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Wily Widget

  Previous Issues

Wily Widget at Work
Illuminated Banister

Wily was reading a magazine when his phone rang.  The call was coming from a large building contractor in a major city. They were going to build some new homes for a subdivision.  The architect specified a touch activated illuminated banister, which was to illuminate the stairs leading to a large basement. At each end of the railing was a metal button.  When the button was touched with a finger, the light would turn on.  When touched again, the light would turn off.  The LED light strip inside the translucent plastic railing was to be powered by 12vac and would draw about 25 watts.
 
Touch Activated Illuminated Handrails
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The home designer did not specify how this kind of lighting control was to be accomplished could Wily help?

There were two different handrail designs.  One design called for a straight rail while the other was L shaped. The total distance between the top and bottom of the railing was 15 feet.  Wily knew that most capacitance switches would not be able to deal with such a large separation between the two ends.  Also, there was not much room inside the two ends of the rail where the touch switch was to be located.  Wires for the two touch buttons and the AC powered LED strip wire were to be fed through one of the brass brackets holding the rail in place.  The original idea was to place the 12vac power supply and the capacitance switch up in the ceiling, well away from the stairs.  But, Wily knew that such a long wire would not be wise in a capacitance type switch circuit.  Some kind of remote but tiny capacitance sensing circuit was needed.  Then, one sensor could be attached to each of the two brass touch buttons and the output signal could then be routed to a main circuit, placed next to the AC power supply some distance away.  Wily had experimented with such circuits in the past and thought that he could use one of those circuits in this situation.  The contractor said that he would need about 100 such systems.  Wily decided to take on the project.

It had been decades since Wily had performed some of the experiments for remote capacitance sensing.  The basic sensor circuit is shown below.  A 20KHz, 12 volt peak to peak triangle shape waveform is used to provide both power to the single transistor circuit and the critical AC excitation signal.  When sufficient capacitance was connected between the transistor base and an earth ground reference, the transistor circuit would begin to route part of the triangle signal to the collector.  The diode turns the pulses into a DC voltage.  That output is then routed to a simple filter network, located some distance away.  After that, it was a simple task to add some additional filtering and an on/off flip/flop circuit.  The flip/flop circuit would then turn on and off a solid state relay.  The relay would switch on and off 120vac power to the 12vac power supply. The touch switch circuit would be powered by a small 12v DC power adapter.

The above circuit is been corrected on 7/5/2012

The complete touch switch circuit is shown below.  

Wily placed the remote capacitance sensor circuit inside a small potting shell, which contained a three screw terminal block and a single turn variable resistor. 

The variable resistor allowed the capacitance needed to activate the switch to be set from about 10pf to 170pF.

The whole sensor assembly measured just about a half inch cube and could easily fit neatly inside the end of the hand rail. 

The two buttons were connected in parallel, so touching either button would toggle the flip/flop.

 

 

 
Remote Touch Sensing Modules  Touch Switch Control Module

 


Second Quarter,  2012    


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