include the feature where thecircuit extends the ON period, so the circuit stays on for a length of time after the finger isremoved. This is shown in Circuits A.

2. Touch a set of pads fairly quickly and the project turns on. Touch the pads again for a shortperiod of time and the circuit turns off. This is called the "Flip-Flop" effect. If the finger is kept onthe pads, the circuit will turn on-off-on-off at a rate of about once per second. This is shown inCircuits B.

3. Touch one set of pads to turn the circuit on and another set of pads to turn the circuit off.This is shown in Circuits C.

CIRCUITS A

Here are a number of circuits that turn on a device when the touch-pad is touched.

Touch Switch http://talkingelectronics.com/projects/TouchSwitch/TouchSwitch-1.html

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The circuit above is the simplest Touch Switch. It is called a "super-Alpha pair" and is actually

identical to a single transistor with a very high gain.Putting a finger on the touch pads turns the top transistor ON and this transistor turns on thebottom transistor. When the finger is removed, the circuit consumes less than a microamp.

The 555 can be used to create a Touch Switch. The only problem with this is the 555 consumes

about 8mA, at all times when the supply is connected. The circuit above turns on the LED whenthe finger is applied and pin t becomes "open circuit." This allows the 10u to charge via the 100kresistor and when pin 6 detects a HIGH, the LED turns off. The finger should be removed beforethis occurs. See below for an ON-OFF touch switch using a 555.


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The Touch Switch circuit above is a very complex design to do a simple task. It is also a very

poor design as the biasing (turn-on) for the output transistor is via a resistor and the outputtransistor is turned off by taking the biasing current to the 0v rail. This is a wasteful design if thecircuit is to be powered by a battery.

The circuit above has a signal "sitting" on the TOUCH PLATE via the oscillator made up of a

Schmitt trigger between pins 1 and 2. The operates as a square-wave oscillator at approximately150 kHz. The oscillator's output gets ac-coupled to R2 that sets the drive level and hence, the

sensitivity for the touch pad. Applying negative excursions of several volts of a square-wavesignal to its gate repetitively drive N-channel JFET Q1 from conduction into cutoff. An


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approximation of the square wave swinging from 0 to 12v appears at Q1's drain. A peak detector

circuit formed by D1; R7 and C4 provides sufficient dc voltage to force IC1B's output to a logic

low.However, if someone touches the touch pad, any added capacitance to ground reduces the acdrive at the FET's gate, and Q1 continuously conducts. The square-wave voltage applied to D1decreases. The voltage on C4 drops below the logic threshold, and IC1B's output goes high. You

can adjust R2 to set sensitivity and compensate for device-to-device variations in the FET's

pinch-off voltage.

The following circuit does not work. It uses a CD 4001

The TRUTH TABLE for a NOR gate is:

NOR GATE

INPUT OUTPUT

0 0 1

1 0 0

0 1 0

1 1 0

We can see from the Truth Table that the output of a gate only changes when both inputs areLOW. For the top gate, pin 1 never goes low so this type of gate will not work.

Try a NAND gate:


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The circuit above does not work. By checking the Truth Table, we see the gates are correct:

NAND GATE

INPUT OUTPUT

0 0 1

1 0 1

0 1 1

1 1 0

But the circuit does not turn off. The reason is the 4u7 is not charge or discharged by anycomponent in the circuit. When the circuit is first turned on, the electrolytic is uncharged and pin 5is effectively connected to pin 3. If output pin is HIGH, pins 5&6 will be HIGH and pin 4 will beLOW. This will make pin 3 HIGH. Both the Touch Wires will be HIGH and touching them will notchange the state of the circuit.We need a component to allow the 4u7 to charge and make pins 5&6 LOW.The next diagram does this:


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The 100k "safety resistors" have been removed as they do not play a part in the operation of thecircuit and the touch wires have been connected to the circuit to have the greatest effect.

CIRCUITS B

The following circuits show a "flip-Flop" effect. The circuit changes state, each time the touchpads are touched.

If a finger is kept on the touch plates in any of the toggle circuits above, the circuit will oscillateON, OFF, ON, OFF at a low frequency. The frequency of 3 sec, 0.5 sec has been identified in


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the top circuit. An improvement to the Toggle Touch Switch above, to keep the charge on the100n, is to use a second gate:

A touch switch can be made with 2 gates from a 4049UB IC, as shown in the following circuit. Ithas proven to be reliable at 6v and 12v. The design has the advantage that the output does notcycle if a finger is kept on the Touch Pads.

CIRCUITS C

These circuits have two touch plates. One touch plate turns the circuit on and the other plateturns the circuit off.


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to seecircuitwork

The TOUCH-PADS deliver current from the power rail to the input of the circuit, via a moist finger.The finger acts as a very high vale resistor. Note the 4M7 feedback resistor that keeps the circuiton when the finger is removed.The circuit above is available from Talking Electronics as a kit. The kit is called TOUCH SWITCH:


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TOUCH SWITCH USING A CD 4011 IC

A TOUCH SWITCH using a CD 4011 is shown in the diagram above.

A simpler version is shown below:

When the circuit is first turned on, the two gates will "race" and the fastest gate will create aHIGH output. It cannot be determined if the LED will light when the circuit is first turned on. Byadding the 100p (shown in red) to the position shown on the circuit, one input of the gate will startwith a LOW and this will make pin 4 HIGH. The top gate will have HIGH on both inputs and theoutput will be LOW. This will turn on the LED. It is not know why the previous circuit used all 4gates of the 4011. The circuit was taken from a kit manufactured by a non-electronics personand he did not investigate the possibility of simplification.Since the output of a CD 4011 is not capable of sinking or sourcing a high current, you can bufferthe output of the gate with the third gate in the chip and wire it as an inverter.


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ON-OFF TOUCH SWITCH USING A 555 IC

For those who like the rugged 555, we have included a 555 ON-OFF touch switch.

TOUCH PADSA touch Pad can be obtained from many different sources. The photos below show a touch padobtained from a toy. Some of the very light touch buttons consist of a small carbon blockmounted in silicon rubber and when the button is pressed, the carbon block touches the pad andreduces the resistance between the two interleaved tracks.

3 TOUCH PADS


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Close-up of the touch pad

This part of the circuit board can be cut away and used as a touch pad for the circuits in thisdiscussion. The pads are already protected from corrosion and form a very good design fordetecting a finger.The important feature of the pad is the number of interleaving fingers as this is equivalent to apair of lines about 12cm long and when a finger is applied, the resistance between the linesdrops to between 150k and 850k, depending on the pressure and moisture in the finger.

HIGH IMPEDANCE CIRCUITWe have already said a touch pad is a high impedance device (circuit), but what does this meanand how does it work?We are going to explain why it must be a high impedance circuit.Below we have four different touch pad circuits. The supply voltage does not matter, howeverwe have shown it as 6v. The main purpose of a touch pad is to reduce the voltage on the"output." Generally this must be15% - 25% of rail voltage to trigger the circuit.

If we take the first circuit "A" and place a finger on the touch pad, the circuit becomes equivalent

to two resistors in series. These two resistors form a voltage divider and the voltage on theoutput is in proportion to the value of the resistances. We will assume the resistance of the fingeris 1M to make the discussion simple. The 5M resistor is not a standard value but s also used tomake the discussion easy to understand. In the diagrams below, the output of thetouch pad is 6v when nothing is touching the pad. When a finger touches the pad, the voltagedrops to 1v. Without using mathematics, we can see the 5Meg resistor is in series with the 1Meg


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finger, making a total of 6Meg. This means 1v appears across each 1Meg and thus the output is1v.

If we apply the same finger to circuit "B," the output voltage will drop to 3v. This voltage may not

be low enough to trigger the circuit connected to the touch pads.

If we apply the same finger to circuit "C," the output voltage will drop to 5.4v. This voltage will not

be low enough to trigger any circuit connected to the touch pads. Let's look at how this voltage iscreated. The two resistors are 100k and 1M in series. If we convert the 1M into ten 100kresistors, each resistor will have the same voltage across it. There are 11 x 100k resistors andthis means very close to 0.6v will appear across each resistor. That is why the output voltage willbe about 5.4v when the finger touches the pad.


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From this we can see the "pull up" resistor must be as high as possible so the effect of a fingerwill reduce the output voltage of the pad to a low value.There is one other important factor to remember.The output of a touch pad must be connected to a high impedance input. The diagram belowshows the gates and a "super-alpha" transistor. These all have a high impedance input.

High Impedance Inputs

Why do we need a high impedance input?Suppose the circuit we are connecting to the touch pad has a low impedance. It will be equivalentto placing your finger on the touch pads. The output will go low and your finger will not be able tocreate a HIGH-LOW voltage change.The input impedance of a gate can be considered to be very high (greater than 10M). When the"super-alpha" pair is connected to the touch switch, the voltage on the "output" of the touch padwill not rise above 1.3v. This is due to the base-emitter junctions of the two transistors.The output of the super-alpha pair will be low. When a finger is placed on the touch pads, theoutput of the super-alpha pair will rise. An alternate circuit for connecting touch pads to a super-alpha pair is shown below:

LATCH CIRCUITHere are two latch circuits using transistors. The first operates exactly the same as the4-transistor Touch Switch above. It can be used with a touch pad. It's another "Building Block" toadd to your collection. The second circuit operates in the same way. When the circuit is firstturned on, both transistors are not conducting. As the input voltage increases to 0.65v, the BC547 transistor turns on and this turns on the BC 557. The BC 557 is connected to the base of theBC 547 and it takes over from the input voltage. The two transistors turn each other on until bothare fully turned on. The supply must the turned off to reset the circuit.


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Here is a Touch Switch circuit from a magazine:

Why use half a chip and a FET to do the same as our 74c14 circuit above?

That's why you need to know how to design circuits, so you don't over-design.See our "Spot The Mistake" article for more over-designed and incorrectly designed circuits. Youlearn more from other people's mistakes than anything else.

USING A TOUCH SWITCH IN A PROJECT

1. DOORKNOB ALARMThe 74C14 (40106) is a hex Schmitt trigger IC with 6 gates that can be used for 6 differentbuilding blocks. Even though it has a "74" marking, it can be placed in a circuit with a voltage ashigh as 15v - all the other 74 series require a maximum of 5v for the supply. (More data on the74C14 can be found in Chip Data eBook.)


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In the following circuit, the gates are used to detect the touch of a door knob and produce anoutput that goes HIGH for approx 1 minute.As the capacitance on pin 1 is decreased, the frequency of the oscillator increases and thismakes it easier for the human body to absorb the signal. Try changing this value as well as thecoupling 5p6 to 22p that connects the oscillator to the detecting gate between pins 3 and 4.

The output of the above circuit can be taken to an alarm. Open the reed switch contacts andconnect the reed switch to the output of the Door-knob alarm.

A suitable alarm can be found in the $2.00 "Junk Shops" for about $2.00 These consist of a piezodiaphragm and a driver circuit consisting of a transistor and COB (Chip On Board) to produce avery loud wailing sound. Some of the devices have an inductor to increase the voltage to about60v to 80v to produce an output of about 90dB. The device we bought had a transformer to drivethe piezo to 80v.The photo shows the device and magnet. The magnet holds a reed switch closed and when thetwo items are parted, the reed switch opens and sounds the alarm.


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The reed switch can be seen in the photo below. It is an uncovered reed switch consisting of twosoft-iron strips that overlap slightly in the centre. When a bar magnet is brought near, the twostrips become magnetised with each forming a north at the top and south pole at the bottom.This means the top strip has a south pole at its bottom and the lower strip has a north pole at itstop. Since unlike poles attract, the two strips will touch each other when a bar magnet is present.

When a magnetic object comes in the vicinity of a magnet, it becomes temporarily magnetisedwith North and South poles. This is shown in the diagram. This is how the two strips of the reedswitch close and "stick together" when the magnet is near.


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The magnetic field of the bar magnet causes the twoparts of the reed switch to become "magnetic."

The side of the alarm showing Chime (Doorbell), Off

and Alarm. See below for a link to these sounds.

The underside of the alarm showing the COB module and the 4 pins from the transformer thatdrives the piezo diaphragm. To hear the "DoorBell" sound and "Alarm" sound, click HERE.or here: SOUND


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Open the reed switch so the Door-Knob circuit can operate the alarm.

2. TOUCH MOTOR CONTROL - by L. W. Brown, Burwood, Victoria, Australia.

The following circuit is suitable for operating a12v motor such as on a display in a shop window.The 50mm x 50mm touch plate can be stuck to the inside of the glass and anyone placing theirfinger near the touch plate (on the outside of the window) will prevent the signal entering thecharge pump section of the circuit and keeping the 10n charged.The circuit will take a few seconds before the 10n is discharged via the 10M and the motor willoperate.

3. TOUCH-ON TOUCH-OFFThis circuit is an extension of the Door-knob Alarm presented above. It turns on an output whenthe Touch-Plate is touched very briefly and turns off the output when the plate is touched for aslightly longer period of time. As the capacitance on pin 1 is decreased, the frequency of the oscillator increases and this


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makes it easier for the human body to absorb the signal. Try changing this value as well as thecoupling 5p6 to 22p that connects the oscillator to the detecting gate between pins 3 and 4.A reader had success when the capacitor on pin 1 was reduced to 5p6 as this increased thefrequency of the oscillator to approx the maximum frequency of the gate and this high frequencywas readily absorbed by the body when the touch plate was touched.

TOUCH-ON TOUCH-OFF SWITCH

This article has covered more than 10 building blocks and shown how to adapt a low-cost item ina junk shop to a circuit you have already designed.It has also covered the concept of a HIGH IMPEDANCE CIRCUIT and FEEDBACK to keep acircuit stable in either of its two states.Even if you think you will never need a TOUCH SWITCH in a future project, the building blockswe have covered can be used in lots of different circuits and if you build them, you will have amuch-better understanding of how they work.

26/5/10


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Home > Circuits > Other > Touch Switch

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A touch switch is a switch that is turned on and off by touching a wire contact, instead of flicking a

lever like a regular switch. Touch switches have no mechanical parts to wear out, so they last a lotlonger than regular switches. Touch switches can be used in places where regular switches wouldnot last, such as wet or very dusty areas.

Schematic

Parts

PartTotal

Qty.Description Substitutions

C1 110uF 16V Electrolytic

Capacitor

R1,

R22 100K 1/4 Watt Resistor

R3 110 Meg 1/4 Watt

Resistor

U1 14011 CMOS NAND Gate

IC

MISC 1Board, Wire, Socket For

U1

Notes

The contacts an be made with just two loops of wire close together, or two squares etched1.

Touch Switch http://www.aaroncake.net/circuits/touch.asp

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close together on a PC board.

When activated, the output of the circuit goes high for about one second. This pulse can beused to drive a relay, transistor, other logic, etc.

2.

You can vary the length of the output pulse by using a smaller or larger capacitor for C1.3.

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Touch Switch

Comments

Add A Comment

jay Touch Switch Tuesday, March 09, 2010 8:00:02 AM

i need help!!!!!! how will be the out put taken and how will be the in put given their is only one output terminal if i want connect a led how can i connect and a switch needs a input supply where

should i give in put can i use ic7400 for nand gate please help me its urgent thank u

anonymous Touch SwitchSaturday, February 06, 2010 10:28:44

PM

I need this to work with batteries... 3.7v, 6v, and 7.2v... all pushing up to 2amps. How can I configurethis to work with all three situations?

Godofredo Gregorio Jr. Touch Switch Monday, June 15, 2009 11:27:16 PM

It's so useful. We can use it in many different way. It can be a touch switch to a 220volt lamp with a

220volt, 5amp relay connected at the output of the circiut. It's great in everyday use....JonjonGregorio-Philippines

anonymous Touch SwitchMonday, February 16, 2009 11:21:20

AM

what the output is going to be?? can i put an led to know if the circuit is working or not?? does thecircuit working by touching wires by fingers or what?? any one knows the answers, please tell me

cuz i am doing this device at the moment. thnks alot.

cuan Touch SwitchMonday, February 16, 2009 11:08:33

AM

can i make a touch switch using two scr??? if can then how does it work. and the circuit only power

by dc voltage so i have a hard time to turn off the scr.. only one touch switch is needed and thetouch switch use to turn and off the bulb

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anonymous Touch SwitchTuesday, December 23, 2008

10:02:21 AM

What am i going to do inorder to make the touch switch just like any other ordinary switches which

"on" state is permanent unless it is turned off???

gardenyu Touch SwitchFriday, November 07, 2008 5:17:59

PM

I wonder where can I buy a fairly big touch switch? ( about 1 to 2 inch diameter)?

anonymous Touch Switch Monday, October 27, 2008 3:59:51 AM

i was thinking about adding one of these touch switches to my shaved door handles, so all you doisun lock the car and hit the touch switch . probally mount it behind a window or where the door

handle used to be.

jazk Touch SwitchThursday, October 09, 2008 1:56:26

PM

anybody know about this title "touch switch" ...what the function and what use for this...?

BeeTouch switch ....Main supply

detectionTuesday, September 16, 2008

11:03:09 AM

Great job and works right away ! I need to get an image of the main supply (220V- 110V50/60Hz)but do not have an access to the hot wires....just the isolated cable I need to get the Freq

(not a Pb) but also the phase (Zero crossing detector) on rising edge.... any idea ?????

The last 10 comments are currently shown. Show All Comments. Add A Comment

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touch switch

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