Low battery indicator; Surft PI 1.2 build

[Daren]

I just finished building Surf PI 1.2 from Silverdog's kit (Thanks Silverdog, great kit!). I wanted to add simple battery low level alarm, that takes as little current as possible and is as simple as possible. Here is the circuit I used. It takes 0.2 mA when battery level is above alarm level (LED off) and about 3.0 mA when alarm LED is ON. I set the alarm threshold for 11V, it may be a little on the high side we'll see. (I use LI-ION 18650 x3). The LT Spice below circuit as is simulates low level at 9.0V. Use multiturn 10K ohm pot instead of R2, then you can adjust level to your needs. I used just regular LED; at 3.0 mA it seemed just bright enough to tell what it was ON.
I'm a "software guy", so if there is something wrong with the circuit please point it out. It was built and tested, seems fine. I don't know what effect temperature will have on it, but it probably not enough to matter.
Click on the images to get a bigger versions.

Daren

[Rayjay]

Hi Daren,
Could you tell me what the specs. on your coil are......inductance, number of turns, diameter, and size wire.

Thank you,
rayjay

[DanZ]

I did something similar to my home built PI. Not as energy efficient as your setup though. I'm running 3, 6 volt SLA's in series and wanted to monitor their condition during a hunt. The values/settings I used monitor their full charge (20.7v) down to where they are at 6 volts each when the lowest LED fades. I may end up going they way you did but being a newbie at MD'ing and home built machines I just wanted to watch what effect the different settings had on battery drain rates. Thanks for the post!

[Daren]

DanZ, initially I wanted something similar to what you have, but Surf Pi takes about 45mA while operating (55mA with sound) and keeping LED on all the time would take almost 10% of battery. I tested the circuit on the bench only, we'll see how useful it really is in the field.
Daren

[Daren]

Rayjay,
I posted replay but went missing.... anyway here are the main points: Coil I use is 255mm diameter (fits standard housing), made from standard CAT5e Plenum cable (Plenum uses teflon wire jacket); There are 3 turns of full cable, CAT5e has 8 conductors so the coil has 24 full turns; the inductance is 410uH. It's wrapped in polypropylene cable wrap and then shielded with Scotch mesh tape. Coil housing was then filled with resin. Coil performs fine. Tested on the wet sand salt beach no false signals at all. This is my fist coil/detector so it is hard to judge the performance.
Here are some test coils:
CAT5e plenum coil with 4 full turns; did not perform as well as 3 turn coil; Inductance was too high...


Flat spiral coil from speaker wire (24awg): this coil is FAST; performs very nice; I did not use it because I don't have housing for it; Inside diameter is CD spindle size;


Daren

[Rayjay]

Daren,
Thanks for the feedback. good luck.

[mohandes]

Rayjay,
I posted replay but went missing.... anyway here are the main points: Coil I use is 255mm diameter (fits standard housing), made from standard CAT5e Plenum cable (Plenum uses teflon wire jacket); There are 3 turns of full cable, CAT5e has 8 conductors so the coil has 24 full turns; the inductance is 410uH. It's wrapped in polypropylene cable wrap and then shielded with Scotch mesh tape. Coil housing was then filled with resin. Coil performs fine. Tested on the wet sand salt beach no false signals at all. This is my fist coil/detector so it is hard to judge the performance.
Here are some test coils:
CAT5e plenum coil with 4 full turns; did not perform as well as 3 turn coil; Inductance was too high...
[ATTACH]29799[/ATTACH]

Flat spiral coil from speaker wire (24awg): this coil is FAST; performs very nice; I did not use it because I don't have housing for it; Inside diameter is CD spindle size;
[ATTACH]29800[/ATTACH]

Daren

for cat5 you turn it 3 times?? how much inductance and res ?

for flat spiral coil , can you tell me to make it ,
regards
and best wishes

[Daren]

flat coil is: 1.7ohm 235uH 30 turns; you find description how to make it on adrianandgenese ; just google "surf pi 1.2 speaker wire coil"
the cat5e cable coil I can't remember the resistance; I know it was 410uH inductance; The turns of able and then you connect ends (1end->2start, 2end->3start, etc, and then 1start and 8end will be your coil connection) of the 8 internal wires to make 24 turn coils. Sorry I can't post pictures now..

Daren