Searching \ for 'Proximity detector' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/index.htm?key=proximity+detector
Search entire site for: 'Proximity detector'.

Truncated match.
PICList Thread
'Proximity detector'
1997\07\02@235515 by Harold Hallikainen

picon face
       I've been asked to look at developing a simple proximity
detector.  I kinda have in mind using a PIC to pulse a series resonant LC
once per minute or so.  The LC would radiate a damped 49 MHz sine wave
based on its self resonance.  The PIC would just drive this LC with a
step that changes state once per minute or so.
       I need to detect this damped sine wave up to 10 feet away.  I'm
thinking of another LC (probably using circuit board traces for L)
driving a single chip RF amp set up as a Schmitt trigger so it only
detects stuff over a certain level.  This would drive a diode detector
which would then probably drive an analog in on a PIC.  Software would
check the timing of the damped RF pulses to identify the particular unit
transmitting.
       Anyone have experience in this area?  I am somewhat concerned
about the whole thing being jammed by a cordless phone but don't want to
build a superhet receiver to try to get more selectivity.  Perhaps a
ceramic or SAW filter after the LC?  Even then, with shared spectrum I'm
likely to find interference.
       Comments?

Thanks!

Harold

1997\07\03@142929 by vanes

flavicon
face
I go through old Everyday with Electronics mags every now and then. I
think I saw a article in one of the 1996 issues that went on about
location-sensing with inductive loops. I'm sorry that I can't be more
specific about which issue it was, but it sounded much like what you
want.

good luck!

Harold Hallikainen wrote:
{Quote hidden}

--
eric van es
spam_OUTvanesTakeThisOuTspamilink.nis.za
cape town, south-africa
http://www.nis.za/~vanes/

1997\07\04@121337 by Gerhard Fiedler

picon face
At 15:44 02/07/97 EDT, Harold Hallikainen wrote:
>        I've been asked to look at developing a simple proximity
>detector.  I kinda have in mind using a PIC to pulse a series resonant LC
>once per minute or so.  The LC would radiate a damped 49 MHz sine wave
>based on its self resonance.  The PIC would just drive this LC with a
>step that changes state once per minute or so.
>[...]
>        Anyone have experience in this area?  I am somewhat concerned
>about the whole thing being jammed by a cordless phone but don't want to
>build a superhet receiver to try to get more selectivity.  Perhaps a
>ceramic or SAW filter after the LC?  Even then, with shared spectrum I'm
>likely to find interference.
>        Comments?

Why don't you measure the inductivity instead of the damping of an LC?
Could be done e.g. with an oscillator measuring its frequency, or a bridge
circuit.

-------------------------------------
Gerhard Fiedler <.....gerhardKILLspamspam@spam@pobox.com>
S‹o Paulo - Brazil

1997\07\05@230430 by Harold M Hallikainen

picon face
On Thu, 3 Jul 1997 19:50:03 +0200 Eric van Es <vanesspamKILLspamilink.nis.za>
writes:
>I go through old Everyday with Electronics mags every now and then. I
>think I saw a article in one of the 1996 issues that went on about
>location-sensing with inductive loops. I'm sorry that I can't be more
>specific about which issue it was, but it sounded much like what you
>want.
>
>good luck!
>


       Thanks for the responses...  Guess I didn't make the requirements
clear.  What we're trying to do is make a very small transmitter and a
very small receiver.  The receiver needs to let us know if the
transmitter is "out of range" (about 10 feet away).  Distance itself is
pretty noncritical.  since this is a very short distance for RF, I was
thinking I could just do a damped sine wave generator by stepping a PIC
output from 0 to 5 volts or back.  This output would probably drive a
series LC tuned to some RF frequency (as allowed by part 15 of the FCC
rules for unlicensed devices).  The idea is that at the resonant
frequency there would be a high circulating current in the LC, causing
radiation from the L as the current decays due to losses.  We're kinda
pulling an RF harmonic off the step waveform.
       That's kinda what I have in mind for the transmitter.  For the
receiver, another LC driving something.  Perhaps a current input type RF
amplifier.  The L would again serve as the receive antenna.  The RF amp
would drive some sort of detector which would drive the receiving PIC
where some coding would identify the unique transmitter.  this is a
little like keyless entry stuff.  Could also use "transponder"
technology, but it seems that a separate transmitter and a separate
receiver is more cost effective here since they would always be sold as
pairs.  I think transponders are typically used where there a bunch of
real cheap transponders and a few relatively expensive units for polling
them (like theft prevention tag systems).
       So... anyone have some ideas in this area?  Cost is very
critical, but if we are only transmitting a few feet, it doesn't seem
that we should have to go to triple conversion superhet receiver design!

Harold

1997\07\06@012621 by Mike Keitz

picon face
On Sat, 5 Jul 1997 23:02:38 EDT Harold M Hallikainen
<.....haroldhallikainenKILLspamspam.....JUNO.COM> writes:
[...]
>        Thanks for the responses...  Guess I didn't make the
>requirements
>clear.  What we're trying to do is make a very small transmitter and a
>very small receiver.  The receiver needs to let us know if the
>transmitter is "out of range" (about 10 feet away).  Distance itself
>is pretty noncritical.  since this is a very short distance for RF, I
>was thinking I could just do a damped sine wave generator by stepping
>a PIC output from 0 to 5 volts or back.  This output would probably
>drive a series LC tuned to some RF frequency (as allowed by part 15 of
>the FCC rules for unlicensed devices).  The idea is that at the
>resonant frequency there would be a high circulating current in the
>LC, causing radiation from the L as the current decays due to losses.
>We're kinda pulling an RF harmonic off the step waveform.

A single damped sine wave is going to be impossible to differentiate from
impulsive noise, which is common in the 49 MHz band.  At the very least,
send a series of several pulses with known timing.  Also at 49 MHz the
typical PCB mounted coil's physical size will be too small to act as much
of an antenna.  Some sort of wire antenna will likely be needed.  You're
also going to have to fight with a lot of strong FM signals from cordless
phones, baby monitors, fire trucks and police cars, etc.  A higher band
would have less interference.  At 312 MHz, about all that could interfere
are garage door openers and the Air Force.

{Quote hidden}

Theft prevention tags are just nonlinear antennas that resonate with the
transmitter's strong signal and generate harmonics which are detected by
the receiver.  The tags themselves are completely passive.  Some of them
have a coil and a diode and others have a piece of mu-metal biased with a
magnet.

Transponder tags which send a code such as for toll payment or
implementing the New World Order use an antenna connected to with a
switch driven from open to short by the code at about a Kbaud.  The
polling system has a transmitter and receiver tuned to the same
frequency.  The transmitter sends a continuous signal and the receiver
looks for AM modulation in the returned signal resulting from the
variation in the reflection from the tag.

Both of these systems make the polling unit expensive because (a)
transmitter power on the order of watts is required and (b) precise RF
components are needed to keep the transmitted signal from overwhelming
the receiver.

>have some ideas in this area?  Cost is very
>critical, but if we are only transmitting a few feet, it doesn't seem
>that we should have to go to triple conversion superhet receiver
>design!

Go with AM in the 312 MHz garage-door opener band.  The transmitter is
just a single transistor oscillator with a printed coil operating as both
the inductor and the antenna.  The receiver is a regenerative design with
a single transistor doing all the RF, typically followed by an op-amp for
baseband gain.  Since the receiver is an oscillator, it is conceivable
that the same circuit could be used for both transmit and recieve.
However, because the receiver is an oscillator, it will likely interfere
with other nearby receivers (garage door opener receivers often use an RF
amp to improve sensitivity a bit and also reduce the regen oscillator
leakage back out the antenna.

1997\07\07@155456 by paulb

flavicon
face
Mike Keitz wrote:

> Go with AM in the 312 MHz garage-door opener band.  The transmitter is
> just a single transistor oscillator with a printed coil operating as both
> the inductor and the antenna.  The receiver is a regenerative design with
> a single transistor doing all the RF, typically followed by an op-amp for
> baseband gain.

 You mean a Super-regenerative (Superregen) receiver.  These things
play
havoc with my UHF CB (467 MHz) when you pass or park near a car with one
of
these burglar alarms!  Very suspicious!

> However, because the receiver is an oscillator, it will likely interfere
> with other nearby receivers (garage door opener receivers often use an RF
> amp to improve sensitivity a bit and also reduce the regen oscillator
> leakage back out the antenna.

 The best ones use an RF Monolithics (or equiv.) SAW filter between RF
stage
and superregen, as well as a SAW resonator locking the transmitter
frequency.

 Cheers,
       Paul B.

1997\07\08@190356 by Andrew Russell Morris

picon face
At 11:02 PM 7/5/97 EDT, you wrote:
{Quote hidden}

It sounds a lot like an electronic (invisible, RF) pet fence to me. You
might want to buy one at any pet supply shop and reverse engineer it. Such
a fence is a transmitter emitting a low frequency (~150KHz) and when the
dog gets within a few feet of a wire buried in the ground around the
perimeter of the property, the dog gets a shock from a receiver in his collar.

Please forgive me if I tell you something you already know, but many people
on the piclist are foreigners and may not be familiar with such devices.
:-) I have designed a proximity detector (capacitance sensing) for a
client, but it only has a range of a few inches (all the application needs).

BTW I also have an electronic security device that is a keyring receiver
that beeps if my briefcase gets more than a few feet away. You can buy a
similar device at a baby supply store to alert you if your child gets away
from you while you are looking away. They can't be expensive. I hope this
gives you some ideas.

More... (looser matching)
- Last day of these posts
- In 1997 , 1998 only
- Today
- New search...