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Static electricity detector

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LAN_403

StaticBuildup

Fri Feb 23 2018, 11:50PM

Registered Member #61695 Joined: Sun Jul 16 2017, 11:22PM
Location:
Posts: 31

Static charge detector. The lights are red for positive charge and green for negative. It is meant to be directional, but the lights can be uneven. Detects charged materials such as clothing, blankets and furniture.

DerAlbi

Sun Feb 25 2018, 11:49PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Cool thingy

My first reaction was "WTF, how..

."
Floating mosfet gates, i guess?
Effectfull. You could even sell it to the strange people that believe in field harmony magic and stuff.

Bjørn

Mon Feb 26 2018, 06:21AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Very nice!

Did you use floating inputs on a microcontroller or transistors?

StaticBuildup

Tue Feb 27 2018, 10:50AM

Registered Member #61695 Joined: Sun Jul 16 2017, 11:22PM
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Posts: 31

Glad you like it.

I used floating JFET gates.

DerAlbi

Tue Feb 27 2018, 06:25PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

How do you zero it ?

StaticBuildup

Tue Feb 27 2018, 11:39PM

Registered Member #61695 Joined: Sun Jul 16 2017, 11:22PM
Location:
Posts: 31

That is a problem. The lights sometimes stay on and I have to dissipate the charge on the gates by touching something grounded and then touching the inputs. I might try to add high value resistors from gate to drain to bleed off the charge.

I have also noticed that when moving away from a charged object, it detects opposite charge. This is noticeable in the video. The pink blanket on the left is positively charged (red), but moving away from it creates a negative charge (green).

DerAlbi

Wed Feb 28 2018, 12:21AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Hmmh, what you describe sounds like leakage current is already influencing stuff, so adding resistors will only make the problem worse.
Against leakage current, you could try to
a) use way bigger antennas, so there is more charge overall thus longer leakage time
b) maybe using a reverse polarized mosfet. Imagine the gate drains allways to the soruce, therefore the gate discharges to gnd. connect an other mosfet (inactive) with soruce to VCC and connect the gates together. This should result in the charge draining to GND be resupplied by the other mosfet leaking VDD to Gate.
Anyhow, no need to be extra fancy here, the effect works

Sulaiman

Wed Feb 28 2018, 09:56PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

In the late '70's I was trying to measure photodiode currents in the sub-pA range using a parametric amplifier, fA leakage currents.
I could not get it stable, then one day standing about 1yd away I moved my hand, and the meter needle flew across the dial.
The plethora of charges on me, my clothes, anything, everything, was enough to induce fA currents in the circuitry... I never managed to make it viable.

I'm not expert in this are but I would not use insulators (plastic box etc.) too near to the electrostatic probe electrode as accumulated charge causes drift.
But

The probe is perfect - it does what it is meant to do - nice.

P.S. I suspect that it would sell better as a ghost/ufo/cosmic-rift detector.

Dr. Slack

Thu Mar 01 2018, 07:11AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I'd second Sulaiman's concerns.

I read an article by Bob Pease (RIP) a while ago (if you don't know his stuff, get googling. If I called him 'the Feynman of analogue electronics', I'd be exaggerating, but not by much) that talked about measuring fA leakage on an electrometer amplifier input. To do this, he used it as a fed-back integrator, with a low leakage capacitor, and measured the drift.

Problem one was the physical volume of the capacitor, the bigger the cap the more frequently cosmic ray strikes generaterd charge pairs in the volume, causing the integrator output to jump.

Problem two was that everything had been built on PTFE, and charge stored on the surfaces took minutes and hours to move around, disturbing the readings while they did. Covering as much of the unnecessary insulator surface with alli foil mitigated that.

StaticBuildup

Thu Mar 01 2018, 12:46PM

Registered Member #61695 Joined: Sun Jul 16 2017, 11:22PM
Location:
Posts: 31

I think the lights stay on because of gate-source capacitance.
JFETs
I will try to add a resistor to discharge that and see what happens.

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