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'110 VAC signal sense'
1995\10\27@082016 by Harrison Cooper

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I've started to play around with trying to sense a switched 110 VAC
signal.  I have brought it into an I/O pin (a diode protected pin),
but have yet to get it to work quite right.  Without a signal, the
input pin appears to be floating.  So I put a 1M pup (weak pup),
and of course, just jumpering by holding adds some capacitance from
my fingers.  It seemed to help, so I tacked it on, but it still
doesn't work - unless I probe the pin with a logic probe.  Now, a
logic probe shouldn't affect the circuit, but experiance tells me
that sometimes it does - there is by default a slight amount of
capacitance and resistive load.  I ran out of time this morning,
but what I was thinking is I need to add a small cap to ground,
such that I have a very slow rise time on a RC network, with the
1M giving a weak pup for no signal conditions.  Otherwise, the
110 hot thru a 4.7M resistor to the pin, with a cap should hold
low whenever the signal is present.

Anyone else played with this ?

spam_OUThcooperTakeThisOuTspames.com

1995\10\27@112152 by Conny Andersson

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At 06.21 1995-10-27 -0600, .....hcooperKILLspamspam@spam@es.com wrote:

>I've started to play around with trying to sense a switched 110 VAC
>signal.  I have brought it into an I/O pin (a diode protected pin),

-- snip -- snip --

>Anyone else played with this ?

Try this one:

               +5v

               |
               |
              ---
              / \
              ---
               |
               |
 in -|160k|----+---|100|-+--- ra0
               |         |
               |        | |
              ---       | |
              / \       | | 20k
              ---       | |
               |         |
               |         |
              ---       ---
              gnd       gnd

The diodes are there for protection (voltagelimiting to 0-5V) and the 20k
are for pull down if no input voltage. The 100 are for extra
protection and the 160k are for current limiting purposes.

Over the 160k there will be a maximum voltage drop of about
160 volt, so it should be dimensioned for maximum allowed
current into ra0 (although a small current will pass the 20k). In this case
no more than 1 mA will pass ra0. (Power dissipation = 0,16W max)

The voltage dividing part (160k/20k+diodes) will give a 5V output to
ra0 when the input voltage exceeds 45V. If the input voltage is negative
all current will float through the lower diode leaving -0,5 V to ra0.

Any comments on this? I have only tried this circuit with voltages lower
than 20V but it should work here as well.

-- Conny

1995\10\27@114727 by BBoles

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    Try app note AN521, Embedded Control Handbook.


______________________________ Reply Separator _________________________________
Subject: 110 VAC signal sense
Author:  Harrison Cooper <hcooperspamKILLspamES.COM> at Internet_Exchange
Date:    10/27/95 6:21 AM


I've started to play around with trying to sense a switched 110 VAC
signal.  I have brought it into an I/O pin (a diode protected pin),
but have yet to get it to work quite right.  Without a signal, the
input pin appears to be floating.  So I put a 1M pup (weak pup), and
of course, just jumpering by holding adds some capacitance from my
fingers.  It seemed to help, so I tacked it on, but it still doesn't
work - unless I probe the pin with a logic probe.  Now, a logic
probe shouldn't affect the circuit, but experiance tells me that
sometimes it does - there is by default a slight amount of
capacitance and resistive load.  I ran out of time this morning, but
what I was thinking is I need to add a small cap to ground, such
that I have a very slow rise time on a RC network, with the
1M giving a weak pup for no signal conditions.  Otherwise, the
110 hot thru a 4.7M resistor to the pin, with a cap should hold
low whenever the signal is present.

Anyone else played with this ?

.....hcooperKILLspamspam.....es.com

1995\10\27@124024 by Dana Frank Raymond

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>Anyone else played with this ?

Connie Anderson has suggested a very good circuit for detecting 117VAC. Some
notes however:

The input pin will see approximately 50 or 60 Hz (whatever the line
frequency), therefore you need to check for a high for at least 11ms (45Hz)
or so to detect the presence of voltage. If that is unacceptable, then use
the following:

My ANSI art is the pits, so here goes. Line to D1 anode. Cathode to 160K
resistor. Resistor to node with D2 anode, 10U 25V Tantalum cap, and 20K
resistor. All three of these devices are connected to GND at their other
ends. The node also has D3's anode, with its cathode connected to VCC. As
well, the node is connected to the input pin via the 100R resistor. This
last resistor prevents pin damage if the capacitor has a charge when VCC
drops during powerdown (it also provides transient protection). This circuit
will sense 117VAC within the first cycle, and the logic high will dissapear
within a few cycles when 117VAC is absent.

One other consideration is important however. To sense mains voltage you
need your GND to be a mains NEUTRAL potential. If this is a problem, then
you need to use either an isolation transformer, or an optocoupler. The
optocoupler is simple: Use a TIL113 with a series resistor and back biased
diode across the LED. The transister is connected common emitter, and the
collector has a pullup resistor to VCC and a capacitor to GND. Keep the
pullup resistor as high as possible so that the capacitor may be a small
enough value to prevent excessive current from being conducted by the
transistor.

Hope this helps.
Regards, Dana Raymond

1995\10\27@131627 by Lee Jones

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On Fri, 27 Oct 1995 06:21:12 -0600, Harrison Cooper wrote:

>I've started to play around with trying to sense a switched 110 VAC
>signal.  I have brought it into an I/O pin (a diode protected pin),

[snip]

On Fri, 27 Oct 1995 16:20:26 +0100, Conny Andersson wrote:

> Try this one:

[diagram removed -- high ohm resistor input with diode voltage limiters]

> The diodes are there for protection (voltagelimiting to 0-5V)

Well, actual range is 0 minus 1 diode-drop to supply (nominally 5V)
plus 1 diode-drop.  You might want to use a schottky diode to limit
the diode drop to .2 to .3 V.  This would give a nominal range, as
seen by the PIC input pin, of -0.3V to +5.3V.


Am I missing something in both postings, where's the return path?
To get a current flow, you have to close the loop.  Are you planning
on tying the digital circuit ground to the 110VAC neutral?  In the
US, mains neutral is supposed to be at earth ground (I don't know
the rules in other countries).  But I wouldn't bet _my_ life on it.

Normal solution to this is to use an isolation transformer.  Rather
than just isolate the 110VAC, you can also use it to step down the
voltage.  I've seen wall-wart units that had around 12VAC output at
low current (50-100mA).  One leg ties to digital ground.  The other
feeds into the circuit Conny provided (with appropriate adjustment
to the resistor values).

I'd provide a diagram, but I don't see how to draw a transformer's
coils using ASCII characters. :-)
                                               Lee

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1995\10\27@161713 by Paul Christenson [N3EOP]

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>I've started to play around with trying to sense a switched 110 VAC
>signal.  I have brought it into an I/O pin (a diode protected pin),

I'm sure that you have read the "see app note AN521" responses.  Going
one better, I'll give the app note (total of two pages) a once over.

The regular input pins are already over/undervoltage protected.  This
is primarily for static protection, but you can take advantage of this
for connection to 110V (child's play, compared to a static spike).

The main concern is that you have to limit the input current.  The app
note recommends a 5 Meg resistor in series, to limit the current to
32 uA peak.  (Remember to use PEAK voltage, which is 162V.)

110V -->>----/\/\/---->>--- I/O pin
              5M

Note: There is no protection to Vcc on the RTCC pin of the 16C5x series.
See the app note for more information.

===========================

Remember that you have to reference everything to AC ground potential.
Most power supplies isolate the output from the input.  However, if you
are using your own power supply, you may be able to sneak some AC
voltage from the low side of the transformer.

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