EMRFD Message Archive 1920

Message Date From Subject
1920 2008-08-11 17:28:31 Art auto power off circuit?
Hi All,

First, I'd like to say that this is an incredible mailing list!!!
Although the posts are very OT in general, the content of the posts
is just outstanding. Thanks to all who maintain and contribute!

My request is another OT post however. Sorry about that::>

Earlier in the week, I bought and assembled an AADE LC meter, it's
PIC based that uses audio frequencies to measure L and C. It is
definitely an A rated piece of gear and I'm thinking about getting
one for the basement workshop (I keep the one I just built in my
office desk). When I get my second AADE meter, my commercially made
Elenco LCR-1810 will become surplus I think::>

Anyway, the AADE meter is wonderful, except for one minor
deficiency........it does not have an auto shut off, if left on over
night, one has to buy another battery for it. If the thing would just
shut off after 10 minutes of non use, it would be an A+ rated piece
of gear (IMHO).

My question to the group is:

Does anyone know how to make a simple circuit that can be integrated
into existing gear that will provide an auto shut off function using
the existing power switch (without having to add a switch)? It's ok
if the timing is approximate (can tolerate big differences in the
shut down time out interval). But, it has to work without adding
additional external switches or external modifications to the hardware.

Any comments appreciated.



1921 2008-08-11 18:09:54 Nick Kennedy Re: auto power off circuit?
I like this circuit a lot --


The "on" switch charges a capacitor that turns on the gate of a 2N7000
MOSFET. You can choose the time delay before shut off by the size of
the discharge resistor shunting the capacitor. (You'd be amazed how
long the thing might stay on if you omit the resistor.) Since you
don't want a separate, momentary switch, just switch ON and back to
OFF and let the delay do its job.

The ground or negative return lead of the load goes to the drain and
the source goes to ground or battery negative.

73--Nick, WA5BDU
1922 2008-08-11 18:22:52 WA0ITP Re: auto power off circuit?
Here's another http://jacksonharbor.home.att.net/batsave.htm
I love this radio stuff !
72, 73 Terry, WAØITP

----- Original Message -----
1924 2008-08-12 01:13:00 Art Re: auto power off circuit?
Thanks to Nick and Terry!

Both replied, both had great info. Both circuits
require an additional monetary contact switch,
which is not quite what I had hoped for. I wanted
something that would operate without drilling
holes to mount extra switches, allowing me to use
the stock power switch (which is not a momentary contact switch).

After I looked at the AADE meter, the solution
was easy. The power switch in the AADE meter is a
DPDT push button switch that is used as a
SPST switch, so there are lots of left over
terminals. I discovered the switch could also be
converted to a momentary contact push button
switch with a simple mod. The push button switch
is just what I need, and since it's already
integrated into the meter, it makes a nice clean modification!

Again, thanks to both of you and to the group.



>Here's another
>I love this radio stuff !
>72, 73 Terry, WAØITP
>----- Original Message -----
1927 2008-08-12 09:16:56 Mike Short Re: auto power off circuit?
Can you tell us what you did to the switch? I have one also, and end up
needing to change batteries when I want to use it.


Mike Short



[Non-text portions of this message have been removed]
1928 2008-08-12 14:26:46 Art Re: auto power off circuit?
Hi Mike,

The existing power switch is a DPDT that latches either on or off.
For an outboard auto shut off to work, it needs to be converted to a
momentary push button switch.

To convert the existing power switch to a momentary push button
switch, remove the u shaped steel wire that is held in place by the
return spring on the switch. One end of the U goes into the switch
near the knob, this end is the stop for the switch in the 'off'
position. Without it, the whole plunger falls out in your hand. The
other end of the U fits into the latching mechanism in the switch,
which is visible when you turn the switch on and off.

To convert it to a momentary push button switch, simply remove the
steel wire U shaped wire completely (be careful not to lose the guts
of the switch which can now fall out as the switch separates into 2
pieces. Insert one end of the U shaped bracket (the end closest to
the knob) back into the original position. But, do not insert the
other end back into the latching mechanism on the switch
body-instead, angle the wire so that it rests against the outside of
the switch body instead of into the latching mechanism on the switch itself.

It's tough to explain, but it is easy to do-it is a 30 second
operation. I wish I could send you a digital photo, but my camera is
very bad at doing close ups.



>Can you tell us what you did to the switch? I have one also, and end up
>needing to change batteries when I want to use it.
>Mike Short