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Capacitance Proximity Switch Technology

Introduction   Applications   Physics of Capacitance   Units of Capacitance   Dielectric Constant Definition        Human Body Capacitance    
Low Voltage Touch Switch Circuits       Excitation Circuits   
Capacitance Change Sensor Circuits       Interface Circuits    
AC Line Voltage Switch Circuits     
Capacitance Proximity Switch Examples (Faraday Switches -- Commercially Available)
Helpful Links

About 15 years ago I designed a number of electronic capacitance operated touch activated switches. The switches were mostly used to control lights and intercoms within jail cells.  Electronic touch activated switches were ideal for such hostile environments, since indestructible carriage bolts, mounted in thick cement walls, could be used as touch buttons.  The touch of a human finger to the buttons could control heavy 120vac and 277vac electrical loads or switch on and off an intercom.

Over the years, I have continued to be interested in this technology and have conducted many experiments using a large number of different circuits. I developed circuits that were powered by supplies, ranging from 1.5 volts DC to 277 volts AC.  Most were designed to produce a logic level output swing, whenever the capacitance change exceeded a minimum level.  Some of these switches were very simple circuits that could be mounted near small metal touch buttons and could send their signals thousands of feet away to an interface circuit.   Inexpensive touch panels, containing hundreds of switches, could be made using such a technique.  The switches could control anything from lights to TV cameras.  I also designed some circuits that produced a voltage change proportional to the capacitance change.  These circuits were ideal for many industrial sensor applications that could monitor the level of a liquid in a tank, detect cardboard boxes moving on a conveyor belt or measure the humidity of air.

Capacitance proximity switches and capacitance sensors have lot of potential uses.  My goal in this discussion is to outline some of the techniques I discovered over the years to detect small capacitance changes and use that information to activate electronic switches.  I invite you to send me your suggestions and questions on this very useful technology.

• Human hand, finger or foot activated switches.

• Indestructible electronic switch replacements for standard toggle or push-button switches.  Great for museums, exhibits, interactive displays, jails, schools or hospitals switches for wheel chairs or for the handicapped.

• Object sensors, including humans.

• Automatic product quality inspection, missing part, empty bottle.

• Fluid level sensors, liquids and powders.

• Lighting controls, touch activated dimmers.

• Hand “hover” switches, activated without making contact with any surface.

• Through glass, wall or door switches.

• Touch sequence switch controls.  Touching one button could control multiple loads.

• Detecting human feet from beneath a floor.

• Non contact object sensors for material handling needs.

• Security alarms, human motion detection, hidden alarm reset switch.

A capacitor is formed by placing two metal plates in parallel with each other, separated by a distance “D” with each plate having an area of “A”.  The capacitance of the capacitor can be calculated using the equation below.  Note that the capacitance is proportional to the plate area but is inversely proportional to the distance between the plates.  When the plates are close to each other, even a small change in distance between the plates can result in a sizeable change in capacitance.  Many industrial capacitance type sensors take advantage of this relationship and can detect very small vibrations and motion.

• C = (K)(A)(8.85 x10^^-14)/D

• C = capacitance in farads

• K = dielectric constant

• A = plate area in square centimeters

• D = distance between plates in centimeters

The Farad is the unit of capacitance.  Smaller units include the microfarad, which is one millionth of a farad and the picofarad, which is a millionth of a microfarad.    Most of the capacitance changes discussed in this section will be listed in picofarads.

When a material, which does not conduct electricity, is slid between the two parallel metal plates the capacitance will usually increase.  The ratio of the capacitance before and after the material is placed between the two plates, is equal to the dielectric constant of the center material.  The dielectric of a vacuum is one while dry air is almost one.  The table below lists the dielectric constant of some other materials. Note that water has a high dielectric constant.  Objects containing water, such as a human body, will therefore dramatically increase the capacitance.

• Vacuum = 1.00

• Dry air = 1.0006

• Bakelite plastic =  5.0

• Dry wood = 1.5 – 5.0

• Paper = 2.0 – 3.0

• Teflon plastic = 2.1

• Paraffin wax = 2.5

• Polyethylene plastic 2.3

• Polystyrene plastic = 2.6

• Distilled Water = 80

• Plexiglas plastic = 2.8

• Rubber = 3.0 – 4.0

• Celluloid plastic = 4.0

• Quartz = 4.0 – 5.0

• Formica = 4.7

• Mica = 4.5 – 8.0

• Pyrex Glass = 4.8

• Window glass = 7.0 – 8.0

• Porcelain 5.0 – 6.0

One application of a capacitance-activated circuit is to form a switch that is triggered by the touch of a human finger to a metal button.  The circuit operates by measuring the capacitance change between the metal touch button and an earth ground. 

The skin of the human finger is thin and the human blood under the skin makes a nice electrical conductor.  The outside surface area of the human body is also large.  The act of touching a human finger to a metal button will therefore form an electrical interface that will produce a sizable capacitance change, if there are any metal objects nearby that are connected to earth ground. This environmental capacitance will depend on many factors.  The most important factors are the type of floor the human is standing on and the type of shoes he or she is wearing. The capacitance change will increase with bare concrete floors and thin sole shoes. Nearby metal shelves, tables, light fixtures, heating ducts and electrical wires will also effect the total capacitance between the human body and earth ground.   Experiments have shown that seldom does the capacitance change measure less than about 30 picofarads.  At the other end of the spectrum, if the human is also in contact with an earth grounded object, the finger/button interface can produce a capacitance change of several thousand picofarads.  In most applications, a minimum capacitance change threshold can be set.  When exceeded, the detected change is used to activate an electronic switch.   With a carefully designed circuit a switch can be triggered with a capacitance change of only one picofarad.

During my research, I did conclude that although an electric power line field was not always nearby, making AC hum switches useless, there almost always was an electrical path, often invisible, to an earth ground.  Water pipes, heating ducts and cement structures all formed earth ground paths.  In almost all cases, I was able to get a capacitance operated touch switch circuit to work reliably, thanks to nearby objects that were connected to an earth ground that I could use as a reference.  A key to the success of these circuits was to measure the capacitance change relative to earth ground using a frequency much higher than standard power lines.  Most of the circuits shown in this discussion will use techniques that detect the change in capacitance between a metal plate or button and earth ground.

After many experiments, I determined that a signal with a frequency higher than the normal 50Hz to 60Hz power lines was needed to consistently measure the capacitance.  Frequencies ranging from 10KHz to 40KHz were most often used.  Such a frequency range is orders of magnitude higher than the power line frequencies, but low enough to keep radio noise emissions to manageable levels.  

The excitation signal is routed to a simple touch sensor circuit.  The sensor circuit is ideally positioned next to the touch button or metal plate.  Placing the sensor next to the metal plate provides high noise immunity.  The sensor circuit can be up located up to a thousand feet from the exciter circuit.

In most cases a filtered square wave signal can be used for the sensor excitation signal.   As shown in figure 1, (a PDF file) such a signal is easy to generate.   This figure has sufficient power to operate 50 touch sensor circuits, while figure 2 (a PDF file)can power hundreds of remote touch sensors.

The signals produced by the circuits are all referenced to an earth ground but do not require a direct earth ground connection.  Instead, a 0.1uF capacitor from the circuit ground to an earth ground is used.

Note that in all cases, the excitation circuit generates a signal that is always positive with respect to circuit ground. The excitation signal has both AC and DC components.  The DC component us used to bias the transistor sensor circuit described below into linear operation.  The AC component provides the needed signal to detect capacitance changes.

More often than not, the touch button or plate is located some distance from the excitation circuit.  After many experiments, I settled on the simple transistor circuit shown in figure 1.  This circuit has many advantages.  The circuit is simple enough that it can be housed in a very small package.  Using surface mounted components the circuit requires a circuit board less than 0.5 cm x 0.5 cm in size.    It has good static discharge and noise immunity.  It also draws negligible power when it is a standby mode and it can be positioned up to 1000 feet from the excitation signal.  In addition, the circuit only requires two unshielded wires. Usually, inexpensive telephone cable will work fine.  Finally, by making the base emitter resistor variable, the minimum capacitance sensitivity of the circuit can be adjusted over a wide range.

The transistor acts as a current amplifier with a minimum capacitance threshold.  When the total capacitance between the transistor base and an earth ground exceeds a certain level, the transistor begins turning on, forming a switch between its emitter and collector terminals.

When the transistor begins turning on, its collector terminal begins tracking the excitation signal that is connected to the transistor emitter.  The diode connected to the transistor collector converts the pulsating DC signal to a direct current voltage, which is routed back to an interface circuit.  The interface circuit and the exciter circuits are usually located near the exciter circuit but do not have to be.

The transistor circuit works best for capacitance changes in excess of 25pf with changes greater than 50pf as a preference.

The output signal of the sensor circuit is a DC level, swinging from zero volts to several volts, when a touch sensor circuit is activated.  However, if the distance between the sensor circuit and the circuit used to detect an activated switch is great, the unshielded wires will often collect a lot of unwanted AC power line noise signals.  I highly recommend adding an interface circuit to process the DC level swing from the sensor circuit, before sending the signal to a computer system or to a power switch circuit.  To remove the AC noise components, a passive RC filter is recommended at the front end of the interface circuit.  The filter circuit not only filters unwanted AC line noise, but also does a fine job of preventing damage to the interface circuit from electrostatic discharge.  The output of the filter circuit can be routed to an N-channel FET or to Schmitt trigger circuit.  The Schmitt trigger circuit does a fine job of converting the slow voltage swing from the sensor to a fast clean logic voltage shift.  The Schmitt trigger action also requires a consistent minimum input voltage level, which helps to prevent false switch action.

The circuit shown in figure 1 shows both examples of an FET and a Schmitt trigger circuit.   I personally prefer the Schmitt trigger circuit but have also used the FET circuit when the distance between the touch switch and the interface circuit are short..

The output of the interface circuit can be used to operate both solid state and mechanical relays.  It can also be fed to logic inputs of a computer system.  I have also used the simple logic circuits to produce a sequence of switch outputs.  A touch sequence can also be used to turn on and off various loads according to the logic circuit.  As an example, the first touch of a button might turn on one light.  The second might turn on two lights, a third touch might turn on three lights and a forth touch might turn off all lights.

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Helpful Links

www.sensorsmag.com    Sensors Magazine
www.qprox.com  Quantum Research Group, developers of the Qprox sensor IC.
www.gordonproducts.com  Gordon Products Inc.