Hi,
Please have a look at the circuit at:
http://www.glacialwanderer.com/_blog.../hb_relay5.jpg.
It is from the article at http://www.glacialwanderer.com/hobbyrobotics/?p=9.

The circuit it to turn on or off a AC bulb by a microcontroller. My
question is, can we somehow safely add a manual override switch near
the bulb, so that even if the microprocessor is not working, we can
turn the switch on/off manually? If yes, how will the circuit then
look like?
Sorry if this is a simple question as I am just starting in
electronics and electrical circuits.

"saurabh9" wrote in message
...
Hi,
Please have a look at the circuit at:
http://www.glacialwanderer.com/_blog.../hb_relay5.jpg.
It is from the article at
http://www.glacialwanderer.com/hobbyrobotics/?p=9.

The circuit it to turn on or off a AC bulb by a microcontroller. My
question is, can we somehow safely add a manual override switch near
the bulb, so that even if the microprocessor is not working, we can
turn the switch on/off manually? If yes, how will the circuit then
look like?
Sorry if this is a simple question as I am just starting in
electronics and electrical circuits.

There should be no problem just adding a switch across the upper and lower
relay connections on the right hand side of the square marked "relay" for a
manual over-ride. It then doesn't matter what the relay is doing. It does
mean, however, that once you've turned the lamp on with this switch, the
microprocessor has no further control over the lamp, until you turn the
manual switch back off. Is that what you need to happen ?

Arfa

On Jul 10, 1:13*pm, "Arfa Daily" wrote:
"saurabh9" wrote in message

...

Hi,
Please have a look at the circuit at:
http://www.glacialwanderer.com/_blog.../hb_relay5.jpg.
It is from the article at
http://www.glacialwanderer.com/hobbyrobotics/?p=9.

The circuit it to turn on or off a AC bulb by a microcontroller. My
question is, can we somehow safely add a manual override switch near
the bulb, so that even if the microprocessor is not working, we can
turn the switch on/off manually? If yes, how will the circuit then
look like?
Sorry if this is a simple question as I am just starting in
electronics and electrical circuits.

There should be no problem just adding a switch across the upper and lower
relay connections on the right hand side of the square marked "relay" for a
manual over-ride. It then doesn't matter what the relay is doing. It does
mean, however, that once you've turned the lamp on with this switch, the
microprocessor has no further control over the lamp, until you turn the
manual switch back off. Is that what you need to happen ?

Arfa

What I need is a manual override, to that my unit is usable even if my
automation system fails. Hmm...maybe I can improve upon your idea and
put a sort of three way switch just on the right of the relay, with ON
(direct connection), off (no connection), auto (connection to the
relay). Do you think there might be any safety issue in this approach?

In article
,
saurabh9 wrote:
Hi,
Please have a look at the circuit at:
http://www.glacialwanderer.com/_blog.../hb_relay5.jpg.
It is from the article at http://www.glacialwanderer.com/hobbyrobotics/?p=9.

The circuit it to turn on or off a AC bulb by a microcontroller. My
question is, can we somehow safely add a manual override switch near
the bulb, so that even if the microprocessor is not working, we can
turn the switch on/off manually? If yes, how will the circuit then
look like?
Sorry if this is a simple question as I am just starting in
electronics and electrical circuits.

Do you really mean two way switching? That circuit means either switch
over-rides the state of the other. Like in hall lights. So your relay
controlled one might switch the light off if already on or on if already
off. The circuit for that is below - but the relay needs to be a
changeover type with three contacts.



L1 L1
0===========0 0===========0============= Line
| \ / |
C 0================================O C
\ / \ /
0===========0 0===========0============= Switch return
L2 Optional L2
Intermediate

If, however, all you wish to do is make sure the light can be switched on
regardless of the relay state, add a switch in parallel to the relay
contacts.

--
*Time is fun when you're having flies... Kermit

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

On Jul 10, 2:11*pm, "Dave Plowman (News)"
wrote:
In article
,
* *saurabh9 wrote:

Hi,
Please have a look at the circuit at:
http://www.glacialwanderer.com/_blog.../hb_relay5.jpg.
It is from the article athttp://www.glacialwanderer.com/hobbyrobotics/?p=9.
The circuit it to turn on or off a AC bulb by a microcontroller. My
question is, can we somehow safely add a manual override switch near
the bulb, so that even if the microprocessor is not working, we can
turn the switch on/off manually? If yes, how will the circuit then
look like?
Sorry if this is a simple question as I am just starting in
electronics and electrical circuits.

Do you really mean two way switching? That circuit means either switch
over-rides the state of the other. Like in hall lights. So your relay
controlled one might switch the light off if already on or on if already
off. The circuit for that is below - but the relay needs to be a
changeover type with three contacts.

* * * * * L1 * * * * * * * * * * * * * L1
* * * * * 0===========0 * *0===========0============= Line
* * * * * | * * * * * *\ */ * * * * * *|
* * * C 0================================O C
* * * * *\ * * * * * * / *\ * * * * * * /
* * * * * 0===========0 * *0===========0============= Switch return
* * * * * L2 * * * * Optional * * * * *L2
* * * * * * * * * *Intermediate

*If, however, all you wish to do is make sure the light can be switched on
regardless of the relay state, add a switch in parallel to the relay
contacts.

--
*Time is fun when you're having flies... Kermit *

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

If I put a switch simply in parallel to the relay, then if by chance
the relay is ON, then wont i not be able to manually turn off the
switch? I guess then I might have to add two switches. Maybe the
other switch will have to be at point x, to cut off relay all
together
tried to illustrate the circuit below.

-------x-------------------- -
| | |
| switch |
relay | bulb
| | |
--------AC-----------------

As usual, the Lady from Philadelphia has to step in and resolve this issue.

It would be a simple matter (as someone else mentioned) to use single-pole,
double-throw switching at both ends to permit either computer control or
local control of the light. (This type of switching has been in use -- I'd
assume -- close to a century. You should be able to find a schematic in any
book on household wiring. It's trivial.)

The "catch" is that the switch positions no don't directly correlate with
whether the light is on or off. (I normally wire the switches so that when
both are up or both are down, the light is on.) So closing the relay
operated by the microcontroller will turn the lamp on or off, depending on
the position of the local switch. Do you understand that?

If this ambiguity isn't a problem, then simply wire up the system as any
two-way lighting system, and you'll have full control at both ends. You just
won't know whether closing the microcontroller relay turns the light on or
off.

Note that if you add a manual _override_ at the light's location, that
_forces_ the lamp on, then the microcontroller _won't_ be able to shut it
off. I don't think that's what you want.

saurabh9 wrote:
On Jul 10, 1:13 pm, "Arfa Daily" wrote:
"saurabh9" wrote in message

...

Hi,
Please have a look at the circuit at:
http://www.glacialwanderer.com/_blog.../hb_relay5.jpg.
It is from the article at
http://www.glacialwanderer.com/hobbyrobotics/?p=9.
The circuit it to turn on or off a AC bulb by a microcontroller. My
question is, can we somehow safely add a manual override switch near
the bulb, so that even if the microprocessor is not working, we can
turn the switch on/off manually? If yes, how will the circuit then
look like?
Sorry if this is a simple question as I am just starting in
electronics and electrical circuits.
There should be no problem just adding a switch across the upper and lower
relay connections on the right hand side of the square marked "relay" for a
manual over-ride. It then doesn't matter what the relay is doing. It does
mean, however, that once you've turned the lamp on with this switch, the
microprocessor has no further control over the lamp, until you turn the
manual switch back off. Is that what you need to happen ?

Arfa

What I need is a manual override, to that my unit is usable even if my
automation system fails. Hmm...maybe I can improve upon your idea and
put a sort of three way switch just on the right of the relay, with ON
(direct connection), off (no connection), auto (connection to the
relay). Do you think there might be any safety issue in this approach?

Yup, Three position switch (view in fixed font)


o - off
\
+----o o----------- Bulb -----+
| |
| o - on |
r | | Plug in wall
e o | |
l \ | |
a o | |
y | | |
+-----+------------------------+

Don't know how other posters have turned this into a two-way light bulb
and intermediate hall/stairway switching situation....

Get someone to check it over. This isn't rocket science, but you will
need a good three way switch panel that doesn't electrocute someone ;-)

--
Adrian C

In article ,
Adrian C wrote:
Don't know how other posters have turned this into a two-way light bulb
and intermediate hall/stairway switching situation....

That's what's the subject says?

Besides - we haven't been told what the circuit is for. Could be a
wireless remote switch in which case a two way circuit would be fine.

--
*A closed mouth gathers no feet.*

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