Presence Detector

[alby]

I am interested in developing a sort of a non-contact human presence indicator, a system that will produce a binary output only (presence/not present). <p>I have winding a wire to form a coil on a specific area and hoping that the presence of a person in that area will change the inductance. I believe that this is the method that is used, for example, to detect the presence of a vehicle at the entrance of a parking garage. However, my preliminary tests do not show any change of inductance due to human presence.<p>Do you guys have any other ideas? At this time, the cost is the critical factor, so I cannot use a light beam or a microwave presence sensor, for example. It has to be something that can be manufactured at a minimum cost.<p>Thanks <p>Al

[rshayes]

Try looking for a capacitance change. The human body is mostly water, which has about the same permeability as free space. In other words, to a magnetic field, you just arn't there.<p>Water, however, has a dielectric constant of about 80. To an electrostatic field, water is very different from free space.<p>Impure water is also highly conductive, so to an electrostatic field a human body looks like a rather lossy capacitor plate.<p>A wire with a bias voltage applied to it through a large resistor will show a voltage change when a conductive body approaches. You will probably need a very large resistance, possibly tens of megohms, combined with a high impedance amplifier, possibly using a small MOSFET.

[peter-f]

Stephen has a good direction for you...<p>Should be easy to look up a circuit.<p>Be careful how you use this... In the '70's, there was a fire in a NYC skyscraper. (1 New York Plaza). The elevator call buttons were activated by (either) capacitance change or heat. When the NYFD responded, the elevators went directly TO the fire... several firemen died.

[Chris Smith]

The 555 touch sensor can be tweaked to do your job. However, it becomes soo sensitive that a hot dry wind or a static build up will also set it off. <p>I have built several of these over the years and have been able to control reasonably the sensitivity from a few inches, to several feet, but found that the wider the field the more false triggers you will encounter. <p>Controlling the field angle with shielding would be the way to go.

[philba]

A lot depends on the application. are you talking about close proximity? say < 20 cm? or are you talking about longer distances? PIR is better for farther distances. <p>Capacitive - http://circuitos.tripod.cl/schem/r110.gif<p>eddy current (kind of what you were doing but using a dual coil) -
http://www.electronics-lab.com/projects ... index.html<p>here's an IR based distance sensor that I have used -
http://www.acroname.com/robotics/parts/ ... D12-15.pdf
It products an analog output inversely proportional to the distance of the object. iirc it can measure up to 80 or 90 CM. I use it with a microcontroller but it could be fed into a comparator with the other half coming from pot based voltage divider so you can adjust trigger distance. they run about $11, iirc.

[josmith]

Or you could go to home depot and buy one for 15 bucks.

[terri]

josmith: "Or you could go to home depot and buy one for 15 bucks."<p>Waddayou, some kinda wise guy?<p>Sheesh! A practical and inexpensive solution on this Board? No using PICs? No plans for building a sooperdooper Theremin for this application?<p>Sheesh!<p>Actually, I went through this. I was going to use the metal front door of my apartment as one plate of a cap, with a foil-lined doormat as the other plate. Not enough capacitance change in that configuration (one plate at right angles to the other plate) to be reliable.<p>Went out and bought a US$10 infra-red yard light sensor. Worked great and cost-effective.<p>Forgive me for being practical.<p>Mea culpa.

[alby]

Philba: Thanks for the information and the links. The first schematic, the one based on the capacitive plate is similar to what I need.<p>However, I don't really understand how it works. The two inverters -on the left side- are configured as an oscillator that will produce a square wave. How does the plate (antenna) play a role in here? I could understand it if the oscillator would not start until it gets some sort of "noise" produced by the proximity to the antenna - plate.<p>Is that how it works?<p>Thanks,<p>Al

[rshayes]

The two inverters on the left appear to form an oscillator with the feedback through the two trimmer capacitors in series. Note that they are quite small in value. The sensing plate is connected betweeen the two trimmers. If the capacitance to ground increases, the additional capacitance increases the attenuation in the feedback path, and, if the circuit is properly adjusted, oscillation will cease.<p>The D1, D2, C3, and C4 for a peak to peak rectifier fed by the oscillator output. As long as the oscillator is operating, the output of this rectifier will be nearly as high as the supply voltage (less a little for the diode drops). When the oscillator stops operating, R2 pulls the output of the rectifier to ground. This change in level is inverted by U1-C, so that its output rises when the oscillator stops. This high level turns on Q1 trhough R3, and the collector current of Q1 operates the buzzer.

[Les Davis]

Has anyone considered RFID tags? The tags come free on many consumer items (eg. videos and DVD), these would be the paper 13.56MHz tags. What we need now is a reader schematic.... Anyone????

[jwax]

Professor- Are you sure of that 13.56 MHz freq for RFID? In the US, that's an industrial RF frequency. I thought RFID was primarily in the GHz range.

[philba]

13.553 to 13.567 is one of the medium rfid frequencies. This frequency allows printing of tag coils so the cost can be reduced. Used for a number of applications.<p>I dont think rfid would work unless you were willing to carry a card or chip your hand like that loonie in seattle...<p>[ May 01, 2005: Message edited by: philba ]</p>

[haklesup]

Simply using a coil in your original post would require that the person be carrying a substantial amount of metal. Then a metal detector like circuit would be appropriate which is what they use to sense vehicles.<p>Check out this website www.bik.com they have a download with schematic, BOM and PCB artwork for a personal proximity sensor that senses moving and stationary objects. This looks like a promising solution though I did not study all the details. It has a well writtem manual and all.<p>A close look in the microwave proximity detectors (used in car alarms and as cheap as $11 on ebay) shows thaT THEY ARE COMPRISED OF A HANDFUL OF CAPS, TRANSISTORS AND AN OP AMP. tHE KEY PART OF THIS CIRCUIT IS A MICROWAVE TRANSISTOR MOUNTED ON A pcb WITH A PATTERN THAT MAKES UP THE antenna and a ground plane made up of some metal foil ion the inside of the box. <p>(sorry for YELLING, I jammed my finger on the caps lock button )<p>Try googling on "Proximity sensor schematic" or add some words like microwave, ultrasonic, IR to zero in on a particular technology. I always add Schematic, Plans or Theory to the end of my search terms and it helps weed out commercial websites.<p>[ May 02, 2005: Message edited by: haklesup ]</p>

[Enzo]

Those inductive sensors in the road detect cars because they are a big hunk of iron sitting in the middle.<p>Snesors are cheap. It would really help if you described the application. A sensor to detect someone in a room is different than one detecting a person standing in front of the door or in front of an ATM. One detecting a person as in sensing their hand on a control panel is yet another. And all these ask for vastly different approaches.<p>In a room motion sensors are cheap, whereas IR detectors have to be finely calibrated. Touch sensing is easy and well documented techniques are out there. Up close sensing can easily be done and cheaply with light sensing. A simple photocell or photo sensor will detect the person blocking ambient light. If it must work in the dark, have a source of IR somewhree in the vicinity.