Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[dave w]

1 farad caps are hard to find I think.

Nay-nay, I say.
http://www.allelectronics.com/index.php?page=search&search_query=1+farad&x=28&y=12

[Brian H]

All you have to do is try and find a place for a .76" Diameter X.3" thick device.
I doubt even a 1F will keep the voltage up if you trigger the motion sensor during battery changes.
Of course if some one wants to buy one and test it for us. That would be great.

[dave w]

I doubt even a 1F will keep the voltage up if you trigger the motion sensor during battery changes.
Of course if some one wants to buy one and test it for us. That would be great.

Sounds like a challenge...I'm on it.   
I'm just not sure if I have any more 1 farads in the junk box. May have to wait until my next order to "All Electronics".

[Charles Sullivan]

Just a few back of the envelope calculations here:

The rate of change of voltage across a capacitor is equal to the current divided by the capacitance.  So for example if the motion sensor should be triggered while changing the battery and draws, say, 10 milliamps (just a wild guess), the voltage across a 1 Farad capacitor would drop at 10 millivolts/second while the sensor is transmitting - about 0.6 seconds every 10 seconds or so.  (As I recall there's a refractory period of about 10 seconds after a transmission during which the sensor won't report further motion.)  If it takes 30 seconds to change the batteries and there are 3 transmissions during that period, one might expect the voltage to drop about 18 millivolts (0.018 V).

However since there is no low-battery signal from a standard X10 sensor; you know for sure you need a new battery when the sensor no longer responds to motion (or to dawn/dusk).  It will have to be determined whether the sensor address programming is maintained at that voltage.

With a capacitor that size, I'd probably be more concerned that the life of a battery would be greatly shortened just by charging up the capacitor in the first place.

[dave w]

The rate of change of voltage across a capacitor is equal to the current divided by the capacitance.  So for example if the motion sensor should be triggered while changing the battery and draws, say, 10 milliamps (just a wild guess), the voltage across a 1 Farad capacitor would drop at 10 millivolts/second while the sensor is transmitting - about 0.6 seconds every 10 seconds or so.  (As I recall there's a refractory period of about 10 seconds after a transmission during which the sensor won't report further motion.)  If it takes 30 seconds to change the batteries and there are 3 transmissions during that period, one might expect the voltage to drop about 18 millivolts (0.018 V).

With a capacitor that size, I'd probably be more concerned that the life of a battery would be greatly shortened just by charging up the capacitor in the first place.

A-a-a-a-w-w-w Chuck, now ya went an took all the fun out. 

I'm still gonna stick a farad in an "eye", if I can find a place to mount it. Maybe I can put it right behind that little window. 

[Charles Sullivan]

I'm still gonna stick a farad in an "eye", if I can find a place to mount it. Maybe I can put it right behind that little window. 

For an experiment, you could just duct-tape it to the outside and leave the battery compartment open for the connection across the battery terminals. 
 

[HA Dave]

You could just solarize your motion sensors: http://forums.x10.com/index.php?topic=20237.msg114109#msg114109

[JeffVolp]

Most of those double-layer supercapacitors have high series resistance, so the back of the envelope calculations don't work very well.  I tried to use them for an intermittent low-power transmitter application myself.  Unfortunately, the series resistance caused the voltage to drop too low to be useful when transmitting.  Most of them are really meant for low current memory backup applications.  I did use some TDK 1 farad units with low source resistance, but they were almost as large as a hockey puck.

Jeff

[dave w]

For an experiment, you could just duct-tape it to the outside and leave the battery compartment open for the connection across the battery terminals. 
 

Now being serious (which does not happen often). I have an Active Eye in the bedroom running off a 3V DC wall wart. Everytime the power flickers I have to reprogram the Active Eye so I have been meaning to try a 1 farad in the empty battery compartment just to see if it would avert some of these re programmings. I guess the test will be a success if I have to reset microwave clock but not the Active Eye. 

Will post the results in the event others are running "XXXXEYES" off wall warts.

[panamon]

...
However since there is no low-battery signal from a standard X10 sensor; you know for sure you need a new battery when the sensor no longer responds to motion (or to dawn/dusk).  It will have to be determined whether the sensor address programming is maintained at that voltage.

With a capacitor that size, I'd probably be more concerned that the life of a battery would be greatly shortened just by charging up the capacitor in the first place.

Your point is well taken. It also makes sense that perhaps by the time you notice the detector's batteries are low, the cap may only serve to speed up the final drain, killing settings that may otherwise have survived had the cap not been added to the circuit.

 

I think what I'll do instead is rig up simple box with 2 batteries and some thin blades (or alligator clips), that will provide good contact with the terminals of the detector thus allowing the removal of the depleted batteries and the installation of fresh ones while the detector is well energized.

I get the feeling that keeping it juiced may provide the best hope for preserving settings.


                             - \_________
                                                |
                                                |
                                                |
                                                |
                                                |
                             + \_____       |
                                          |     |
                                          |     |
                                          |     |
                                          |     |
                                          |     |
                                          |     |
                                          |     |
           ________________      |     |
           |                        |     |     |
    |--- | -                    +  |--|     |
    |     |_______________ |           |
    |                                            | 
    |       ________________           |
    |      |                         |          |
    |---| +                      - |-------|
            |_______________ |
(Be sure to apply the blades to the right side of the sensor facing you.)
       
I use DooMotion and there is a feature that flags a sensor as suspect if no motion is detected after a customizable number of hours.

If this approach works, it would be easier and cheaper than adding caps to all my sensors and would not reduce battery life. This could also work with the many MS10A's I have deployed.

(All my batteries are fairly fresh, so if someone tries this before me, please post back and let us know if this works.)

[BoyntonStu]

A few short wires soldered to + and -  and 2 alligator clips on the wires from your 3 Volt source and you are done!

You can also use the leads to measure the voltage to determine if the batteries need replacing.