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Old Feb 18, 2006, 11:29 PM
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LiPo Alarm For The Blade CX

In my quest to find a low voltage alarm for my BCX I think I've found the solution.
I've been reading as much as I can about the BCX and specifically the Lipo that comes stock with the BCX. Thanks mostly to Mike abcd's postings on the BCX Lipo, I take it that the ideal time to quit is when the battery reaches around 7.4volts.
http://www.rcgroups.com/forums/show...&postcount=1388

I thought that Jimsky's Microscreamer might fit the bill but he designed this for planes and since I'm flying in the house in close view of the BCX I didn't think I'd need the audible alarm.

So my search has taken me to Bruce Abbott's design of the "Simple Low Voltage Warning Alarm"
http://homepages.paradise.net.nz/bhabbott/lvw.html
I've breadboarded the circuit and it works quite well testing it with a regulated variable DC power supply and a fully charged, then discharged BCX Lipo. The only changes to Bruce's design that I've made were to change the values of the R2 and R3 resistors to 3.9K and 2.0K respectively. This will have the alarm trigger when the Lipo voltage drops to 7.38 volts. I did use the bright LEDs as per Bruce's suggestion as they're much easier to see. I plan to simply place the LEDs in the canopy and tail as I think they'll be visible through the stock BCX body and I'll use wirewrap wire (very thin gauge) to run the LEDs to the front and back. I'll post pics.

One question,... as you can see by the circuit, when the battery voltage is above 7.38V, the TL431 closes the connection between its anode and cathode effectively closing out the LEDs from the circuit and turning them off. In doing this it will place a 2.2K resistance between the leads of the Lipo.
I would guess that this is a safe situation but I'd appreciate it if the electronic gurus could verify this.
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Old Feb 19, 2006, 11:43 AM
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Voltage divider correction

Mike abcd, on the multi rotor forum, reminded me that the alarm must trigger reading the load voltage so I'll restore Bruce's original values for R2 and R3 so that the alarm triggers at 6.0V.
I'm still not sure about the potential Lipo short situation when the alarm is in the untriggered state.
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Old Feb 22, 2006, 11:22 AM
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Rodono,

Placing the 2.2k resistor across the Lipo will only result in about 4 mA current (< 30 mWatts of power loss). This will not make any harm at all. Of course you cannot have the Lipo alarm connected to the battery during storage - this will flatten the battery after some days.

/Pete
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Old Feb 22, 2006, 05:50 PM
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Thank you Pete!
This simple little alarm, therefore works great!
Rod
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Old Feb 06, 2015, 01:16 PM
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Hello!
Nice little thing!
This can work with 1s lipo?
possible?
Thank you!
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