capacitive proximity detection

The simplest thing to build is a basic single transistor RF generator and a frequency counter with decision logic. L ~ 10uH C ~ 47pF, a 1m rod as antenna. This arrangement can reliably detect a human body from about

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

for the above type of circuit?

any sort of antenna(s).

range, and they work on the idea of sensing a change in near field RF reactance (human body makes a good capacitor to Earth) and/or loss factor (human body is partially conductive)

sharply resonant with some self-compensating tuning/amplitude control since the small changes due to human body movement are masked by the bulk properties or the entire RF environment near the antenna. Bridge circuits, varactor diode, PLL, XTAL frequency reference....I have seen various techniques employed.

electric field intensity change measurements is rather tricky in that your power source wires, and the processing circuitry itself, becomes unavoidably coupled into the measurement, unless battery operated and optocoupled.

settled in on capacitive proximity due to some unique conditions of the application that can't be avoided. So let's please not diverge off the topic OR if you feel compelled to bring up Passive Infrared, Doppler Radar, or UltraSonic then create a new thread.

given complete flexability of antennas you can choose from many different frequencies. You can extend the rnage beyond 1MHZ and include al the ism band, 433 mc 918 mcc. 2.4 GHZ, etc... Higher frequency makes for smaller and more directive antennas, if you need to "point" in a given direction.

at 1-2 meters away you are not technically a capacitive snsors. as those are typicall thought of as memebrane switches and the simialr 1mm to 100 mm spacing. and of couse raltive to the wave number in an rf system.

for the above type of circuit?

any sort of antenna(s).

range, and they work on the idea of sensing a change in near field RF reactance (human body makes a good capacitor to Earth) and/or loss factor (human body is partially conductive)

sharply resonant with some self-compensating tuning/amplitude control since the small changes due to human body movement are masked by the bulk properties or the entire RF environment near the antenna. Bridge circuits, varactor diode, PLL, XTAL frequency reference....I have seen various techniques employed.

electric field intensity change measurements is rather tricky in that your power source wires, and the processing circuitry itself, becomes unavoidably coupled into the measurement, unless battery operated and optocoupled.

settled in on capacitive proximity due to some unique conditions of the application that can't be avoided. So let's please not diverge off the topic OR if you feel compelled to bring up Passive Infrared, Doppler Radar, or UltraSonic then create a new thread.

~$1.00 for wireless motion sensor on Ebay..

D from BC

OK. Analog Devices makes some very sensitive and impressive capacitance-measuring chips that should work just fine for you.

for the above type of circuit?

any sort of antenna(s).

range, and they work on the idea of sensing a change in near field RF reactance (human body makes a good capacitor to Earth) and/or loss factor (human body is partially conductive)

sharply resonant with some self-compensating tuning/amplitude control since the small changes due to human body movement are masked by the bulk properties or the entire RF environment near the antenna. Bridge circuits, varactor diode, PLL, XTAL frequency reference....I have seen various techniques employed.

electric field intensity change measurements is rather tricky in that your power source wires, and the processing circuitry itself, becomes unavoidably coupled into the measurement, unless battery operated and optocoupled.

settled in on capacitive proximity due to some unique conditions of the application that can't be avoided. So let's please not diverge off the topic OR if you feel compelled to bring up Passive Infrared, Doppler Radar, or UltraSonic then create a new thread.

a one foot distance is pretty much an extreme range for a capacitive sensor.

Why not us an IR motion detector instead? these do require a warm moving target, but otherwise are simple and cheap.

Otherwise, Analog Devices have some new capacitive sensors that have insane differential sensitivity. One zillionth of a wumptofarad.

Freescale (nee Motorola) has had the MC33794 e-field chip out for some time, and a cheaper version as well, but I don't know about the range. The main target application, IIRC, is to detect the size of a human sitting in a car seat in order to attempt to avoid killing them with airbag deployment.

Best regards, Spehro Pefhany

What if he's trying to detect lawyers? ;-)

sensor.

Pah! Leon Theremin did ten times that, five decades ago!

Tim

-- Deep Fryer: A very philosophical monk. Website @

an extreme range for a capacitive

Leon Theremin had a very interesting life and although not as widely popularized as Nicola Tesla, I put him in the same catagory as others who were "Ahead of Their Time". Theremin was famous for his musical instruments that could be played by moving your hands/arms/feet in proximity to straight wire or large diameter loop type antennas, thusly named the "Theremin". He had some other contributions to science and technology but they show in obscure references only with out much detail. He was not into publishing his works or going after financing in such a big way as Tesla was. It's about time for a revival of the Theremin since nowadays synthesizer techinques are so much easier and cheaper and so much more can be done than was possible in the days of the Moog.

Staying on topic though...I'm sensing human body movement on the other side of a wall so the IR and Ultrasonic suggestions are a NO GO !

Capacitive Proximity is a loose term and near field RF effects could be argued to be both Electric and Magnetic Fields simultaneous as either one will respond to a partially conductive body. It takes a lot more energy to create a decent strength RF magnetic field a meter or two away than it does an electric field.

I'm presently going along the lines that Vladimir Vassilevsky suggested with single transistor oscillator, although I find I don't need any long ferrite loopstick, but rather just a tiny ferrite drum core. My L values are around 1mH and C values around 10pf or essentially the tiny inductor's self-resonance. This makes the wire antenna very sensitive to nearby capactive/conductive object effects. The wire antenna needs to be a few feet long though. Ground is essential. Kicking out a field thats a few volts per meter appears ample strength to overcome background noise, and such a circuit can run at 3V at 500uA which makes it tiny and battery power capable.

Seems little advantage between detecting frequency or amplitude changes so I'm staying with amplitude changes for circuit simplicity. I'm making use of positive feedback of the AM detected output back into the transistor bias and this gives an over-all boost in sensitivity, much like a Regenerative or Reflex style AM or Short Wave receiver. I guess this method becomes useless if there's any nearby radio station or shortwave transmitter, but I don't need long term continuous reliabiliy and the rare false alarm is tolerable. I'm thinking to do an array of these sensors to track movement between various zones. The odd spurious single zone trip, can be ignored, as can coincidental simultaneous all zone trips. I might even make the detection differential so it only looks for crossover from one zone to the next.

Giga Hz stuff always gets me into trouble! Maybe it's because my prototyping techniques are bad but I always get microphonic sensitivity from home-brew giga hertz circuits. Cutting and shaping micro-stripline from scratch is such a pain! Even the meniscus size from the amount of solder used can radically shift circuit parameters, and the idea of pure inductance and capacitance seems to fade away at those frequencies where everything you do inescapably involves lossyness and tuned lengths? If I touch that stuff anymore, it's with pre-assemble modules only. Seems it's still expensive and hard to find raw parts that are useful in the 1-5 giga hertz range? A BITCH to work with in all aspects IMHO. Giga Hz stuff bounces/refracts and suffers high loss trying to penetrate most wall structures made of modern construction materials. VHF might work though.

Thanks guys for the chip maker tips. Thought I had searched those already but I must have missed something. Nothing in the IC realm I found so far gave any app notes for distances or 1-3 meters so I did not look into them further.... If you got a part # comes to mind or you have used something before? that would help me zero in.

Any further tips or pointers along the lines of single transistor RF oscillators and circuit topologies specifically for near field RF or Capactive Proximity for humand body movement sensor or detection? There's been some high-end Car Alarms where that technology has been employed but I can't seem to find much details. There's a few patents that refer to fence or perimeter type Intruder Alarms but they don't give circuit details.

This is one of those technologies on the fringe that seems to have all sorts of potential but never gets commercialized or exploited in a big way. I wonder why?

I'm pleased to see the regulars in this group are still around!

Greets to you :-)

~Terry Moreau