Need, maybe found, low voltage latching circuit

All,
I have an expensive measuring tool that is powered by the ubiquitous
SR44 silver oxide battery. The tool doesn't get used much but when I
do go to use it the battery voltage has always dropped enough to cause
the display to flash. So I always remove the battery when I put the
tool away. This is a pain. I thought about putting in a tiny toggle
switch but there is not room in the electronics enclosure. There is,
however, room for a couple small ICs and a couple passive components
if distributed around inside the enclosure. The tool does have an
on/off switch but this only turns off the LCD display. I could
re-purpose this momentary switch, by cutting a couple traces, for
actuating a latch circuit. The tool draws 43 microamps when on and 30
microamps when "off". I found this circuit:
http://www.discovercircuits.com/DJ-Circuits/Low-Voltage-Latching.htm
and I think it would work but I am not sure if it will also draw power
when "off". Is the circuit in the link above appropriate? Is there a
better one?
Thanks,
Eric

 

[George Herold]

On Friday, May 9, 2014 12:25:37 PM UTC-4, et...@whidbey.com wrote:

All,

I have an expensive measuring tool that is powered by the ubiquitous

SR44 silver oxide battery. The tool doesn't get used much but when I

do go to use it the battery voltage has always dropped enough to cause

the display to flash. So I always remove the battery when I put the

tool away. This is a pain. I thought about putting in a tiny toggle

switch but there is not room in the electronics enclosure. There is,

however, room for a couple small ICs and a couple passive components

if distributed around inside the enclosure. The tool does have an

on/off switch but this only turns off the LCD display. I could

re-purpose this momentary switch, by cutting a couple traces, for

actuating a latch circuit. The tool draws 43 microamps when on and 30

microamps when "off". I found this circuit:

http://www.discovercircuits.com/DJ-Circuits/Low-Voltage-Latching.htm

Hmm well you are going to drop some voltage with Q1... not sure how much..
100-200 mV maybe.

And then the logic inverters will also draw some current just sitting there.
(I don't know how much.)

I like JL's magnet/ reed relay idea.
(it wouldn't have to be a latching relay)
Maybe a NC switch so with the magnet in place the switch opens.

George H.

and I think it would work but I am not sure if it will also draw power

when "off". Is the circuit in the link above appropriate? Is there a

better one?

Thanks,

Eric

 

[John Larkin]

On Fri, 09 May 2014 09:25:37 -0700, etpm@whidbey.com wrote:

All,
I have an expensive measuring tool that is powered by the ubiquitous
SR44 silver oxide battery. The tool doesn't get used much but when I
do go to use it the battery voltage has always dropped enough to cause
the display to flash. So I always remove the battery when I put the
tool away. This is a pain. I thought about putting in a tiny toggle
switch but there is not room in the electronics enclosure. There is,
however, room for a couple small ICs and a couple passive components
if distributed around inside the enclosure. The tool does have an
on/off switch but this only turns off the LCD display. I could
re-purpose this momentary switch, by cutting a couple traces, for
actuating a latch circuit. The tool draws 43 microamps when on and 30
microamps when "off". I found this circuit:
http://www.discovercircuits.com/DJ-Circuits/Low-Voltage-Latching.htm
and I think it would work but I am not sure if it will also draw power
when "off". Is the circuit in the link above appropriate? Is there a
better one?
Thanks,
Eric

How about a mercury switch from an old thermostat? Store the gadget upside down
to turn it off.

Or a CdS photoresistor in series with the battery, so it turns off if you keep
it in a dark box.

There are latching reed switches, too. Turn on/off with a magnet.

Tiny slide switch?

What is the measuring tool?



--

John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation