reading signal from mains...

[computer]

Hi,

I have a low voltage microcontroller circuit up and running and I'd like to be able to control it via a standard theatre dimmer (for this application, it makes sense).

the input would be 230v but it could be dimmed. originally I was thinking a mains relay would work but if it was dimmed via PWM it might make the relay flick on/off badly. is there a nicer way to test? Also is it possible to do vague readings as in percentage? so I can have say 5 different levels: 0%, 20%, 40%, 60%, 80% and 100%?

thanks

 

[Nigel Goodwin]

Hi,

I have a low voltage microcontroller circuit up and running and I'd like to be able to control it via a standard theatre dimmer (for this application, it makes sense).

the input would be 230v but it could be dimmed. originally I was thinking a mains relay would work but if it was dimmed via PWM it might make the relay flick on/off badly. is there a nicer way to test? Also is it possible to do vague readings as in percentage? so I can have say 5 different levels: 0%, 20%, 40%, 60%, 80% and 100%?

I think you need to post a LOT more details, I don't really understand wha you mean?. As for it making sense for your application, if you don't give details on that application how are we supposed to offer suggestions?.

 

[computer]

Ok fine.

A standard theatre dimmer will PWM a 230v output (usually at 15amps).

I need to make an effect for a show and would like to be able to control it via a usual dimmer. The effect section is all sorted out and is controlled by a microcontroller to some relays. It takes it's own 230v 'hot' supply (ie, 13amp socket). There's a transformer to make 5v for the ICs too.

I want to be able to plug another plug into a dimmer socket, and sense whether it is on or off. I thought a relay but if its PWM it might cause a relay to click on/off rapidly, not good, so maybe a capacitor across the relay coil might stop that? It would be a 230v coil relay so I need properly rated components.

A plus point is if you suggest another way (perhaps some sort of opto-isolator or something) is if it could give me a general analogue reading, ie, instead of just on or off, I could detect say about 50% (so when the desk is outputting 50% to the dimmer my circuit can read that as well), so I could tell the MC to do a different function perhaps?

Does that explain more?

 

[Nigel Goodwin]

Ok fine.

A standard theatre dimmer will PWM a 230v output (usually at 15amps).

PWM is used for controlling DC power, lighting controllers are usually AC, and so use phase control for dimming - where a triac (or thyristor, depending on design) is only triggered a specific amount of time after zero crossing, the later you trigger, the dimmer the light.

I need to make an effect for a show and would like to be able to control it via a usual dimmer. The effect section is all sorted out and is controlled by a microcontroller to some relays. It takes it's own 230v 'hot' supply (ie, 13amp socket). There's a transformer to make 5v for the ICs too.

I want to be able to plug another plug into a dimmer socket, and sense whether it is on or off. I thought a relay but if its PWM it might cause a relay to click on/off rapidly, not good, so maybe a capacitor across the relay coil might stop that? It would be a 230v coil relay so I need properly rated components.

A mains AC relay should be OK, you would have to find the actual 'drop out' point of the relay by experiment. This would give relay OFF at zero brightness, and turning ON as you increased the power through the dimmer. Going back down it would turn back OFF at a far lower point.

A plus point is if you suggest another way (perhaps some sort of opto-isolator or something) is if it could give me a general analogue reading, ie, instead of just on or off, I could detect say about 50% (so when the desk is outputting 50% to the dimmer my circuit can read that as well), so I could tell the MC to do a different function perhaps?

Yes, so it's really just to save running a cable?. You could certainly generate a varying DC voltage from the dimmer signal, it's probably easiest (and safest) to use a low voltage transformer. Take the output of the transformer through a bridge rectifier onto a capacitor, this will smooth the voltage - you could then monitor this voltage using the A2D in the PIC (assuming it has one?) - you would also need to place a load resistor across the capacitor to discharge it as the dimmer is turned down. Values for the capacitor and resistor are probably best arrived at experimentally.