Need expert advice for unusual LED circuit.

[PRN.JRA]

I apologize if this is not the correct forum.

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.

 

[D.A.(Tony)Stewart]

Electric Fields are inherently high impedance and impulse EMF are exceptional wide bandwidth. What range of levels and frequency are you hoping to detect in uV and Hz?

A cheap FET gives the impedance required but further signal amplification is required to trigger a latch such as a small signal SCR to turn off an LED driver, thus at least 3 or 4 low cost transistors or at least one specialized IC is required TBD based on your detailed application requirements.

 

[PRN.JRA]

Hello SunnySkyguy, thank you for your help so far.

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.

Thank you for your time.

 

[BradtheRad]

The FET detector is so sensitive that all the led's would light if you hold your hand near the unit while you are carrying a static charge.

If your charge is great enough, all the led's would light when you stand some feet away.

 

[PRN.JRA]

The FET detector is so sensitive that all the led's would light if you hold your hand near the unit while you are carrying a static charge.

Perhaps I could use some sort of insulative inhibitor around each independent antenna in order to severely limit the detection range of each individual FET?

I am more confused about how the component logic would work to disable a previously powered LED, and then keep it off until the circuit resets. This is something I don't know how to do but is probably exceedingly simple for actual electrical engineers.

 

[BradtheRad]

Perhaps I could use some sort of insulative inhibitor around each independent antenna in order to severely limit the detection range of each individual FET?

If you wish, you could experiment with degrees of static charge. There might be a range of voltage... not too high, not too low... where you could turn on one led without turning on its neighbors. You would need to maintain that voltage on your hand at all times. Your entire body would need to be at that voltage. I have never heard of anyone trying to do this.

Certain substances might insulate the static charge better than others. Another thing to try is to put conductive shields around the sensors. This gets very unwieldy.

I am more confused about how the component logic would work to disable a previously powered LED, and then keep it off until the circuit resets.

A silicon controlled rectifier is the component that comes to mind. Once turned on, it continues to conduct until power is cut off.

 

[PRN.JRA]

Your entire body would need to be at that voltage. I have never heard of anyone trying to do this.

Yes this is a unique endeavor. The bodies that will interact with this array will have a static charge due to unusual circumstances.

A silicon controlled rectifier is the component that comes to mind. Once turned on, it continues to conduct until power is cut off.

I will look into silicon controlled rectifiers then, thank you for that advice!

 

[BradtheRad]

My memory needed to be refreshed. Bill Beatty constructed a small panel containing many fet charge detectors.

'INEXPENSIVE FET ELECTROMETER ARRAY'

http://www.amasci.com/electrom/e-field2.html

'Build this sensor panel and SEE the e-fields around charged objects'

http://amasci.com/electrom/electrom.html