By: asgard

2013-01-31 13:52:42

This is J.R. Stoner's blog post.

This is a picture of my muse, Mr. Humphrey Lyttleton Katz or Chairman Humph, for short.

He is saying here, "Go away. You're so boring.  Get over yourself."

Actually Humph is probably the neediest cat I ever had, in his own weird way.

Enough about Humph.  Here is a listing of some of the projects I have been working on, of late:

This one is called the Powah.  It is just a little on/off toggle with an indicator LED.  It is designed to mount on one end of my breadboard, so as to reduce the wear and tear caused by excessive insertions of power bus leads.  The board is approx 1 cm^2.

Pretty nifty, eh?  I searched around and the closest thing to this is a power switch from Pololu, which I did not find that interesting (too large and did not have the 100-mil form factor. Here is a schematic:

It is very simple, as you can see.  The main limitation is that the power for the IC is tied to the input, so the DC level is limited to 6V.  Turned on, the current loading from the Powah is 3.06mA. Turned off, the loading is unmeasurable with my crusty old B&K multimeter.  If I can sell enough of these, and perhaps some other ones I am working on I can spring for a nice Fluke one.

Here is an alternative design I have been noodling right now:

The main advantages here are:

The MOSFETS are complementary and on the same die, so they are matched.  And they are much beefier so the current passage has more safety (I would be comfortable passing at least 2A through this device).

The Zener limits the gate power supply to 5.1V, so the input switching can go all the way to the breakdown limit of the P-channel MOSFET, which is 20V.

Turning on the switch requires only a short button press, while turning off the switch requires a longer button press, as a fail-safe.

The inherent hysteresis of the Schmitt front-ends on the gates means that the reset signal on the second pair requires a time interval before the latch signal is active.  This is a free way to debounce the circuit.

The main disadvantages are the increased complexity and part-count requires the board real-estate to also increase.  I have managed to fit the whole works on a 0.8 x 0.5 inch board.  There are, to my knowledge, anything like a 74LVC2G7002 device out there.  I wish there were, as I think I could fit 2 SO8's better than the SO14.  Here is a 3D rendering of what that board is likely to look like:

Anyway, enough of my blather.  For now.


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