Make a touch operated power supply! Its MAD! WHY DOES IT TURN ON??

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martakz's picture
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Joined: Dec 20 2003
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Hi all

This might be useful for someones hack.

In my attemps to make 2 atx psus turn on at the same time I made this circuit.

For testing purposes I linked it to one psu only. The 3.3V line went to the atx green sense line. The ground wire went to the same psus ground line. The 5v wire which is soldered to the transistor was left unconnected to the psu.

Anyway...i fired it up...the psu did not turn on. Which is good that is what it is suppost to do at the start! I then picked up the 5v line...and to my surprise the psu turned on! On further experimenting...i found that I could turn the psu on and off by just tapping the 5v line. One tap, on. One more tap, the psu turned off!

Its mental. For some reason the transistor is being triggered by my body. A friend suggested that my body is proving a current and has a potential difference with the psus ground. Even so...if I earth myself, the psu still turns on. If I am earthed this should not happen, as the path from me to the ground through earth is easier for electricity, than the path from me, through transistor to psu.

I then wired in a 10 megaohm resistor, in series, between me and the 5v line. Even so...it still turns on, even through the monster resistor.

I am totally baffled. On the plus side, if you wired this to a metal case...one tap and the computer would turn on. I think this effect would get better if you use a darlington pair...

Now to another off bit. This transistor is acting like a latching relay. One tap its on...one tap off. If my body is really suppling charge, then the psu should turn off when I release the line, yet it does not...only if I tap it again.

I would be greatful if you have any ideas??

The transistor is a spare n-channel power mosfet I had lieing around. The psu is a 300w atx powersupply. The 1000ohm resistor is really a 10000ohm variable resistor.

Please reply!

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sourapple's picture
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Joined: May 27 2004
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Yes your body does provide current

Yes your body does provide a current. It cannot be grounded otherwise your heart would fail. It needs a small amount of electricity to beat, as does your brain. Your body creates it so you cant get rid of it.

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martakz's picture
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I had no idea that the transi

I had no idea that the transistor was sesitive.. Its an n-channel POWER transistor...it should not be so sensitive. Also, when connected to a powersupply, I am having problems turning it off. The only way is to remove the 3.3V line and ground. Could this be an earth leak of some kind?

On second thoughts...maybe its static electricity? That might explain why the resistor had no effect...

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Joined: Dec 20 2003
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maby that has sumthin to do w

maby that has sumthin to do with the power button on the keyboard??? im just guessing here though and I have no idea why you would have the same effect on it

martakz's picture
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Nope. Nothing but one atx ps

Nope. Nothing but one atx psu was connected.

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Joined: Dec 20 2003
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Keyboard power

Thatwas just a short between two pins on the ADB plug.
;D

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Joined: May 5 2004
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Whicn two pins on the adb plu

Whicn two pins on the adb plug were shorted to do this?

aj

tmtomh's picture
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Joined: Dec 20 2003
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2 and 4

Pins 2 and 4, when shorted together, send the power-on signal.

According to Apple tech docs, if you're looking at the end of an ADB port, 2 is the upper-left hole and 4 is the lower-left hole.

Which means if you're looking at the end of an ADB cable, 2 and 4 are upper-right and lower-right, respectively.

Matt