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> Wec #2 - Auto Power Off Circuit
Posted: May 23, 2010 09:36 pm
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Welcome to The Weekly Electronic Challenge No#2:

Auto Power Off circuit

Imagine the following scenario:

You own a couple of old battery powered instruments which have no auto power off feature. So, if You forget to turn one off after using it, You'll come back to find a dead battery. So, the challenge is to build an add-on power off circuit.


1. Time delay: somewhere between 1 and 10 minutes. Doesn't need to be adjustable, and doesn't need to be super accurate. The fixed time value could be set when constructed, using a fixed component value or jumper setting.

2. Power Handling: Enough to power a typical battery operated multimeter. Let's say about 100 mA just to be safe. Should also be compatible with whatever voltage the meter uses. Let's say 1.5 to 15 volts.

3. Power consumption: Should consume zero power when the device is powered down (but see item 5), and minimal power when the device is powered up.

4. Size: Should be as small as possible so that it can be fit into an existing meter case.

5. Operation:
Method A (easy): The simplest option is to have an added momentary pushbutton to turn the power on, and then the circuit will power down after the specified time elapses.

Method B (difficult): The deluxe option is to have the circuit detect current flow to the meter which tells it that the meter has been switched on, and then after the specified time elapses, it interupts power to the meter. It then keeps power interrupted until the user manually switches the meter off, at which time the circuit resets. With this second option, it would be necessary for the timer circuit to consume some power while it has the meter powered down. However, the less power consumed, the better.

6. Should not interfere with or affect the normal operation of the meter.

7. You may use any components you find useful for this, both digital and analog.

The rules above are member issued, meaning it's more of a GUIDELINE rather than actual strict rules, so we're not picky, if you don't meet power requirements etc, doesn't really matter that much, as long as you manage to make some kind of auto-power-off, you're in smile.gif

You guys have from NOW until next Sunday around the same time this post was posted, until voting starts.

Good luck and above all - HAVE FUN! smile.gif


If my soul was software, I could program myself and you.
PMEmail Poster
Posted: May 29, 2010 12:16 am
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Earlier today I found the perfect candidate for this challenge: a cheap DMM that uses an expensive 12V battery and has no auto power off function. The meter is branded "Draper", part number is DMM7. I still don't have a decent camera, so yeah, I had to use a phone and so the pictures and video are bad quality, out of focus and all that. Sorry.

About to be gutted
Opened up
First auto power-off prototype

Schematic (everything to the right of the red arrow is my circuit):

user posted image

Power consumption? The leakage of the FET plus the leakage of the capacitor and the 10M resistor in series. Not too sure what that will add up to, maybe some picoamps...

Here's a video: http://www.youtube.com/watch?v=_XWngH75A08

Peace, Rouslan
Abdullah M.A.
Posted: May 29, 2010 10:07 am
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Hi guys, I finished from building "Auto Power Off Circuit", the circuit is typically
is big, but we can reduce from it's options to make it small, this is circuit may
do not carry all specifications of the requirements, because there was not too much
time, anyway, below you will see a picture and a video about what I did.

user posted image



"A scientist can discover a new star, but he cannot make one. He would have to ask an engineer to do that."
"For an optimist the glass is half full, for a pessimist it's half empty, and for an engineer is twice bigger than necessary."
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