minimal mcu recommendation given following requirements

hi,

this is my first post here, and a rather long one at that. have read the stickies, and done a search before posting. if in spite i miss something, plz excuse and point it out

i am a programmer by training/education, and have limited (but exciting) experience using Arduino (Duemillanove USB to be precise), but very limited understanding of anything more than basic electronics -- especially so for analog electronics. willing to learn though, and thus these questions.

objective: design mcu based ckt that acts as slave, accepts short-range RF signals from a main controller (master), and turns on/off mains electrical equipment (230VAC-50Hz), including equipment like lights, fan, TV, music-system, refrigerator, and even water pump/motor (rated at 1.5kW). also it should be able to send back acknowledgement, and few other pieces of data s.a. overall health, status etc., to the master. this slave device should ideally be mains powered, and have optional battery backup.

From slave standpoint, I think MCU needs atleast 3 GPIO pins -- 1 pin to control (on/off) device (+ into the Relay), 1 pin for serial rx and 1 for serial tx. If it doesn't have a USART, I could fallback on bitbanging. the rx/tx pins connect to some RF module. For the RF part, my search tells me the 433/315MHz ASK may be the cheapest option for short-range comm. (basic command/response).

The master controller, for now might be a PC with a 433MHz/315Mhz usb dongle, but eventually a MCU based independent design.

I am aware that the above is a common objective of home-automation devices, but I'd like something DIY.

questions:

1. Does the description of slave circuit above sound reasonable or is it incoherent ?

2. for the slave-device, what mcu could lead to cheapest overall BOM considering that the mcu has a reasonable C/tiny-C/small-C programming environment. The question of cheapest needs qualification. Idea is to see if possible, make a commercial venture out of this , so consider something produced in say 5-10k lots.

3. any existing reference design that could be leveraged for this sort of thing ? designing a complete circuit from scratch isn't exactly something I have high hopes of achieving.

all pointers / suggestions welcome.

thanks,
jay

hi

according to me the atmel AT89c2051 controller will do(in terms of cost). you may found many cheaper controllers which may not be available in india(i think you are an indian).but if the quantity is so much, you will get good pricing from the vendors.
any way you have to do the hardware interfacing for the control unit.(you said you are going to control 1.5KW motor).you have to interface 1 transmitter and 1 receiver to the controller because the modules are available only for 1 purpose.the range will be about 30 meters if you connect the good antenna and of line of sight.
for better range you can use tranceiver modules from TI. but they are costlier compare to the ASK modules. here also the quantity does matters.if you don't want the data back from the slave, there could be some other options.
my question is if all the outputs are working on 230VAC, why you need a battery back up?

ml

Hi,

Well your idea is sound, but as for manufacturing such a system think you will find there are many similar systems on the market though perhaps with not as much feedback, if turning on a fridge needs any ?

Look at systems like X10 developed for doing just this kind of thing.

Have used those ASK modules but you need to use an encoding method (typically Manchester) otherwise your signals will be lost in the mass of other signals at that frequency.
You might find Bluetooth a better method.

Thanks @microlab.

microlab said:

according to me the atmel AT89c2051 controller will do(in terms of cost).
you may found many cheaper controllers which may not be available in india(i think you are an indian).but if the quantity is so much, you will get good pricing from the vendors.

Click to expand...

India it is . A very quick price check at Element14 and couple of other distributor shows the price to be still close to about $2. Could you / someone experienced with current volume pricing tell me if it can be brought down to $1 level ? If so, at what kind of volume !! My earlier search had shown me some PIC's available at $0.70 range for about 10units. Of course, I didn't get around to do a apples-to-apples comparison...

microlab said:

any way you have to do the hardware interfacing for the control unit.(you said you are going to control 1.5KW motor).you have to interface 1 transmitter and 1 receiver to the controller because the modules are available only for 1 purpose.the range will be about 30 meters if you connect the good antenna and of line of sight.

for better range you can use tranceiver modules from TI. but they are costlier compare to the ASK modules.

Click to expand...

Doing a quick search earlier had actually found 2 different type of such transceivers -- one claiming to be able to do 30mtrs and another upto 100mtrs. Is this TI module mentioned do 100mts ?

microlab said:

here also the quantity does matters.

Click to expand...

Any idea on what could be the volume price (just to have an estimate) ? My idea is to fit the entire bill-of-material(right?) within a certain range, so this query.

microlab said:

if you don't want the data back from the slave, there could be some other options.

Click to expand...

Could you plz elaborate as to what other options are possible ?

microlab said:

my question is if all the outputs are working on 230VAC, why you need a battery back up?

Click to expand...

Summer is around the corner, and many places will start having erratic 2-4 hours power cuts. I thought this let's me know if power is there or not ! Although on second thoughts I am beginning to question it's value. Maybe have this kind of sensor specifically for the power presence check with a tx on module !

Reading a bit more on Edaboard, I am seeing lot of discussion around Powerline communication for similar applications. Looks very interesting ! However, I am guessing that the system goes dead during power-failures. Given the resistance (I'm assuming it to be quite high) of normal household electrical wiring, I guess attempting comm. on something like 12V battery power would be impossible !

---------- Post added at 22:54 ---------- Previous post was at 22:36 ----------

Thanks @wp100.

wp100 said:

Well your idea is sound, but as for manufacturing such a system think you will find there are many similar systems on the market though perhaps with not as much feedback, if turning on a fridge needs any ?
Look at systems like X10 developed for doing just this kind of thing.

Click to expand...

Commercialising the idea is not my primary objective ... satisfaction of doing something useful is! Of course, if commercialisation helps a cause, all the more better.
I did run into X10 and bunch of others... just after posting this. Looks very interesting, but as you mention I saw that it doesn't seem to have any feedback.
Turning fridge on/off doesn't need feedback for sure, however turning on/off an old AC, that has fixed thermostat(right?) might be interesting. if in the same unit I can add a temperature sensor, it can help me turn it into a climate control solution as well. something, I think I can't do with X10... I might be wrong, haven't spent enough time reading about it.

wp100 said:

Have used those ASK modules but you need to use an encoding method (typically Manchester) otherwise your signals will be lost in the mass of other signals at that frequency.

Click to expand...

Okay. Do I need to do the manchester encoding in software? And, don't I need something to uniquely identify each such modules from the central controller and back ? Would it be something like a uniqueId hardcoded in program memory ? Or is there some other way ?

wp100 said:

You might find Bluetooth a better method.

Click to expand...

Would love to do that... in fact, that was my first thought. Would've let me control the switch directly using my mobile, but 2 worries crept in.
1. Wouldn't that require a much higher capability MCU, and add significantly onto the overall BOM cost ?
2. Bluetooth range seems to be pretty limited. Solution seems to be Zigbee, but zigbee apparently needs software stack, and thus overall BOM becomes expensive.
3. Both of those were taking the complexity beyond my reach ... it already seem to be fairly complex.