Hi, Please have a look at the circuit at:
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
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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?
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.
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Dave Plowman dave@davenoise.co.uk London SW
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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.
Yup, Three position switch (view in fixed font)
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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.
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Dave Plowman dave@davenoise.co.uk London SW
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