We have drawn up a schematic using EasyEDA and we are going to try to get the PCB made at JLCPCB.
I posted the schematic on the EasyEDA forums just to make sure it was correct before moving onto getting the PCB done but I was just told it was wrong and he claimed a BC557 was a NPN transistor and not a PNP and no one else has chipped into the thread, so I am just asking if the following schematic of just the BC557 is correct in your knowledgable eyes. Is it No1 or No2 or something else.
Thanks
In reply #21, circuit no 2, the BC557C PNP transistor that drives the LED: I do not see how its base-emitter junction can be forward biased to draw base current so the LED can light up.
Its something else.
No 1 is wrong - because the poliarities are incorrect
No 2 is wrong because a: you have omitted a stage of amplification, and b: there is no curreent supply to the base of the transistors.
and your approach is wrong. You dont understand transistors, so build it on a breadboard to get it working before you CONSIDER getting a PCB.
Also the circuit is not a charge detector.
Try this
You can use an nte457 jfet which has an input capacitance of about 5pF
The charge on a capacitor is Q = CV so if the voltage is 1V the charge detected is 5pC
More interestingly you can use a FET input op amp to build an electrometer that will MEASURE charge (both polarities)
But that's the point of this particular design, isn't it? I've seen other versions where the base of the first transistor floats like this, just being connected to the antenna, so presumably it's known to work. Mind you, I haven't tried it myself.
My concern is that this is surely going to respond to AC fields from the house wiring, as much as any electrostatic field. Have I misunderstood?
You see the BC547 based circuit at the top of this thread is everywhere, it detects household AC etc and it detects a negative charge (or whatever people might call it). Now with my grandson we just wanted to detect the opposite charge, in this case a positive charge (or whatever people might call it). So I just got the PNP version of the BC547 which is the BC557 and tried to replicate as I have not seen much on the internet for designs etc.
I originally asked here to see what I need to reverse and then drew up a schematic based on this and posted a question on the EasyEDA forum to check the schematic was correct before creating the PCB but was told by someone that the BC557 was not a PNP transistor and I need to also change the orientation but from what I could see the BC557 was PNP device. However no one else on the EasyEDA thread helped so I am now really confused as I don't know much about transistors.
So I drew up the 2 mini schematics to ask here hoping you all are more knowledgable but even now there is some disagreement to which is correct No2 or something else which is more confusing to me
I have build the circuit the circuit on a breadboard initially from the original advice in this thread before creating the schematic though I can really test it as I have tried to create positve static charge but I can not tell.
If I touch the BC557 antenna lead then the LED does light up but so does the negative LED on the BC547 when touch so I can't tell if the orientations are correct on the BC557 as I know nothing. I have tried the comb and plastic rubbing and that kinda seems to work but not 100%
I know there is better devices like the CB4001 or JFET or MOSFET etc, but just want to get this one design sorted in our heads before jumping onto other things.
So, the concensuss is no2 schematic is correct for the PNP BC557 with the Vcc going to the emitter (and not the collector as it would do in the NPN BC547).
Your NPNs are correctly cascades, the PNP's however have no base drive at all. PNP circuits should be the vertical mirror image of NPN circuits, all +ve and -ve swapped over.
Sorry, could you explain a bit better as I don't understand, are you saying that I now have to change the vcc/gnd on the BC557 side ie as per No1 in the little schematic in post number 21
Perhaps if someone could put me out of my misery and draw the schematic for me on a piece of paper etc.
I thought I was being precise enough, but apparently not. The circuit is reflected, the power rails are not. Otherwise nothing would have changed! Where NPN sees +ve on the collector, PNP sees -ve on the collector, etc etc.
Sorry MarkT, I do struggle with electronics. Just wished I learnt this when I was at school 40 years ago but I choose IT instead and became a programmer.
I am more monkey see monkey do type of person these days due to my medication, I struggle with learning some concepts and slowly forgetting others. Thanks for your help and also to everyone else.
Its still not a charge detector, and BOTH circuits will detect EM radiation equally. AC yes, DC no - not really as the amount of charge shown by something like an electroscope would very quickly be dissipated by flowing through the BE junctions.
To detect charge you need a very high input resistance as given by a JFET or better a MOSFET.
