HELP ! (Please !) - Very basic switch / LED question

[[email protected]]

Hi,

I am hoping to modify an audio mixing desk so that when a latching
pushbutton switch is depressed, a miniature LED will illuminate to
indicate activation (as it is currently very difficult to see with
black (non indicating) pushbuttons on a black background used in dim
lighting conditions !!).

The problem I have is that the switches are DPDT latching, and BOTH
poles are currently used to route audio signals. I presume that it is
not possible to connect an LED to one of these poles as it will
introduce voltage / current where the original design wasn't expecting
it....

I had thought about miniature DPDT relays... re-route one 'pole' of
the audio signal off the existing switch onto one pole of the relay,
use the freed-up pole on the existing switch to drive the relay and
use the spare pole on the relay to switch the LED. However, with
thirty switches to deal with, this could be expensive and cause quite
a power drain.

Is there any easier way (transistor ?) to switch an LED when you only
have a DPDT switch (with both poles used for other things !) ??

Many thanks for any help you can give.

Cheers,

Kev.

 

[Paul Burke]

I am hoping to modify an audio mixing desk so that when a latching
pushbutton switch is depressed, a miniature LED will illuminate to
indicate activation ...
The problem I have is that the switches are DPDT latching, and BOTH
poles are currently used to route audio signals. ...
Is there any easier way (transistor ?) to switch an LED when you only
have a DPDT switch (with both poles used for other things !) ??

Does the switch have a bit that sticks out the back when it is in the
depressed position? If so, you could use that to operate a microswitch
to turn the LED on/off.

Paul Burke

 

[Andy]

Hi,
...
Is there any easier way (transistor ?) to switch an LED when you only
have a DPDT switch (with both poles used for other things !) ??

Many thanks for any help you can give.

Cheers,

Kev.

If you're looking for a pure electric/electronic method (i.e. not
involving mecanical fixtures around the switches), it will of course
depend on the caracteristics of the audio signals that are routed by the
switches and the circuit stages before and after.

Can you caracterize them?

For example, if both the preceding and the following stages were
AC-coupled (net very probable, I think), you could try to inject a small
current on one side of the switch and sense for it on the other side.
The disadvantage would be some 'click' during the commutation.

Another, more complex approach would be to inject an sine wave of a
given frequency not within the working range of the desk's circuits, and
sense for it on the other side of the switch.

-- Andy

 

[Luhan Monat]

Hi,

I am hoping to modify an audio mixing desk so that when a latching
pushbutton switch is depressed, a miniature LED will illuminate to
indicate activation (as it is currently very difficult to see with
black (non indicating) pushbuttons on a black background used in dim
lighting conditions !!).

The problem I have is that the switches are DPDT latching, and BOTH
poles are currently used to route audio signals. I presume that it is
not possible to connect an LED to one of these poles as it will
introduce voltage / current where the original design wasn't expecting
it....

I had thought about miniature DPDT relays... re-route one 'pole' of
the audio signal off the existing switch onto one pole of the relay,
use the freed-up pole on the existing switch to drive the relay and
use the spare pole on the relay to switch the LED. However, with
thirty switches to deal with, this could be expensive and cause quite
a power drain.

Is there any easier way (transistor ?) to switch an LED when you only
have a DPDT switch (with both poles used for other things !) ??

Many thanks for any help you can give.

Cheers,

Kev.

Hi,

How about just replacing the switch with one that...

a) fits in the same hole.

b) is a toggle/handle type that is easy to see which way it thrown.

 

[Mark Jones]

Hi,

How about just replacing the switch with one that...

a) fits in the same hole.

b) is a toggle/handle type that is easy to see which way it thrown.

c) is a 3PDT switch.

 

[NSM]

Hi,

I am hoping to modify an audio mixing desk so that when a latching
pushbutton switch is depressed, a miniature LED will illuminate to
indicate activation (as it is currently very difficult to see with
black (non indicating) pushbuttons on a black background used in dim
lighting conditions !!).

Have you tried a booklight with doublesided tape? Sometimes easy works well.
Otherwise you are looking at replacing all the switches.

 

[[email protected]]

OK, I think I can replace the switches with multi-pole changeover
types, now that I've found more suppliers than RS and Maplin !

I'm a bit confused though.. according to this site :
http://www.kpsec.freeuk.com/components/switch.htm , near the bottom,
there is a pic of the kind of switch I need - labelled "Multi-Pole
Switch". However, the text says the pic is of a SIX pole changeover
switch. It's got twelve tags... !?? Huh ??

My logic says that if the switch is a 'Changeover' that means it's
double-throw, right ?

So if it's laid out like this :-

1 2 3 4 5 6
1 2 3 4 5 6

Then pins 2 and 5 in each row are the common connections and depending
on the switch position will connect 2-1 and 5-4 OR 2-3 and 5-6 in each
row.

Am I misunderstanding this ? ....or have they just got the wrong
picture there and are actually showing a FOUR pole changeover (or
possibly a SIX pole 'on-off' type).

Cheers,

Kev.

 

[Rich Grise]

OK, I think I can replace the switches with multi-pole changeover
types, now that I've found more suppliers than RS and Maplin !

I'm a bit confused though.. according to this site :
http://www.kpsec.freeuk.com/components/switch.htm , near the bottom,
there is a pic of the kind of switch I need - labelled "Multi-Pole
Switch". However, the text says the pic is of a SIX pole changeover
switch. It's got twelve tags... !?? Huh ??

My logic says that if the switch is a 'Changeover' that means it's
double-throw, right ?

So if it's laid out like this :-

1 2 3 4 5 6
1 2 3 4 5 6

Then pins 2 and 5 in each row are the common connections and depending
on the switch position will connect 2-1 and 5-4 OR 2-3 and 5-6 in each
row.

Am I misunderstanding this ? ....or have they just got the wrong
picture there and are actually showing a FOUR pole changeover (or
possibly a SIX pole 'on-off' type).

That pic doesn't show it all - it's a PCB switch, and the lugs on the top
seem to be for convenience:
http://www.rapidelectronics.co.uk/images/siteimg/78-0500e.pdf

Good Luck!
Rich

 

[[email protected]]

I don't see what you're getting at... I know it's a PCB switch -
that's what I need. I was questioning my understanding of which pins
connect to which when operated. The pic in my url shows a switch with
twelve connectors (forget the lugs). If my understanding of how these
work is correct then the one shown is a FOUR pole switch - common and
two alternate connections per pole = 3 x 4 = 12. OR it's a six pole
single throw (i.e. on-off) switch - 2 connections per pole = 2 x 6 = 12.

 

[Rich Grise]

I don't see what you're getting at... I know it's a PCB switch -
that's what I need. I was questioning my understanding of which pins
connect to which when operated. The pic in my url shows a switch with
twelve connectors (forget the lugs). If my understanding of how these
work is correct then the one shown is a FOUR pole switch - common and
two alternate connections per pole = 3 x 4 = 12. OR it's a six pole
single throw (i.e. on-off) switch - 2 connections per pole = 2 x 6 = 12.

Well, you could always ask the guys who are selling them.

Good Luck!
Rich

 

[Gary J. Tait]

OK, I think I can replace the switches with multi-pole changeover
types, now that I've found more suppliers than RS and Maplin !

I'm a bit confused though.. according to this site :
http://www.kpsec.freeuk.com/components/switch.htm , near the bottom,
there is a pic of the kind of switch I need - labelled "Multi-Pole
Switch". However, the text says the pic is of a SIX pole changeover
switch. It's got twelve tags... !?? Huh ??

My logic says that if the switch is a 'Changeover' that means it's
double-throw, right ?

So if it's laid out like this :-

1 2 3 4 5 6
1 2 3 4 5 6

Then pins 2 and 5 in each row are the common connections and depending
on the switch position will connect 2-1 and 5-4 OR 2-3 and 5-6 in each
row.

That is the way that sort of switch usually works.

Am I misunderstanding this ? ....or have they just got the wrong
picture there and are actually showing a FOUR pole changeover (or
possibly a SIX pole 'on-off' type).

It is indeed a 4 pole double throw latching pushbutton switch

 

[Bill Jeffrey]

Methinks it is a typo. What I see is exactly what you think it is - a
FOUR-pole double-throw switch. There are several minor errors like this
in the rest of the page.

Bill Jeffrey
=============================

 

[[email protected]]

Thankyou both for clearing that up ! I thought I was getting senile
for a moment then !