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Cold cathode touch buttons.

Discussion in 'Lighting' started by Clive Mitchell, Mar 10, 2006.

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  1. It would appear that in the heady days of valves and smoke, that
    elevators had a brief love affair with cold cathode touch buttons.
    These were basically neon devices that were held at just under their
    strike voltage with a DC supply that also had AC referenced to earth so
    when a finger was placed on the end of the tube (via a spring and
    plastic disk) it would cause the tube to fire and not only call the
    elevator, but illuminate too.

    Once the elevator had answered the call it would reset the button by
    biasing it in a manner that interrupted the discharge.

    Does anyone have a picture or link to data of one of these interesting
    sounding tubes? The veteran elevator engineers refer to them as cold
    cathode touch buttons.
  2. The real dampening factor was when they discovered that they were
    triggered by adjacent flames. When a fire occurred in a building the
    elevator promptly went to the blazing floor and opened it's doors.
  3. Are you sure those were cold cathode touch buttons?

    Many elevators in the US have buttons that are activated
    with just a touch, or even without a touch if your finger is
    close enough. Before solid state electronics the button was
    tied to the tank circuit of a small vacuum tube Class-C
    oscillator. Touching the button would detune the oscillator,
    causing the anode current to increase. The increase in
    current would be used by the control system as notification
    that the button has been "pressed".

    Later solid-state versions dispensed with the oscillator and
    just use a high-gain amplifier followed by a detector. When
    a finger touches the button it acts as an antenna, picking
    up a number of signals including EM waves at the power line
    frequency. These are amplified and detected and then sent
    to the control system to indicate that the button had been

    Vic Roberts
    To reply via e-mail:
    replace xxx with vdr in the Reply to: address
    or use e-mail address listed at the Web site.

    This information is provided for educational purposes only.
    It may not be used in any publication or posted on any Web
    site without written permission.
  4. The ones in question were used extensively by Otis and had a tube number
    of 2040. They were definitely neon filled and as part of the operation
    glowed when tripped thus serving as both sensor, indicator and latch.
    These were vintage devices, but are still found on some rare lifts.
    The current breed of touch dimmers use a reference to the mains. The
    touch plate is connected to the circuitry by a chain of high value
    resistors, and when the pad is touched the circuitry detects the tiny
    current flow to ground through the person touching the button.
  5. Some of the early elevators used metal buttons pushing leaf switches via
    a plastic bush. The only thing insulating the metal button from 110V
    (or perhaps 240 in the UK) was a piece of fish paper as was common in
    stacked contacts at that time. When the fish paper wore through it was
    possible to get a significant shock fro the button. :)

    In the neon sensors the touch plate itself was apparently a plastic disk
    with a metal plate and stud embedded in it, which interfaced with the
    tube via a spring.

    I did experimentally build a touch panel that used a PIC16C54 processor
    to detect finger contact with plates by simple means of referencing the
    PIC to the mains via a chain of resistors so that you could directly
    change the state on the inputs by grounding the touch plate with your

    By the time I had added input protection circuitry and anti-static
    protection the current needed was high enough to cause discomfort if you
    touched one of the sensors and an earthed metal object at the same time.
    Not a good marketing feature. :)
  6. I built a touch panel when I was in my teens. I used a square wave
    generator, the output split into two and fed via a pair of resistors
    to both inputs of an XOR gate. The output of the gate remained low
    as both inputs were the same. Also connected to one of the inputs
    was the touch plate. If you put your finger on it, your capacitance
    to earth delayed the square wave signal at that gate, and you got a
    stream of pulses out of the XOR gate at twice the oscillator frequency.
    With the oscillator running above 20kHz, you only had to put your
    finger near the touch plate for it to work, and so it could actually
    have a thin insulating cover.
  7. Of course, nowadays they'd use something like this...
  8. But you actually have to touch this screen. Using the
    proximity switches you could always test your coordination
    and amuse yourself at the same time by trying to activate
    the switch without actually touching it :)

    Vic Roberts
    To reply via e-mail:
    replace xxx with vdr in the Reply to: address
    or use e-mail address listed at the Web site.

    This information is provided for educational purposes only.
    It may not be used in any publication or posted on any Web
    site without written permission.
  9. Apparently so as many geeks seem to have amused themselves gauging the
    sensitivity of the buttons by hovering their finger slowly towards it
    until it lit then moving on to the next button in line.

    The subsequent elevator stops and needless machine wear were well worth
    the scientific analysis I'm sure. :)
  10. Elevators Pty Limited [EPL] Australia (later merged into Kone Elevators, did
    develop and use cold cathode buttons from the mid 1960's till about 1970 on
    landings and in cars. These were also thermosensitive with age whereupon the
    landing buttons would false fire after the HVAC shut down - so the cars were
    going up and down all night until the temperature stabilised. This wouldn't
    have been so bad - but one day there was a fire and the landing buttons
    called the lifts to the fire effected floor (a whole lot of retrofits
  11. To combat the thermal sensitivity issue many of the button boxes were
    fitted with a small heater resistor in the bottom.

    Makes you think that while the buttons were a cool idea, they were
    somewhat counterproductive compared to a simple push button.
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