Re: Problem applying a mosfet

default wrote:
On Fri, 1 Feb 2008 18:35:02 -0800 (PST), Winfield
<winfieldhill@xxxxxxxxx> wrote:


default wrote:

mosfet in the circuit. Mosfet is IRLZ24 type, N channel in
the ground or minus leg of the camera.

The little darling in question is a Saitek "Pocket Cam X" I opened
it up soldered wires to the battery tabs and shutter button and ran
them out the case to a connector.

Intent is to use a pair of D batteries external to the camera. No
external power jack except it will accept 5 V via the USB cable -
but that wastes a lot of power.

No, this baby and their megacam power up and initializes when
batteries are put in - the reason there's so much radio control,
aerial photo, stuff on the web for hacking into it - and the low
price.

I'm running a battery life test on it now it has taken 257 pictures
and still counting over 2 days on 3 AA NiMH cells. Camera takes ~7.5
seconds to power up and get straight, shutter actuates, 6.5 seconds
for pix to go into memory, then every thing goes to sleep until the
controller turns the mosfet on again.

Goal is 900-1200 pictures over a week during daylight for time lapse.
Time delay per picture adjustable over the range of 1 minute to 8
minutes per pix.
--

That's the Aiptek PocketCam X, right? Very interesting
behavior. What are a few good websites to learn more
about it? It sounds quite useful for making a smart
uP-controlled time-lapse camera, or a trail camera for
shooting wild animals. We have a coyote family active
in our backyard that we'd like to know more about. I'd
like to understand a bit more about what they're up to.

Here's a table of some of the logic MOSFETs in my stock
drawers, with your part at the top for comparison.
(Id* is the maximum mosfet current for Id=100C.)

Id* Rds @ Vgs
part Vds A m-ohm
------- --- --- --- ---
irlz24 55 13 105 4.0
fqa33n10L 100 23 43 5.0
irlz44n 55 33 35 4.0
irl3715L 20 38 15 4.5
irl2505 55 74 13 4.0
fdp7045L 30 75 4.8 4.5
fdp8030L 30 - 3.6 4.5

As you can see, there are far better parts to choose from,
than the one you picked. Sorry! However, none of these is
specified for operation at 3.0V gate voltage, let alone 2.0V,
etc., end-of-life battery voltage. Looking at the fdp7045L's
data*** plots, we can estimate Ron might increase to about
10 m-ohms at Vgs = 3V, and perhaps 20 or 25 m-ohms at Vgs =
2.5V, but below that the on-resistance climbs dramatically.

One good solution for you is to create a small gate-voltage
power source, which would say double the battery voltage,
for the low-power purpose of running the mosfet's gate, and
perhaps a few other things that need more voltage. A part
like the 7660 can accomplish that task in one step, from
voltages as low as 1.5 volts. From $1.30 at DigiKey.


Thanks, I'll look into the chip. Camera one - I used a 10 KHZ pulse
train out of the picaxe to switch an NPN transistor for a little HV
supply to goose the mosfet into conduction. It was a tad unwieldy
with about 10 parts just for that function, but it does work well.
With the current requirement next to nothing it may be possible to
just use the IO pin to toggle the inductor directly (with a diode to
protect the 'axe).

Camera one is in a waterproof case and has a mount that slides into a
channel on the front of my kayak or motorcycle.

Cameras

http://aiptek.com/

The PocketCam X is vastly superior to the smaller camera for
resolution, light level correction, current drain, and speed. Code:
R-PKX Price:$19.99 +$5 for a 512mb SD card (it has internal memory as
well, the SD card boosts the number of pictures you can take from 20
to 640 high res shots) Comes with a tripod, small camera bag, usb
cable, software. I think there's a built in microphone and it will
take AVI videos. Focus is adjustable but not automatic, but the depth
of field is great from about 10' - infinity in the long range mode and
down to 9" or so in close up mode.

My first camera is the mini pen cam (they were calling it the mega pen
cam but changed the name and color) Code: R-PCM13B Price:$9.99. It
comes with a reasonably good belt loop case, microphone and earphone,
and a stand that serves as a tripod. The resolution is nowhere near
the PCX camera but it is small, very light, and cheap enough to
practice on. It says it takes 50 shots - I usually get 60 and even 62
before the memory is full.

Picaxe is the time lapse controller - just BASIC to program costs ~$3
08M and 14M versions cost the same, but the 14 pin has more I/0 and
just as easy to apply. You would have all of $20 invested for a
"development system" including three controller chips. Programming
software is free, you need a PC and serial cable. This thing is a lot
of fun . . . http://www.rev-ed.co.uk/picaxe/ for software and forum

http://www.phanderson.com/picaxe/picaxe.html for parts in the US -
he charges ~$10 for three 08M or 14M parts, includes the programming
resistors, shipping is only $1.95.

You don't need all the hoopla - a basic solderless breadboard , serial
cable, and holder for 2 or 3, 1.5 V batteries is all it takes. It was
harder to figure out how to get the usb to serial cable to work with
windows than it was to use the picaxe.

The radio control forums are excellent sources for camera
modifications - they send them up with planes, kites, rockets, and
balloons.

I got the disassembly and wiring instructions from these sites

http://www.designsoft.com.au/ahome/rc/jazz/jazz.html for modifying the
dolphin jazz camera - identical to the pocketcam X

http://www.laureanno.com/RC/aiptekmod.htm how to hack the megacam

*Nice* background information on your project - the camera &
PICAXE stuff is enticing. :-)

Ed
.

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