RE: [misterhouse-users] How to watch for a specific voltage with Weeder Digitial I/O board?

[Shane H. <sha...@pa...>]

I'm no great hardware engineer but I'd use a comparator.

Take a look at:
http://home.cogeco.ca/~rpaisley4/Comparators.html
for example.

Basic idea would be that you would feed the -ve input to the comparator
with a reference voltage (the threshold that you want to detect).  This
could be done by a voltage divider.  The +ve input would be driven by
the line you want to detect the voltage on.  If the +ve goes above the
reference it switches the output on. If it is below, the output is off.

Probably easier to run everything from 5V.  Hence comparator supply
would be 5 volts and your reference voltage as well.  This would mean
dividing down the input line with a resistor network so it never
exceeded 5 volts.

As example - say your input line had a maximum voltage of 20V.  Use a
resistor network to drop this to a maximum of 5V ie. 1/4.  Clearly your
thresholds would also be reduced by the same amount.  Resistor voltage
divider information should be easy to find on the net if you are new to
that - or simply ask a few more questions.

HTH
Shane

> -----Original Message-----
> From: mis...@li... 
> [mailto:mis...@li...] On 
> Behalf Of Kirk Bauer
> Sent: Monday, 24 November 2003 2:17 p.m.
> To: mis...@li...
> Subject: [misterhouse-users] How to watch for a specific 
> voltage with Weeder Digitial I/O board?
> 
> 
> Okay, you would think that with my degree in Computer 
> Engineering I could figure this out for myself, but I'm just 
> not very good with 
> electronics.  I'm hoping somebody can give me a idea of what 
> kind of circuit I should build and then I can do the calculations.
> 
> Sure, I could do this with an Analog input card, but not only 
> are those more expensive, but they have fewer inputs, and it 
> seems a bit overkill. Plus, I don't have an Analog input card 
> laying around.
> 
> So, here are the three things I want to monitor:
> 
> Garage door "safety sensors" (light beam to keep it from 
> shutting on something, as seen at the two terminals on the 
> garage door opener:
>    No obstruction: 6.17V, 20mA
>    Obstructed: 6.52V, 14.5mA
> 
> Garage door opener light (I found that the voltage on the 
> terminal going 
> to the open button changes when the light is turned on, and 
> I'd like to 
> monitor this):
>    Lights off: 17.30V
>    Lights on: 16.26V
> 
> Door sensor hooked up to alarm system:
>    Door closed: 2.97V, 2.96mA
>    Door open: 12.8V, 0mA
> 
> So, I basically have one Weeder Digital I/O board.  I have 
> three voltages I want to monitor.  Each has their own ground. 
>  Each has two 
> positive voltages representing the two states. 
> 
> So, how do I turn, for example, 16.26V into a 0V input to the 
> Weeder board and 17.30V into a 5V input to the Weeder board?  
> 
> I"ll accept an answer of 'just buy an Analog input board' if 
> that is my best option.
> 
> Thanks for any help you can provide.
> 
> -- 
> Kirk Bauer <ki...@ka...>
> http://linux.kaybee.org | www.autorpm.org | www.logwatch.org
> 
> 
> 
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