Water bed thermostat light

[MattyF]

I have a Tech Mirage water bed temperature controller that's about 20 years old. It has a light on it that is presumably to show when the heater is on. I assume that the light is neon. Everything seems to work OK except that the light will only shine if I shine another light on it, e.g. an incandescent lamp or an LED torch. The neon light then flickers on and off until I move the torch away, when it then stays off completely. I think it has always done this, certainly for at least 10 years.

It would be nice if the neon light stayed on, so I can tell if the power has been unplugged somehow. Does anyone know what the heck is wrong with the neon light? The thermostat is switching about 70 watts. I can't get at the thermostat easily to look at it without emptying the water bed, which then involves a lot of work and sleeping in another bed etc etc.

[Brian Gaff]

Erm, how can one have a light that only works when light is shone on it. that is simply not possible unless this has a photocell deliberately intended to do this. Why would one want to? Brian

[Huge]

Untrue. It's a known failure mode of mains neons. I had a neon nightlight do it (I may still have it somewhere) which prompted me to read up on the phenomenon. The light being shone on it provides just enough energy to allow the neon to strike.

[jkn]

That's a fascinating phenomenon your Neon Lamp is showing ...

The potential across the rods must be just below the 'striking' potential f or some reason, and the added energy of the photons from your lamp causes s triking to occur. I think Einstein got a Nobel Prize for explaining that on e - (though I could be mis-'explaining' it).

I would keep it as a souvenir/demonstrator rather than trying to fix it.

J^n

[Dave Liquorice]

Well if it's an ancient neon it may have leaked over the years and no longer be able to strike properly from just the applied voltage. External stimulus (a light source) provides enough energy for it to strike.

Wired ended neons are cheaply and easily available. Just replace it, I'm sort of surprised that the control unit can't be got at without draining the bed, if only from the "water and mains don't play nicely" POV.

[MattyF]

Now that I have an explanation about the neon I just won't worry about it. There is a large piece of furniture next to my bed by the thermostat and I'd rather not move it. The thermostat has a tube running under the water bed mattress and is attached to the base. It can't be moved without draining the waterbed. The other night I unplugged what I thought was a heater and plugged in a TV. Unfortunately I had unplugged the water bed instead, which I discovered in the middle of the night. I was so cold I had to have a hot shower, and sleep in the guestroom bed.

[MattyF]

I suspect Franck and Hertz got the Nobel prize for the discovery. A pity, I'd rather it was Einstein.

[jkn]

Hi Matty Looks like you're right; I was thinking that it seemed akin to the photo- electric effect, which was what Einstein got the Nobel prize for (eg. not relativity, which most people would have said, I think).

Although I knew of the experiment I didn't know it by name as the Franck-Hertz experiment, so thanks for that.

Cheers J^n

[Geo]

It is worn out. The old nixie tubes used to do the same thing when they were used (instead of LEDs or LCD) in test instruments. Often a blank decade could be coaxed into displaying by shining a torch into it. Buy new neon lamp (Maplin/ebay?)