Help needed to add a two way switch to this circuit

[saurabh9]

Hi,
Please have a look at the circuit at:
http://www.glacialwanderer.com/_blog/blog2008/04_April/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.

 

[Arfa Daily]

Hi,
Please have a look at the circuit at:
http://www.glacialwanderer.com/_blog/blog2008/04_April/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

 

[saurabh9]

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" fora
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?

 

[saurabh9]

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 switchedon
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        [email protected]           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-----------------

 

[Adrian C]

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 ;-)