What I am trying to construct is probably exceedingly simple, yet currently outside my ken. I am seeking advice concerning an unusual circuit concept. I am looking to build this device as simply and cheaply as possible, because I will need to construct a large number of these. So here is what I’m trying to build:
An isolated circuit primarily consisting of an LED and an EMF detector. When the circuit is powered on, the LED lights up. This LED has a tiny antenna near it that detects EMF disturbance. When the EMF field is disturbed, the LED will turn off. The LED will then stay off until power is cycled on the circuit. Once power is cycled the circuit resets and the LED lights up again.
I understand I could use something like a FET to do this for the EMF detection, but I prefer not to do this with a FET if possible. I say that because buying a lot of Fairchild FETs to create an array the size I need would be expensive. So if there is another way to do this without a FET that would be great. I have no idea how to do the component level circuit logic beyond that because I am an IT specialist not an electrical engineer. I do plan to bus power the array btw.
Any ideas or prototype circuit designs for this concept would be greatly appreciated. The simpler the better! Thanks for your time.
PRN said: I understand I could use something like a FET to do this for the EMF detection, but I prefer not to do this with a FET if possible. I say that because buying a lot of Fairchild FETs to create an array the size I need would be expensive.
What is the EMF disturbance (amplitude) you are trying to detect? Not wanting to spend at least 10 cents for a FET per trigger approaches witchcraft or the metaphysical realm. IMHO
There are capacitive, magnetic, IR, light sensing circuits, etc. Your using EMF as you say, which to me is basically a area filled with a radio frequency energy, might be challenging. Thermal drift and tuning come into play. I like your imagination so tell us more. You need a bigger budget.
Christopher
What I am trying to construct is a variation of something like this in mass multitude:
http://www.eskimo.com/~billb/emotor/chargdet.html
http://amasci.com/emotor/fetdet1.gif
If I understand things correctly, in that design the LED lights up when the FET is switched by EMF disturbance. Let us consider this device “1 pixel”. I want to create a grid like array of “pixels” that consist of LEDs individually triggered by nearby EMF disturbance. I would want the EMF detection to be sensitive enough that when someone places their finger near a respective LED “pixel”'s micro-antenna it would turn off that individual LED. So in a way you could say I’m making a drawing tablet of these LED pixels to be drawn on by using one’s finger. The complexity lies in the fact that the LEDs need to be all lit initially, and turned off as they are triggered independently, and then stay off until power is cycled on the entire circuit.
I do not know if a FET such as the one described in the article I cited is powerful enough to detect a single finger’s nearby presence to it, in enough magnitude to trigger the transistor switch. Then there is the issue of individual EMF antenna placement in such a way that the LED itself does not trigger its own self off. Obviously my electrical engineering knowledge is only rudimentary. That’s why I’m seeking expert advice on this project. There may very well be a far simpler way to construct this circuit application than I am currently realizing. I just need to create an array of lit LED pixels that can be independently turned off by the presence of a nearby finger’s EMF disturbance.
I know the FET would be very sensitive but perhaps an insulative inhibitor around its antenna could reduce the detection field significantly, providing a localized range of 1cm or so near its respective LED.
As far as money goes I could throw $1000 at this project no problem. The issue is once I get one of these arrays working I may need to mass produce them, and in that case cost per device matters. That’s why I’m trying to keep build cost down.
If you have any advice I am listening with mind wide open.
It is not clear what you might mean by an “EMF disturbance”, but the simple FET device in the links merely detects the presence of nearby objects that carry a static electric charge, i.e. either an excess of free electrons, or a deficit of free electrons, depending on how you hold it.
Static electric charge is accumulated, for example, when you walk across a carpet, pet the cat or comb your (dry) hair. Is that what you want to detect, and if so, why? Note that if your finger is not charged, the detector won’t respond.
PRN said: I do not know if a FET such as the one described in the article I cited is powerful enough to detect a single finger’s nearby presence to it, in enough magnitude to trigger the transistor switch. Then there is the issue of individual EMF antenna placement in such a way that the LED itself does not trigger its own self off.
I believe your examples are using capacitive coupling to transfer a voltage onto the FET junction. This is similar to touching an amplifier/speaker input and getting that loud hum. EMF may be the wrong term to use. Also in your examples static discharge will give that approach a very short life. Probably X Y scanning is the better approach but that is out of my realm.
Christopher
