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