Hi,

I am trying to design an audible/visual indicator that beeps every 5 minutes when a light switch is turned on.

The complete circuit has to fit behind a light switch plate and so a transformer cannot be used.

Any ideas or pointers would be really useful.

Thanks.

In article ,
M.Joshi wrote:
I am trying to design an audible/visual indicator that beeps every 5
minutes when a light switch is turned on.

The complete circuit has to fit behind a light switch plate and so a
transformer cannot be used.

This can't be done unless you have a neutral at the switch position. When
the switch is on there is no potential difference between the terminals
therefore no power source.

Any ideas or pointers would be really useful.

--
*I will always cherish the initial misconceptions I had about you

Dave Plowman London SW
To e-mail, change noise into sound.

Dave, I was thinking just that after submitting the post! Could the earth be used instead of neutral for low currents? Is that a good idea?

Otherwise, I was considering a coin battery solution where the current flowing through a wire is sensed.

Otherwise, I was considering a coin battery solution where the current
flowing through a wire is sensed.



Don't try to measure current through the wire, it's far easier to
measure voltage across the switch. Zero voltage means the switch is on.

"Dave Plowman (News)" writes:

In article ,
M.Joshi wrote:
I am trying to design an audible/visual indicator that beeps every 5
minutes when a light switch is turned on.

The complete circuit has to fit behind a light switch plate and so a
transformer cannot be used.

This can't be done unless you have a neutral at the switch position. When
the switch is on there is no potential difference between the terminals
therefore no power source.

It could be battery or supercap powered.

Or, if the box is grounded, there will be line voltage between the
load side and the box in a correctly wired normal (not 3-way) circuit.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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Important: Anything sent to the email address in the message header above is
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In article ,
Sam Goldwasser wrote:
This can't be done unless you have a neutral at the switch position.
When the switch is on there is no potential difference between the
terminals therefore no power source.

It could be battery or supercap powered.

Might be difficult to fit all this inside a switch backing box?

Or, if the box is grounded, there will be line voltage between the load
side and the box in a correctly wired normal (not 3-way) circuit.

I hope you're not recommending this? In the UK it would be forbidden - not
least because it unbalances the line and neutral.

--
*Gun Control: Use both hands.

Dave Plowman London SW
To e-mail, change noise into sound.

Dave, Could you expand a bit more on what you mean by "unbalances the line and neutral." please?

Thanks.

In article ,
M.Joshi wrote:
Dave, Could you expand a bit more on what you mean by "unbalances the
line and neutral." please?

In the UK, neutral is connected to earth at some point - the last possible
one being at the incoming main. After that, they are separate. Use the
earth cable in the house as a return and you are now not drawing equal
current on line and neutral. And RCDs work by looking for a difference in
the current flow between line and neutral, so you don't want to impose a
load which does just that. Of course, if it's only a couple of milliamps
it's unlikely to matter, but in principle it's a bad idea.

--
*Cover me. I'm changing lanes.

Dave Plowman London SW
To e-mail, change noise into sound.

Dave Plowman (News) wrote:
In article ,
Sam Goldwasser wrote:

This can't be done unless you have a neutral at the switch position.
When the switch is on there is no potential difference between the
terminals therefore no power source.


It could be battery or supercap powered.


Might be difficult to fit all this inside a switch backing box?


Or, if the box is grounded, there will be line voltage between the load
side and the box in a correctly wired normal (not 3-way) circuit.


I hope you're not recommending this? In the UK it would be forbidden - not
least because it unbalances the line and neutral.



UK electrical boxes are incredibly tiny compared to those in the
US/Canada. Our standard boxes are 2.5" deep but you can get them up to
4" deep in the standard 3.5"x2" size used for single switches and
receptacles.


Connecting between line and ground would not be legal by code, but if
the circuit is designed carefully it would not be a danger either. I'd
lean towards powering it with a battery personally, much safer for the
novice.

"Dave Plowman (News)" writes:

In article ,
Sam Goldwasser wrote:
This can't be done unless you have a neutral at the switch position.
When the switch is on there is no potential difference between the
terminals therefore no power source.

It could be battery or supercap powered.

Might be difficult to fit all this inside a switch backing box?

Or, if the box is grounded, there will be line voltage between the load
side and the box in a correctly wired normal (not 3-way) circuit.

I hope you're not recommending this? In the UK it would be forbidden - not
least because it unbalances the line and neutral.

I did not address the regulatory issues. But, I assumed he was talking
about a few mA of current just to operate a flashing high brightness LED,
the the line balance wouldn't be a practical issue unless it tripped the
RCD/GFCI/whatever.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.

Dave Plowman (News) wrote:
In article ,
M.Joshi wrote:

I am trying to design an audible/visual indicator that beeps every 5
minutes when a light switch is turned on.


The complete circuit has to fit behind a light switch plate and so a
transformer cannot be used.


This can't be done unless you have a neutral at the switch position. When
the switch is on there is no potential difference between the terminals
therefore no power source.





There's almost always a neutral in the switch box, occasionally I see an
arrangement with power coming into the light box and a separate switch
drop but it's fairly rare. Also it'd be possible to power the reminder
with a battery in that case, it wouldn't have to draw much power. If the
lamp is incandescent you could even use a rechargeable battery or memory
backup capacitor to power it and "steal" power to charge the battery
when the switch is off.

In article MzWsf.11569$X86.8911@trnddc04,
James Sweet wrote:
This can't be done unless you have a neutral at the switch position.
When the switch is on there is no potential difference between the
terminals therefore no power source.

There's almost always a neutral in the switch box, occasionally I see an
arrangement with power coming into the light box and a separate switch
drop but it's fairly rare.

Right. In the UK the latter is the norm. Lighting circuits are radials and
the feed goes from one ceiling lamp to another, with the switch drop from
that lamp. Of course if you have floor outlets for table lamps etc you're
more likely to find a neutral at the switch.

Also it'd be possible to power the reminder with a battery in that
case, it wouldn't have to draw much power. If the lamp is incandescent
you could even use a rechargeable battery or memory backup capacitor to
power it and "steal" power to charge the battery when the switch is off.

Yup - although it might be tight to fit all that within the switch back
box. In the UK, some are so shallow there's just enough room for the cable.

--
Is the hardness of the butter proportional to the softness of the bread?*

Dave Plowman London SW
To e-mail, change noise into sound.

On Thu, 29 Dec 2005 09:02:49 +0000 (GMT), "Dave Plowman (News)"
put finger to keyboard and composed:

In article ,
M.Joshi wrote:
I am trying to design an audible/visual indicator that beeps every 5
minutes when a light switch is turned on.

The complete circuit has to fit behind a light switch plate and so a
transformer cannot be used.

This can't be done unless you have a neutral at the switch position. When
the switch is on there is no potential difference between the terminals
therefore no power source.

What about using a dimmer switch set to about 90% duty cycle? The
circuit could grab some power during the 10% off time.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

This is not being built for a blind person - This light switch often gets left switched on all the time as it controls a light outside.

It already has a neon light on it which seems to dim when the light is switched on?

If I build the circuitry into a separate box next to the light switch, is there an easy way to sense when the switch is on/off?

In article ,
M.Joshi wrote:
If I build the circuitry into a separate box next to the light switch,
is there an easy way to sense when the switch is on/off?

In the UK lighting switches are usually single pole, so if that's also the
case in the US change to a double pole one and use one pole for the
sounder.

--
*If at first you don't succeed, redefine success.

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (News)" wrote in message
...
In article ,
M.Joshi wrote:
If I build the circuitry into a separate box next to the light switch,
is there an easy way to sense when the switch is on/off?

In the UK lighting switches are usually single pole, so if that's also the
case in the US change to a double pole one and use one pole for the
sounder.

--
*If at first you don't succeed, redefine success.

Dave Plowman London SW
To e-mail, change noise into sound.
why not use one of the movement sensor type lights

On Tue, 3 Jan 2006 12:27:55 +0000, M.Joshi
put finger to keyboard and composed:


Franc Zabkar Wrote:
On Thu, 29 Dec 2005 09:02:49 +0000 (GMT), "Dave Plowman (News)"
put finger to keyboard and composed:

In article ,
M.Joshi wrote:
I am trying to design an audible/visual indicator that beeps every 5
minutes when a light switch is turned on.

The complete circuit has to fit behind a light switch plate and so a
transformer cannot be used.

This can't be done unless you have a neutral at the switch position.
When
the switch is on there is no potential difference between the
terminals
therefore no power source.

What about using a dimmer switch set to about 90% duty cycle? The
circuit could grab some power during the 10% off time.

This is not being built for a blind person - This light switch often
gets left switched on all the time as it controls a light outside.

It already has a neon light on it which seems to dim when the light is
switched on?

The neon is probably wired across the switch contacts. When the switch
is off the neon sees the full mains potential. When the switch is
closed the potential is zero.

If I build the circuitry into a separate box next to the light switch,
is there an easy way to sense when the switch is on/off?

Here's my suggestion:
http://www.users.on.net/~fzabkar/WarnLight.JPG

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Are you building this for a blind person? Just curious.

On Wed, 28 Dec 2005, M.Joshi wrote:


Hi,

I am trying to design an audible/visual indicator that beeps every 5
minutes when a light switch is turned on.

The complete circuit has to fit behind a light switch plate and so a
transformer cannot be used.

Any ideas or pointers would be really useful.

Thanks.


--
M.Joshi