Adam,
From what I am thinking, a op-amp would be a battery drain unless I use a
high dollar low current device.  Perhaps coupling the raw signal (from what
ever) to a np cap directly into a port pin of a sleeping PIC.  Wake on
change and... Sounds too easy and cheep...
A photo-transistor...  If I use a high value resistance on the collector, it
would limit the current draw.    Or on a CDS I see one in the Allied catalog
with a range of 100K - 10Meg... That is a little extreme, what is the input
impedance of a PIC input....  Guess it is time to play and see what I can
do.. I am trying to find the trip voltages for a PIC input and current
leakage...  Ok current leakage is 1 uA plus or minus.  Now the trip voltage,
looks like 0.15VDC & 0.85XVDD.  Too large a range for passive AC coupling..
I will get back to this later when I have more spare time.

Best regards,
Giles


>From: "M. Adam Davis" <adampic@UBASICS.COM>
>Reply-To: pic microcontroller discussion list <PICLIST@MITVMA.MIT.EDU>
>To: PICLIST@MITVMA.MIT.EDU
>Subject: Re: [PICLIST] [OT] Crocking Frog, People sensor
>Date: Mon, 20 Aug 2001 21:30:48 -0400
>
>It sounds to me as though you've got the basic idea.
>
>Use a CDS cell or phototransister in a configuration where it outputs a
>voltage corresponding to light level.
>
>(simplified)
>This voltage level goes two places
>1) into a diode/capacitor high pass filter which then leads to an opamp
>2) through a diode into the other opamp input
>
>The output of the opamp is the derivative of the signal, ie how fast the
>signal is changing.  A fast change (from light to dark) results in a
>high voltage, where a slow change results in very little voltage.  A
>change from dark to light is a high voltage in the opposite polarity.
>This can go directly to a schmidt trigger.
>
>Saturation is not an issue if you pick the right components for the
>light levels you're expecting, and even then it doesn't matter much.
>
>If the signal is steady, the charge on the cap is equal to the signal
>from the light sensor, and the opamp has a difference of 0 to multiply.
>This is the case whether the sensor is saturated or not.  If the sensor
>is saturated, then the light can change dramatically within the range of
>saturation and not trigger the device, but that is a simple problem of
>determining reasonable light levels to design for.
>
>-Adam
>
>
>
>
>Giles Honeycutt wrote:
>
>>OK, their is a 100 ways to do this, but I don't want to re-invent the
>>wheel.
>>  Are their any reference designs for something like the crocking frog
>>that
>>makes noise when someone walks in front of it?  Are they using a
>>photo-resistor and sensing change with a low pass filter?  Are they
>>using a
>>photo diode? Photo Transistor?  If so, how do they avoid total
>>saturation?
>>
>>Best regards,
>>Giles
>
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