The Design Corner
    Introduction to Capacitance Proximity Switches
    
 Updated on:  Friday, December 18, 2015 08:20 AM

 

Units of Capacitance  
Dielectric Constant Definition   
Human Body Capacitance  

Excitation Circuits
Capacitance Change Sensor Circuits 
Interface Circuits  

 
1. Excitation Circuits

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.

2. Capacitance Change Sensor Circuits

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.

3 Interface Circuits

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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