You actually have an AC detector. My physics instructor will tell you if your detector finds a positive charge, there will be an equal negative charge very close by. So if you find one, you have found the other.
Paul
While cascaded ("Darlington") transistors will do this, a CD4049 which will work at 9 V or a 74HC04 at 5 V may be even more sensitive.
How you distinguish a positive or negative charge is another matter, but one way may be to use a 74HC4066 to initially or periodically clamp the input to the 5 V supply and then see if the output switches (from LOW to HIGH) when you bring the antenna close to your charge.
To change any circuit from a NPN transistor to a PNP transistor, simply use the same wiring but reverse the supply voltage. You also need to reverse the direction of the LED.
However, that circuit will just detect any AC signals so the description of positive charge detection is not exactly correct. AC signals are all around us, from the mains and from radio stations. If you want to see this just touch the tip of an oscilloscope probe without connecting the ground to anything and see the whopping signal you are picking up.
I don't know the sources as I am just building this static detector with my grandson as a project after seeing a positive/negative build on youtube using a fds8958a chip. https://youtu.be/tl0-_zDOc1E?t=99
We tried build this but my soldering skills are a bit poor as my hand shakes and the fds8958a is a so8 so the pins are quite small and delicate. I did try using a small PCB adaptor to solder to but still found it tricky, so I looked for a different solution and found the NPN BC547 Array which is easier to put together on a breadboard. So I then need to build a PNP version and used the 547 opposite which is the BC557.
Thanks Mike, we will try this out.when my grandson comes over Friday.
As I just said to TomGeorge, we tried a building Electrostatic Polarity Detector expriment from a youtube video using a FDS8958A but damaged a few of them due to shaking hand soldering
I have a "static" charge detector left over from my electronic assembly service company. It is a commercially made device and it really does indicate static electrically charged devices.
IT is totally metal enclosed with the exception of a sensing port which has wire screen covering the port. It is powered by two 9 volt batteries and has several op amp ICs inside and a meter to indicate relative voltage.
It does work and it does eliminate all AC signals. It cannot sense positive from negative.
Yours will have to be enclosed just like mine, if you want to actually indicate only static charges.
Good luck,
Paul
Nice video ... I have to admit that I did not understand you project until I have seen it
The FDS8958A is a pair of MOSFETs while you want to use bipolar transistors. And as it is about static charge, I would expect that the LEDs would go on ... but not for long because there is acutally a current flowing (through base pin of BC547/557). This is avoided by using MOSFETs because their gate is isolated, i.e. the charge remains ... have you considered this?
I have tried using 2 different MOSFET's a week ago, the IRFZ44N and the IRLZ34N as I have a hundred of these sitting around but could not get these to work. Perhaps I'll take another look at these MOSFET's
I really wished I did electronics back in the 80's instead of doing IT. But I have been tinkering with Ardunio for about 10 years but my knowledge is lacking as I can't seem to retain information due to medication I am on. So always a noob!!!
Without further checking, but those are not "small signal" MOSFETs. That it, they will have higher gate capacitance than those of the FDS8958A. Gate capacitance must be loaded to a certain voltage before MOSFET starts conducting. Could be a reason why not working, look for MOSFETs with smaller gate capacitance (something around 100pF) and low V_GS threshold (< 3V) voltage.
Indeed power devices are a hopeless choice, the capacitance will utterly swamp any signal.
The link I provided uses MAX4322 devices which are only 3pF capacitance, but I suspect using JFET or MOSFET input opamps would fare even better as these have much lower bias currents.
Just checked the data sheet - they have relatively high gate capacitance in 3-digit-pF range. So a MOSFET like ZVNL110A, ZVN4206 (n-channel) or ZVP2106A (p-channel) should do it - or something similar.
Variations on that circuit can be used as an ionization detector (e.g. for radioactive particle detection). Works quite well, although much less sensitive than a Geiger counter.
Different styles of CMOS have different front ends. The old CD4xxxUBE chips (unbuffered RCA CMOS chips from the 1970's) were incredibly sensitive, but as a
consequence also very vulnerable to static damage. Modern CMOS tends to have a
cascade of input stages and hysteresis to give better performance for digital applications, but are probably less effective for sensitive analog tasks.