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June 2017 - Auto Cut-Off Timer
I was looking for a way to save the battery in my multimeter. I kept leaving it on and PP3 batteries can be quite expensive nowadays.
Sprat 156 came up with the answer. An idea from Bob Kellogg AE4IC.
There were a few sources of the LMC7221 on EBay but some of them look purposely misleading as the description ends "1000 pcs". Which would lead you to think you were going to get a thousand of them - and you weren't - so be careful.
I found a USA seller that looked reasonable. He was - and delivery was very prompt.
He was open to an offer of five dollars too! (and that was for 10 of them).
I etched a PCB suitable for surface mount components.
Here is the design view:
and the populated PCB which measures 35mm x 18mm.
The file for this board which was created using the toner transfer method can be downloaded here.
The software that I use is “Sprint Layout”. It isn’t free, but it isn’t expensive either.
http://www.abacom-online.de/uk/html/sprint-layout.html
But this is the free software for sharing...
http://www.abacom-online.de/updates/Sprint-Layout60_Viewer.exe
Once installed in my meter it turned itself off after 3:30 (mins), and once off, there was no measurable current drain on the battery. I eventually used two 0.22uF capacitors (in parallel) for C1 which set the timing.
As you can see, there is plenty of space to add Bob's circuit to the multi-meter.
To the left of the battery was a convenient moulding so I drilled that through to the front and added a virtually flush push switch.
