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Apr 15, 2004, 05:50 PM
Trying to defy gravity...
jimsky's Avatar

MicroScream: $5 Audible Lipo Alarm

I have a few Plantraco DSP4-SC combination receiver/ESC in my planes. I really like these aside from one drawback, no low voltage lockout feature. So far no problems, but I'd like something to prevent an overdischarge lipo situation.

I didn't like the LED style lipo alarms, so I designed what I really wanted: a cheap, lightweight, very loud audible alarm that will tell me without a doubt it's time to land the plane and swap battery packs. Less than 5 bucks of parts and about 20 minutes with a soldering iron you can have one too. I've call this alarm the MicroScream.

I know many say, you will know when" time's up" due to preformance roll off if you don't have an ESC with a low voltage feature. But with this little alarm no need to even think about it. When you hear the alarm, land the plane.

I selected a small 85 decibel piezo alarm, it's pretty darn loud. Just a few simple parts are's all outlined on the attached schematic. The last one I built weighed only 2.9 grams and I wasn't even trying to do a lightweight build. See the attached photo of it on my scale.

I've tabulated the schematic for a 2 cell and 3 cell version, but there's no reason why it can be modified for additional cell usage.

I've learned a lot here and figured I'd like to give something back. So here you go: print the schematic, order you parts and have fun!

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Apr 15, 2004, 06:50 PM
Registered User
Andy W's Avatar
Very cool..
I'll move this with redirect over to DIY .. I think a reference to this thread in the LiPo sticky thread would be worthwhile, however..
Apr 15, 2004, 09:09 PM
Permanently Banned
Larry3215's Avatar
Very cool! I would like to use this with hi voltage setups as well. What would the values be for 4S, 5S, 6S, 9S, 10S etc. Is there a formula to use to calc the values?


Apr 15, 2004, 09:47 PM
Trying to defy gravity...
jimsky's Avatar
There are two things that need to be adjusted for additional cells. The value of R1 to set the alarm trip level, and R4 to drop voltage so X1 doesn't see more than 7 volts maximum.

The voltage developed by the R1/R2 divider that connects to the "Refrence" pin of the TL431 wants to be 2.5 volts when the battery is it's warning point (that would be 3 volts per cell used). Select R1 to drop the appropiate voltage using Ohm's law.

The piezo I picked has a maximum voltage of 7 volts, so a dropping resistor is needed as the battery voltage goes above 2 cells. I used the specs on the piezo as a starting point, but then trimmed the resistor via testing.

I'll work up a more encompassing tabulation sheet and add it to this thread.

Apr 16, 2004, 10:15 AM
Registered plane abuser
Smarticle's Avatar
Very nice, I was looking for another little project, after building my lipo charger. This will work well with my 5A gws esc and experimental foam planes. Thanks jimsky.
Apr 16, 2004, 12:26 PM
Trying to defy gravity...
jimsky's Avatar
Regarding information for additional cells, I'd like to review this a bit more. There's an issue regarding the maximum voltage the gate of the 2N7000 can tolerate. With high cell count battery packs this would need to be addressed.

Presently I only own brushed motor 2 cell configuration planes. I know technically you can stack up as many cells as you like, but what it the practical maximum number of cells that are typically used (barring science project builds)?

Apr 16, 2004, 01:45 PM
Registered User
blueprints's Avatar
Would it be possible to hv adjustable voltage trigger e alarm like variable resistors or sort? for e.g setting it to trigger @6.2V for 2 cell configuration. Thanx.

Apr 16, 2004, 02:20 PM
Trying to defy gravity...
jimsky's Avatar
Sure you could add a pot. But that's weight, cost, size and is it really set right? When I was designing this alarm light weight and small size was my goal, so I went with fixed resistors. Personally I think 3 volts per cell is a good alarm setting, here's my logic.

Testing my SuperFly-E with a GWS 350 motor, 8060 prop and Tanic 2 cell 2200 battery pack I found the following. With a fully charged pack the battery voltage dipped 0.75 volts going from "motor off" to "motor full on". This can be thought about as the "load regulation" of the battery pack, the voltage change from "no load" to "full load" due to it providing current to the motor.

This needs to be taken into account when selecting the alarm trip point, otherwise you'll be restricting your flight time. You can always land earlier before the alarm sounds, and my guess is most people will, due to degraded preformance. The alarm is there to tell you must to land, but it still has some saftey margin to boot.

My thinking is as follows:
You've been flying quite a while (motor on - loading the battery) and the alarm sounds in the air indicating the battery voltage has hit 6.0 volts. You back off the throttle, the battery voltage increases (less loading) and the alarm will actually go off. Land the plane, disconnect battery. If you were now to measure the actual battery voltage you will find it's higher than 6.0 volts due to the load regulation of the battery. Happy, well exercised but not over depleted lipos.

As long as you don't fly around till the cows come home with the alarm sounding (pulling the battery well below 6.0 volts) you should be fine. When the plane starts to get sluggish, keep it close by and do frequent fly bys to listen for the alarm. When you hear the alarm, land the plane and you should be fine.

Apr 16, 2004, 03:09 PM
Registered User
blueprints's Avatar

I muz say u've really gave me quite a run down. Right i guess ure right. This was designed with weight n size to keep in mind. Can't wait to build my own. Juz like to know wad does Yageo mean. 3.57K by parallel connection u mean? Also would it be absolutely critical for e resistor values to be exactly precise for the alarm to work. Thanx for sharing ure great idea. Appreciate it.

Apr 16, 2004, 03:18 PM
Registered User
blueprints's Avatar

Kinda difficult to get fixed value resistor such as 3.57K here in singapore. hv to put them in parallel or something.... gonna be messy n bulky... where'd u got ure resistors?
Apr 16, 2004, 04:06 PM
Trying to defy gravity...
jimsky's Avatar
Yageo is the manufacturer of the 1% resistors that Digikey sells.
R1 & R2 form the voltage divider that sets the alarm trip point, that's why I used 1% resistors. You could cobble up values using series/parellel combinations, but it will increase size and weight. The 1% resistor values I specified are all industry standard values. Radio Shack doesn't carry 1% resistors, but any real electronics distributor should. As indicated in the notes section of the schematic, you can buy all the parts at:

Apr 17, 2004, 02:47 AM
Registered User
This would be great for hand launch gliders with li-polys, where you dont have any noticable perfomace drop(no motor, just servos).
Thanks, Rob
Last edited by robert harik; Apr 17, 2004 at 02:57 AM.
Apr 17, 2004, 11:45 PM
Registered User
HELModels's Avatar
That is neat!

I do have a question about controlling the cathode current on the TL431 with R3. How is the value of R3 selected?

It looks like 2N7000 Gate must be considered somehow? I realize Vout= (1 + R1/R2) * Vref and that is how R1 and R2 are picked. I'm just curious and also realize there is a minimum of 1mA cathode current, which R3 allows.
Apr 18, 2004, 10:07 AM
Trying to defy gravity...
jimsky's Avatar
Cathode current for the TL431 is specified as a minimum of 1mA and a maximum of 100mA, so pick R3 accordingly.

I did mention in a previous post that the maximum voltage the gate of Q1 sees must be considered for higher cell configurations.

Apr 28, 2004, 05:17 AM
Registered User
Is there some alternate part numbers that we can use for U1 and Q1?

I am having problems finding these devices in Australia.


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