I have discovered the fatal flaw in my pull switch and humidistat bathroom
I cannot tell if the humidiatat is on or the pull switch is on.
I have worked out the wiring (which is how I first proved that the
humidistat was on most of the time) but I would like to fit a small LED
which only lights when current is passing through the switch.
To do this I need a 240v LED which can be mounted in a small hole in a
Looked in Maplins but can't find a 240v LED - although most don't seem to
have a voltage rating.
That's because an LED is a DC current operated device. For normal DC
operation they require a series resistor the value of which depends on the
voltage it's running off and the current you wish to drive it with. For AC
use, you require a diode as well.
It's more normal to use a neon indicator, but if you'd prefer the look of
an LED it's easy enough if you can solder.
I *think* RS components do mains LED indicators ready made - they
certainly exist, I've got some.
*Change is inevitable, except from a vending machine.
Dave Plowman firstname.lastname@example.org London SW 12
Wire a mains neon ie one *with* a series resistor, between the switched
contact and neutral; connect the series resistor end to the live (switched
wire) side. Modify the case so you can see the neon, obseve safety at the
A LED would have to have 228V dropped across it's resistor, at about 10
mill thats about 2.3 watts - definitely not the way to go
You could use 3 or 4 series rectifier diodes and wire the LED + resistor
across these - that would give you a suitable voltage drop to power a
small LED. You would also need a reverse diode to bypass the LED so that
when the reverse phase comes along your LED does not go pop!
Not true, LED's are only tolerant to a small reverse voltage, much less
than a normal diode. If you are working from a 9 volt PP3 then they will
be OK connected backwards. At 240V mains they will be destroyed by the
If the electrical option is too complex for this reason, there may be
a mechanical solution. The last time we were in the US, the ceiling
fans had a little mechanical doodah on the pull cord to show what
state the fan was in. Although now that I think about it, the fans
(and the doodahs) were designed with 1 off state and 3 on states for
the different speeds. So even if you could find them here (unlikely)
they probably wouldn't be much use for a 2 state device... Sorry!
[See previous posts]
I already have an external on/off switch with both neon and mechanical
The issue is the internal switch (tiny) within the fan body.
I need a very small indicator light to show if power is coming via the
switch or the humidistat - both of which are built into the compact fan
So the indicator has to be integrated into the fan body.
Maybe I didn't explain myself well... The mechanical indicator is
attached to the pull string of the internal fan switch, so that it
would track (and indicate) the state of this internal switch. So it
would indicate whether the humidistat was being over-ridden by the
manual switch. Sounds very clunky, but actually worked fine on the
overhead fans. I expect these are an over-the-counter accessory sold
in the US, but may not be available in a two-state on-off form.
Did you get your answer yet, David W.E.?
I read most of the thread but couldn't see any simple answer...
As far as I am concerned, you need to look at the *current* flowing
through the switch (or the humidistat), as I guess they are connected
I saw a circuit using a LED and a few diodes, which in effect will
monitor the current. It was used as an indicator for a normal (no
neutral) 240V light switch.
Quote: (for 'Lamp' read 'Motor')
fixed width font
--|>|-- = DIODE
A bit heavy on component count but lights evenly independent of lamp
load, choose diodes to match full lamp current. LED is low current
All parts are live, including the LED. For safety don't poke the led
thro the panel, use a plastic holder.
Extra components would be required to meet BS/VDE safety.
Unquote... (from uk.diy - "Power an LED from a light switch?")
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