posted 02-28-2000 12:57 AM            

Please read this first before continuing:
http://www.geocities.com/SiliconValley/Hills/6490/mc460modc.txt

He is addressing the safety issue of modifying a mini controller. He makes a very good point,and since I had first hand experience with what could go wrong, I must agree. If any component breaks down, say during an electrical storm which produces a surge, there is the potential that line voltage could be placed on the switch leads which after a modification are extended to reed switches. Remember there is no transformer providing isolation from the line in the mini-controller, everything is hot, that's probably why X10 put a transformer into the front end of the Powerflash. To see this go to: http://www.geocities.com/ido_bartana/
click on "modification to X10 modules" and then scroll to "Powerflash Mod Module" and open "Schem: S.M. Bloom". It would be wise for anyone considering modifiying this unit to place small, low voltage SPDT relays on/at the mini controller, to control the Unit switch ON/OFF connections, and have SPST reed switches control the SPDT relays, powered by a low voltage transformer. This should provide adequate isolation from the AC line. It shouldn't effect any function or use of the project except for a slight increase in cost. Please review the following string for more info on this topic:
http://www.homeautomationforum.com/ubb/Forum16/HTML/000003.html

posted 02-28-2000 09:11 AM               

Carol is right on this, it should somehow be isolated. How difficult would it be to put in opto isolators I wonder?

posted 02-28-2000 03:44 PM               

At Rick's request I'm poppin' into the middle of the discussion so if I've misunderstood the concerns I'll be happy to reconsider my conclusions.

Personal safety must be a consideration when interfacing 120vac devices. I have to agree that that some isolation or integration is called for if you want to bullet proof the modified mini controller. As to what kind of isolation, that is really a matter of application.

Remember that many many consumer devices use no transformer in the power supply and that often the components that could fail and cause potential exposure to line voltage will also cause a cascade of component failures that will render the unit inoperable thus removing the potential hazard. X10 devices are a pretty fair example of this scenario.

INTEGRATION:
If your whole project using the modified MC is self contained in a box or chassis or if all devices can be hardwired to a common power source I would simply use low voltage/current signal diodes that will blow like a fuse if a fault occurs. A 1N914 comes to mind.

I would feel the same if I were running a dedicated single pair cable to a dedicated door switch or other isolated dry contacts. Use a standard bell wire with a 600 volt rating. DO NOT use your cat5 network!

ISOLATION:
Transformers, relays or optical isolators can also be very effective and would be my first choice if I were interfacing other 120vac devices that might result in a difference in voltage potential between the two units.

The assumption that all relays are noisy is a product of poor design on the part of X10. Relays in general are quite common in household appliances and you would never know they were there unless you looked for them. In this case the discussion is about reed relays and there can be little to no noise relating to the use of this style of relay.

There is sufficient room in the mini controller to add interface relays but they will require a wall wart or some other transformer isolated low voltage DC power supply to accomplish the purpose. Optoisolaters have the same external power requirments of relays and often require additional resisters, diodes or capacitors to build out an interface circuit.

This is a kind of general overview and my opinion. You are free to disagree or to introduce a specific circuit that we can discuss and make application to. Hope this helps.

My .02 worth,

posted 02-28-2000 05:34 PM            

Whether an optoisolator or relays are used they will need a power supply since they are both active devices. I'm not real familiar on how to with configure optoisolators but here is how it might be configured. 2 optoisolators drawing about 20ma each would be require per mini module Unit switch, one for ON the other for OFF. The switching side of the optoisolator would have to be configured to switch the Logic=High of 17.5Vdc present in the minicontroller and the other for the Logic=Low of Ground to the pins of the IC. SPDT reed switches with a 3 wire run to toggle the two optoisolators for each Unit switch would be required. A 250ma wall mounted power adapter should be plenty to operate the optoisolators and provided power for LED's @ 20ma each (total of 10 LEDs) in circuit with the optoisolators to give a visual indication of the ON & OFF optoisolator status, for a total of 200ma of power drawn at any given time, 100ma for 5 optoisolators and 100ma for 5 LED's. This power consumption would be a constant 200ma because at least one optoisolator and one led for each Unit switch would be active at any given time.

If relays are used such as the Radio Shack 275-245, SPDT subminiature relay at $2.19 each, they would draw 72ma @ 5Vdc, with contacts rated for 2 Amps @ 125VAC. The contacts would connect to the 3 contact points of the Unit switches at or near the mini controller. These relays are hardly audible, with only a low click during contact operation. This interface would seem to be easier to configure (than the optoisolator approach) and SPST reed switches with a 2 wire run required for operation of the relay. A 500ma wall mounted power adapter should be plenty to power all five relays, with additional capacity to place LED's @ 20ma each, in circuit with the relays to give visual indication of the relay coil energized status, for a total of 460ma, assuming all relays are energized. 460ma for 5 relays and 100ma for 5 LED's. If all relays are not energized power consumption naturally would be zero.
If a reversal of the final operation of the SPST reed switch is required the connections to the NO & NC on the SPDT relay can be reversed to accomplish this.

So if you are not up on optoisolator configuration the SPDT relay arrangement seems the way to go in my opinion. But I would like to see any other application to make a comparison.

The important part in all of this is to insure that the mini controller, the reed switches and wiring, and YOU are isolated!

posted 02-28-2000 10:10 PM               

I forgot about those reed relays Carol, they are really small too.

But although I'm not poor, my fun challenge is to find the cheapest way! That diode idea sounds good, but then again why not fuses? Solder them in line and that could keep the cost down quite a bit.

posted 02-29-2000 05:24 PM               

At least in my current design, some button is always pressed down, so that would mean one or more relays would always be on.

Ignoring the power consumption, would this be a problem with those reed relays?

posted 02-29-2000 10:33 PM               

I will say that reed relays will be MUCH simpler, and don't consume much more power, what was the analysis, 500ma for relays vs 250ma for optoisolators? And the relay draw COULD be much less, depending on how many have their coils energized at a time. I don't want to discourage you from trying, I would too, but I think in the ned I'd just use relays.
In Radio Shack they have a series of small books by Forest Mims, the Engineers Notebooks. One of them is on optoelectronic devices. It's probably worth picking up. If you have just a basic understanding of schematics, I don't think the material would be over your head. I gues I'm in 'practicing' engineer mode now, don't remember all the formulas needed to calculate limiting resistors and pull downs and pull ups, I would just look on the data sheet for the aprticular part and see what they recommend. And these books are excellent cookbooks - they have the basics for various circuits involving a particualr component, what you do once you know how to use a device is up to you. Anyone doing mods and home brew projects like this should grab those books.