Re: A father / daugther project :-) simple door monitor
- From: ehsjr <ehsjr@xxxxxxxxxxxxxxxx>
- Date: Thu, 09 Nov 2006 19:44:58 GMT
Markus Zingg wrote:
Hi Ed
I think you are making it far more complex than it
needs to be. The voltage drop in the wires to the
doors/windows should be such a small percentage
of the total drop due to the limiting resistor
that it can be ignored. You'll have about 17 ohms
in 100 feet of wire. For example, if you run 12 volts
out to the door/window switches, and use 1K limiting
resistors for the LEDs, you'll get about 10 mA. That
translates to a voltage drop in 100 feet of wire of
.017 volts. You may have more variation in the combination
of the limiting resistance tolerance and the Vf variation
in the LEDS, than variation caused by the different lengths
of .2 mm wire.
Thanks for pointing this out. Some wires are likely to be in the 300
feets (or even more) range. If I understand you right, this would
tripple the voltage variation to about half a volt versus say nearly
nothing with shorter wires with switches located more "nearby". I just
intend to end up with an appliance where the connections can be easily
interchanged later, alas where I don't have to adjust resistors to
have a similar birghness of the LEDs.
Aside from that, if you do decide to go with the more
complex circuit, I don't think the idea of making
each LED independently switchable to indicate closed
or open status is good. I think they should be consistent;
that is, if led lit = open that should apply to all
leds or if led off = open that should apply to all leds.
You have a point here. However, the reason to have it this way comes
from the fact that we most likely will have different switches for
windows and/or doors based on their different construction. We will
make simple switches ourselfes and it's handy being able to implement
them as closers or openers so as they fit a given door/window
construction best.
So, with the jumpers we actually have a tool in our hands to make the
display consistant with the logical state of the doors/windows
independant of the kind of switches used.
Markus
Aha - your last paragraph explains. Nice idea. As to the
length of the wire - yes, running 300 feet with a 10 mA
LED current would give you about a 1/2 volt drop. That may,
or may not, cause discernable difference. It will certainly
be minor. With 300 feet of wire at 17 ohms per 100 feet,
with a 1K limiting resistor, you'll have 1051 ohms resistance.
At 12 volts, that will translate to ~.0097 mA through the LED,
figuring 1.8 Vf. If the wires were very short, the resistance
would be negligible, so you'd have 1000 ohms resistance.
That translates to .0102 mA - a difference of .0005 mA.
Getting back to the homemade switches: you can make either
kind of switch, and mix normally open and normally closed,
and still make lit = open (or lit = closed) with either a
normally closed or normally open switch and your jumper idea,
without the need for the inverter. You just change where the
LED plugs in to the header. Drawn below is lit = open
+12 -------[1K]---+---[LED]---+---Gnd
| |
| / |
+---o o----+ Normally Open
When door opens, switch opens, removing short across LED
+12 -------[1K]---+ +---[LED]---Gnd
| |
| |
+---o--o----+ Normally Closed
When door opens, switch closes, completing circuit to LED.
You can also do it vice-versa where lit = closed - just
use the same diagram and swap the switches.
Ed
.
- References:
- A father / daugther project :-) simple door monitor
- From: Markus Zingg
- Re: A father / daugther project :-) simple door monitor
- From: ehsjr
- Re: A father / daugther project :-) simple door monitor
- From: Markus Zingg
- A father / daugther project :-) simple door monitor
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