Oh what it would be like to have a steady hand. It seems like just about every time I go to work on soldering something my hand starts shakking.Awe, c'mon...... they're not that small.
I routinely solder SSOP IC's with nothing more than a $10 wal-mart iron, so stuff like this isn't really a challenge for me anymore (yes, I'm bragging).
Here's an example of what a $10 iron and a steady hand can do......
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I have some of these small FETs from TI (forgot which ones exactly). I'll try wiring one up later and let you know how if goes.
Mosfet for 3.7 volt
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I can get digikey but the shipping cost of just one mosfet is about 18 bucks.
I have it wired as this pdf
http://www.discovercircuits.com/PDF-FILES/fingersw.pdf
Thanks for your help Scot
I'd start by just seeing if the FET as appropriate switching characteristics for what you are doing.... later you can move on to the "touch" capability.
You are using an N-channel FET, so you're using the right circuit. Forget the 47kΩ resistor and wire the gate directly to V+. If that won't power an atty, then you'll need a FET with different switching characteristics.
You are using an N-channel FET, so you're using the right circuit. Forget the 47kΩ resistor and wire the gate directly to V+. If that won't power an atty, then you'll need a FET with different switching characteristics.
Already tried it, doesn't work.
virtually same load across atty.
looks like i need a different FET.
One last thing Scotbee.
I understand only some of the specs of the mosfet.
But only some.
What am I looking for when trawlling through the UK sites.
Thanks again
Peter............
Peter,
Here is a datasheet for a P-channel MOSFET that I have used in these applications:
http://www.vishay.com/docs/72023/72023.pdf
Note that the gate drive is described as "logic level", and the gate threshold is only about 1V.... giving plenty of margin when working on a 3.7V circuit.
The parasitic resistance is low (about .04Ω), and it can handle over 3A of current.
And it's a really, really tiny package (SOT23).
Let me emphasize again that this is a P-channel FET, meaning you'd use a slightly different switching schematic.
Here is a datasheet for a P-channel MOSFET that I have used in these applications:
http://www.vishay.com/docs/72023/72023.pdf
Note that the gate drive is described as "logic level", and the gate threshold is only about 1V.... giving plenty of margin when working on a 3.7V circuit.
The parasitic resistance is low (about .04Ω), and it can handle over 3A of current.
And it's a really, really tiny package (SOT23).
Let me emphasize again that this is a P-channel FET, meaning you'd use a slightly different switching schematic.
I'm new around here, but I am an electronics student. For SMT soldering, I've been having pretty good luck using solder paste, a needle for application, and harbor freight heat gun with a narrow attachment (AKA ghetto hot air rework station). It takes some practice, but I haven't fried a single component yet, though i have singed a few edges. However, this is nowhere near the damage/wasted components I'd be looking at had I tried an iron. If you don't have solder paste, you can also tin the pads with solder, though placement is more challenging. If you're only doing single-side PCBs you can also use a single-burner electric hot plate.
BTW, if you register on TI's site as a student, you can get a ton of free samples.
This has been a BIG help in creating LED drivers (another little hobby of mine)
Maxim is another great company for free samples as well, though I'm not sure if they have many mosfets, if any.
BTW, if you register on TI's site as a student, you can get a ton of free samples.
This has been a BIG help in creating LED drivers (another little hobby of mine)Maxim is another great company for free samples as well, though I'm not sure if they have many mosfets, if any.
Hi Jason
You don't need a resistor for it to work.... but !!
A resistor will be needed to stop it working.. lol
As soon as you connect the battery without the switch being activated the gate will be 'floating' ....
The gate taken directly to the battery (depends on circuit if its + or - )via the momentary switch will activate the circuit and then when you release the switch the gate will again be 'floating' now if it floats in the right (or wrong direction... depending how you feel about it) the mosfet will switch on by its self, so we have to add a resistor to hold the gate at the opposite of what we want when we switch the mosfet on.. so in short, something in the area of 5 -10k should suffice between the gate and the opposite end of the battery that the switch is connected to.
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