help with pin pointer

[kosacid]

im trying to make a low powered pin pointer using a 70t03gh fet, you can salvage lots of them of motherboards, they are smd and switch at 3v so i though i would use them to run of a pp9, but here is my problem if i use a coil less than 500ohm the fet heats up and if i use a dampening resistor less than 500ohm that heats up to, so how do i work out a balance between the coil and fet so its not killing the fet and battery



using a 600ohm coil im getting about 2" but that's a lot of wire lol

[Qiaozhi]

im trying to make a low powered pin pointer using a 70t03gh fet, you can salvage lots of them of motherboards, they are smd and switch at 3v so i though i would use them to run of a pp9, but here is my problem if i use a coil less than 500ohm the fet heats up and if i use a dampening resistor less than 500ohm that heats up to, so how do i work out a balance between the coil and fet so its not killing the fet and battery

[ATTACH]37846[/ATTACH]

using a 600ohm coil im getting about 2" but that's a lot of wire lol

The 70t03gh is an N-channel enhancement power MOSFET, so you'll need to invert the output of the 555 timer.
From your schematic, you have:
t1 = 0.685 * (100k + 4k7) * 100n = 7.18ms
t2 = 0.685 * 4.7k * 100n = 321.95us
T = t1 + t2 = 5.50ms
f = 1/T = 133.3Hz

In this case, TX-on is longer than TX-off. Also, with such a long "on" pulse, no wonder it gets hot.

[kt315]

http://www.geotech1.com/forums/showt...detector-Pirat

[Skippy]

There are standard techniques to change the 555's output to make it 'inverted'. They usually use diodes to seperate the charge and discharge paths of the capacitor. Take a look online for the 555's datasheet, application notes etc, you should find it.
some stuff here:
http://www.electronics-tutorials.ws/...scillator.html

[kt315]

he has to learn 'simplest pi detector' matter Skippy. information is full. http://www.geotech1.com/forums/showt...light=simplest

[kosacid]

@Qiaozhi and Skippy thanks for the help some times a simple mistake gets you head scratching lol, i knew it wasn't the gates fault not fully switching, last thing i though about it being on to much, thing is i tested it in a oscope lol i should have twinged on talk about staring you in the face(i blame working on a bara and a surf lately) i used a lm358 for the test but i might go for a pic type i have a programmer and know C
@kt315 i did read both threads they were intresting
but hey motherboards are a good source of fets so why not use them

[kosacid]

ok got it working fine only problem now is the coil lol, i never thought about it and there no pin pointer coils in the coil section, i have experimented but not much success, i tried a tube type only detects inside the coil lol a circular one its not very pointy lol, but you get a good 3", off to research some thing i totaly forgot about

[kt315]

kosacid@

always try to search what people made before you thinking to do. maybe your idea exists in last.
so what you only try to do i see like a project
http://www.md4u.ru/viewtopic.php?f=19&t=10344

[kosacid]

well i like to try my self first best way to learn when im done ill post pics and details of my work, i don't think its going to be big because it will run of a pp9 and it will fit in a small box, i based it on the pirate simply because the parts are cheap and no need for a programmer, i run it for 4 hours it was still working so the power problem is fixed, but good info kt315

[Skippy]

It's worth considering a 'sideways on' coil, rather than an axial type. Something about 3cm x 8cm should still be practical for a handheld probe.

[kt315]

for viewing

[kosacid]

ok all done i get a good 3" to 4" on a coin ill post layout soon but i decided to go for a ferrite core now its pointy lol but here is a few tests i done
i did try to squish the 2" coil but it just went on feed back mode lol
https://www.youtube.com/watch?v=qKqrLI38odg
not too bad but i made it pointy
https://www.youtube.com/watch?v=5V6W_UkHTg4
im going to fit a vibrator to it as well and a buzzer
just need to find a good lay out prog now

[nick holas]

hi kosacid
how about your layout??
did you finish your pinpoint???
thanks

[Koala]

ok all done i get a good 3" to 4" on a coin ill post layout soon but i decided to go for a ferrite core now its pointy lol but here is a few tests i done
i did try to squish the 2" coil but it just went on feed back mode lol
https://www.youtube.com/watch?v=qKqrLI38odg
not too bad but i made it pointy
https://www.youtube.com/watch?v=5V6W_UkHTg4
im going to fit a vibrator to it as well and a buzzer
just need to find a good lay out prog now

Well done.

[kosacid]

its my first time using that program but i think its out ok
for the coil i used 10mm ferrite 150 turns of 0.6mm you can get them out old radios and the fets from old motherboards
the fets you get just solider pins to them, if the numbers don't match just look them up make sure they are npn they all do the same job
same with the other transistor its a general one

here is the coil

i also included the pcb file if anyone wants to help make it better post the ideas or diagrams
pinpointer.zip