How do those "touch" lamps work?

[jock]

You know the ones I mean - They go from dim-brighter-brightest-off every time you touch the any metal surface of the lamp.

Does anyone out there know? Am I on the wrong forum, or are you just not interested in my wee problem?

 

[breezer]

Does anyone out there know? Am I on the wrong forum, or are you just not interested in my wee problem?

you only posted jut over 4 hours ago.

they work by capacitance, its basicaly a dimmer which instead of a rotary knob to adjust dimness, uses a capacitance sensor to tell it when to dim

 

[Inspector]

breezer - do they work by sensing the path to earth through the person touching it?

 

[Aletank]

I have one of these.
I've had it about a year now and I'd say about 5 times, I've come down of a morning and its been on !! - lol , The slightest electric surge must trigger them. Also says timer plugs/switches can't be used with them.

 

[Big_Spark]

breezer - do they work by sensing the path to earth through the person touching it?

I believe your right Inspector. The ones I have had dealings with have all been resistance operated, using the body's natural conductance to change the resistance along the two conductors.

However I do believe there are some that do use variable conductance for the control as well.

 

[ZenStalinist]

I believe it uses some sort of FET feedback latch.... I'll delve into my old schoolbooks if you want to know because we did about touch sensors in Electronic Systems.

It picks up on the microvolts of electricity your body generates, so you basically touch the metal area on the lamp, and that gives it enough potential difference. Could be wrong.

It's really more of an electronics issue than an electrics issue.

 

[Aletank]

I remember years ago(about 20) lol, My Nan had a Television before remotes was out properly and to change channel you didn't have to touch the button properly on the TV. My mum use to tell me it works from the heat off your finger - lol
These may work similar

 

[ryanjuk]

Have a look at this.

Touch-sensitive lamps almost always use a fourth property of the human body -- its capacitance. The word "capacitance" has as its root the word "capacity" -- capacitance is the capacity an object has to hold electrons. The lamp, when standing by itself on a table, has a certain capacitance. This means that if a circuit tried to charge the lamp with electrons, it would take a certain number to "fill it." When you touch the lamp, your body adds to its capacity. It takes more electrons to fill you and the lamp, and the circuit detects that difference.

 

[kendor]

I believe it uses some sort of FET feedback latch.... I'll delve into my old schoolbooks if you want to know because we did about touch sensors in Electronic Systems.

It picks up on the microvolts of electricity your body generates, so you basically touch the metal area on the lamp, and that gives it enough potential difference. Could be wrong.

It's really more of an electronics issue than an electrics issue.

Bang on apart from the bit about your body generating the volts, basically the body picks up electricity from surrounding electrical equipment acting as a sort of aerial, this noise if you touch an oscilloscope probe's tip can be seen it is around the 50HZ mark and the touchplate/lamp itself is tuned to pick up this frequency and the microvolts from your fingertip(signal) (your finger joins onto this single wire/aerial) , if you were to shield yourself completely from outside electrical influence then you would not be able to switch on the lamp, not easy in this day and age of being swamped by electrical fields.

That is not to say that some switching circuits don't work by capacitance and quite a few do work that way as explained in the post above(there are different types out there) but a lot of the lamps use the properties of the good old 555 timer chip(incorporated as part of a dimming circuit) to be able to be triggered from the noise from your fingertip when incorporated in the correctly applied tuned circuit

Capacitive sensors in themselves are not touch sensors but "proximity" sensors and as such will trigger from an object being close within it's sensor range(if the plate was big enough you would activate the lamp just by walking passed it) as well as actually touching it, this is why you find them used a lot in the processing industry(ie as part of a PLC counting circuit on a conveyor belt).