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'[EE]: Looking for advice lowest cost hand sensor'
2008\02\17@092731 by cllow2020

picon face
a ultra sound transcerver with PIC should do the job. simple school project

On Feb 17, 2008 2:57 PM, Cedric Chang <spam_OUTccTakeThisOuTspamnope9.com> wrote:

{Quote hidden}

> -

2008\02\17@093948 by cllow2020

picon face
or a simple metal detector using nand gate, but you's hand have to ware
something. its more immune compare to unltra sound.

On Feb 17, 2008 10:27 PM, cllow2020 <.....cllow2020KILLspamspam@spam@gmail.com> wrote:

{Quote hidden}

2008\02\18@013727 by Xiaofan Chen

face picon face
On 2/18/08, Jinx <.....joecolquittKILLspamspam.....clear.net.nz> wrote:
{Quote hidden}

The circuit is kind of similar to my first project as a
real electronics engineer back in 9 years ago. It is
a capacitive level sensor (actually it can detect something
like cement as well). It is also using the simple oscillator
(based on CD40106) and (quasi-)peak detector. And
yes it is good enough to detect pico-farad changes.
The calibration of the detection threshold is done with
a poti.

The major problem with the circuit at the time was
the RF immunity of the peak detector. After fixing
the other problems with 3 PCB layouts, only with the
4th PCB layout I figured out how to solve the RFI problem.
I added a small resistor in series with the diode (after
the comparator) and finally it passed the tests.

Xiaofan

2008\02\18@020922 by Jinx

face picon face
> The circuit is kind of similar to my first project as a
> real electronics engineer back in 9 years ago

I mentioned RF and Theremins. They are basically the same
as the capacitance method but use a much higher frequency.
One alarm circuit I was thinking of has a simple RF oscillator.
The output is split - one path goes to an op amp/comparator,
the other to an antenna (PCB or wire). The signal on the
antenna goes to the other terminal on the op amp/comparator.
A body approaching the circuit loads the antenna, or absorbs
the signal, I forget which, causing a change in the op amp and
thus triggering the alrm section. The one I made was quite simple
and stable and could detect movement in a room


2008\02\18@101152 by David VanHorn

picon face
> I mentioned RF and Theremins. They are basically the same
> as the capacitance method but use a much higher frequency.
> One alarm circuit I was thinking of has a simple RF oscillator.

Theremins actually use two different approaches.
The pitch section uses hand capacitance to detune a resonant "antenna"
circuit, and shift oscillation of that section against a reference
oscillator, producing the difference frequency as the pitch output.

The volume section runs at a different frequency, and uses the same
technique to shift the oscillator across a tuned circuit, resulting in
a varying output level, which is used as the volume control voltage.

The fun part with the theremin is the "antenna" circuit, which is
designed to maximize and hopefully linearize the pitch changes for a
very small hand capacitance change.  I stand about 2' from the
antenna, and on the low notes, my pitch hand ends up essentially on my
chest.  It's surprisingly sensitive, and fiendishly difficult to play,
let alone play well.


I'm sort of surprised nobody mentioned this other approach that meets
the criteria:

A hot solder iron.  Just listen for the scream.   :)

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