Cold cathode touch buttons.

[Clive Mitchell]

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.

 

[Clive Mitchell]

Sorry I can't help here. Just wanted to say that they don't seem
particularly "old" to me. I remember them at University Hall's
elevators at U of I, back in 1990, and I loved playing with them.

At the time I couldn't figure out how in the world they worked (without
pressure), so it was often that students looked at me very curiously,
when I was missing my floor and after I had already pressed around a
dozen or so, to discern more details.

Perhaps people's curiosity for them, acted as a dampening mechanism for
more modern installations?

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.

 

[Victor Roberts]

[snip]

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.

Sorry I can't help here. Just wanted to say that they don't seem
particularly "old" to me. I remember them at University Hall's elevators at
U of I, back in 1990, and I loved playing with them.

At the time I couldn't figure out how in the world they worked (without
pressure), so it was often that students looked at me very curiously, when I
was missing my floor and after I had already pressed around a dozen or so,
to discern more details.

Perhaps people's curiosity for them, acted as a dampening mechanism for more
modern installations?

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
"pressed."

--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
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site without written permission.

 

[Clive Mitchell]

Are you sure those were cold cathode touch buttons?

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.

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 "pressed."

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.

 

[Clive Mitchell]

Had the mechanism been a little more terse, like with a direct
reference to mains, the touch plate connected to the circuitry through
a 1 Ohm resistor, then I would have been able to tell for sure.

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
finger.

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.

 

[Andrew Gabriel]

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
finger.

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.

 

[Clive Mitchell]

The cheap mains dimmers tend to use small earth leakage currents as
noted by a previous respondant but the most common method is similar to
your XOR method but rather uses (say the technique of) a D flip flop.
The system clok goes direct to the D input and to the CP input via a
resistor. The touch pad is on the CP input. When not touched the FF
output is 0 since the data to clok edge set-up time is (just) too short
and the rising (clocking) edge cloks in a 0. Any delay on the edge to
CP (from the RC delay caused by the finger-contact capacitance) will
clok a logic 1. You get the basic idea which, when refined, can readily
(cheaply) be made auto-correcting for different ambients, is
non-galvanic (eg will work thro glass, with gloves on) etc, etc.

Of course, nowadays they'd use something like this...

http://www.qprox.com/

 

[Victor Roberts]

Of course, nowadays they'd use something like this...

http://www.qprox.com/

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
http://www.RobertsResearchInc.com
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.

 

[Clive Mitchell]

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

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.

 

[New Directions In Building Services \(Australia\)]

[snip]

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.

Sorry I can't help here. Just wanted to say that they don't seem
particularly "old" to me. I remember them at University Hall's elevators
at
U of I, back in 1990, and I loved playing with them.

At the time I couldn't figure out how in the world they worked (without
pressure), so it was often that students looked at me very curiously, when
I
was missing my floor and after I had already pressed around a dozen or so,
to discern more details.

Perhaps people's curiosity for them, acted as a dampening mechanism for
more
modern installations?

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
"pressed."

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
followed).

 

[Clive Mitchell]

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 followed).

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.