Vehicle as capacitive sensor - Page 1
Forum section: electronics / beginners

author: PODF
author_rank: Contributor
timestamp: November 08, 2016, 08:00:48 am
content: Hiis it possible to transform a normal car into an capacitive sensor? So if anybody touches the car the sensor is activated? even if the person is "touching"/scratching the car with a key? or even stabbing with an knive into the wheels?it only has to work in a "parking mode". a g-sensor could detect if the car is sitting still for 5minutes and then activates/calibrates the capacitive sensor.can this work or is this bs becuase the car is a faraday cage? or the sensor is too big so it picks up lots of noise? or it needs an separate antenna outside the vehicle?battery-life in dashcams are minimal... so i want low-power hardware that is an "bad-people-detector" and then can wake-up a dashcam. standby-time should be weeks... not hours...any ideas?cheers

author: Zero999
author_rank: Super Contributor
timestamp: November 08, 2016, 08:56:53 am
content: I don't see how this could be done, without an external connection to earth or another large metal object.

author: tatus1969
author_rank: Super Contributor
timestamp: November 08, 2016, 10:44:10 am
content: thinking loud... if you add another metal plate to the picture, you would form a capacitor. Apply RF AC between car and this plate, and measure the current. This should change when someone touches the car. Question is how big does this plate need to be, in order to obtain a reasonable signal. High frequency will probably also help, but you don't want to pick up radio signals either. Next problem would be wetness, which could short out the car to earth.

author: Andy Watson
author_rank: Super Contributor
timestamp: November 08, 2016, 10:55:02 am
content: I think you need to penetrate the Faraday cage with an electrode - could be as simple as a radio aerial. Then build yourself an oversized Theremin.

author: Rerouter
author_rank: Super Contributor
timestamp: November 08, 2016, 10:58:01 am
content: you need a reference plane to measure against, but technically if you used something to charge the frame of the vehicle up to static discharge voltages you could measure whenever something brushes past it and discharges it, it would also have the perk of giving static shocks to the perp repeatably as they try and do anything.

author: Ammar
author_rank: Regular Contributor
timestamp: November 08, 2016, 11:27:10 am
content: Some ideas, I haven't tried any of these.Maybe the gaps where the windows are could behave as poor slot antennas. You could feed either side with a coaxial cable and drive at a frequency of lambda/2, where this is the distance between either side of the window. You would need to measure transmission/reflection on the coax and see what happens when the car is touched.Maybe a rod somewhere inside the car could form a bad coaxial cable with the outside of the car. Touching the outside should change the characteristic impedance of the coaxial cable.Maybe a bad patch antenna using the roof of the car and some ground plane underneath on the inside?Connect the ground of a signal to the chassis and try feeding different points of the car's surface. You might get some fun surface currents.Just ideas. Non have been tested.

author: PODF
author_rank: Contributor
timestamp: November 28, 2016, 09:24:09 am
content: Quote from: Rerouter on November 08, 2016, 10:58:01 amthat would be funny

author: Rerouter
author_rank: Super Contributor
timestamp: November 28, 2016, 09:57:33 am
content: To accomplish it, you would just need one of those gimmicky ground straps to contact the ground, and generate a positive or negative voltage in relation to it, (not tied to the vehicle chassis), it's a poor conductor so your unlikely to harm someone, and could use something like a camera flash circuit to charge the car up. (Or the strap technically either would accomplish a differential to ground as most tires insulate pretty well up to a few hundreds of volts.