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  

 

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.

 

 

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