MicroScream: $5 Audible Lipo Alarm - Page 2 - RC Groups
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Apr 28, 2004, 05:45 AM
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Try http://www.farnellinone.com/ for parts in Australia
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Apr 28, 2004, 07:34 AM
Trying to defy gravity...
jimsky's Avatar
U1, the TL431 is the heart of the device. I don't think you can easily replace it and use the existing circuit. I did make an identical device using a separate zener reference and comparator. It used more parts, was larger, heavier and consumed much more current.

Q1 could be replaced with a BJT instead of a logic level FET, but you would have to address the high OFF voltage of U1 (assuming it was still used). This would require modifications to the circuit.

So yes other parts can substituted, but changes will most likely be required to address the differences.

Apr 29, 2004, 04:45 AM
Registered User
Geoff_S's Avatar
Futurelec stock the TL431CLP but not Q1.

To be honest, I'm not sure where these guys are, but they have an Australian office and offer domestic shipping prices. I've ordered components from them before without drama.

As clipclop mentioned, Farnell is another option but the postage tends to be a bit more expensive (quick though).
Apr 29, 2004, 06:36 PM
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imeins's Avatar
Ignore this - my brain was in neutral !!! Just saw all the other Farnell posts.


Try the highly overpriced but "have everything" people at Farnell here in Oz. www.farnell.com etc.

May 05, 2004, 12:40 PM
Graham Dyer
pardus's Avatar
Brilliant Idea, don't like the guessing game much
I have a 6S2P LiPo pack and I'd like to modify this circuit for 25.2V (18v alarm), Unfortuantely my knowledge of electronics is a little limited.
What would I need to change?
Thanks in advance
May 05, 2004, 01:43 PM
Trying to defy gravity...
jimsky's Avatar
The 2 cell and 3 cell version of this alarm was my goal, and that's what I designed. The reference to higher cell counts was "just thinking out loud", I have not reviewed this, nor have I done any calculations, prototyping or testing. In fact, all I presently own are 2 cell battery packs.

The more I've thought about higher cell count versions of this alarm, the more issues of concern I have found with this simple circuit. I'll give this some though but I'm afraid more than just resistor values might have to change.

Circuits distilled down to utter simplicity are economically elegant, but these application focused creations tend so loss their ability to be flexible. In essence they were designed to do what they do...with limitations.

Things that were not issues before, become problems needing solutions. The problem is, you don't know what all the problems are until you make one to test it and see.

I'll try to find some time to give this some thought, and perhaps prototype a circuit.

May 05, 2004, 01:58 PM
Graham Dyer
pardus's Avatar
Thanks Jimsky, I appreciate it. I'm about to head off to my garage to start construction of the 3S one. I was hoping that a few resistor values might be all that needed changing for 6S but I can see that at such high voltages more things might have to change. Why the big manufactures like FMA haven't done something like this, I don't know - it seems like the way to go

My ESC's highest cutoff is 12v so my LiPo's will be damaged by the time it gets anywhere near that but of course the motors performance will be severely degraded by the time the batteries reach 18v so I should notice but I would, (like many) like to know exactly when it gets there.
thanks again
May 05, 2004, 06:39 PM
Registered User

From our very own:

Will this do?


and only $24.95

May 07, 2004, 06:44 PM
Registered User

I have tried building this circuit but the buzzer is always on.

How critical are the Voltage divider values?
What voltage does the Reference pin require to turn on/off?
Currently I have 2.53 for a 2 cell battery.

Any help appreciated.
May 07, 2004, 07:20 PM
Trying to defy gravity...
jimsky's Avatar
If you use the 1% resistor values shown, taking the reference tolerance and divider tolerance into account the trip point accuracy should be less than or equal to +/- 2.5% of 6.0 volts (2 cell) and 9.0 volts (3 cell). I think having accurate trip points are important, that's why 1% resistors were specified.

The internal reference is 2.495 volts, nonimal.

One issue might be is you didn't follow appropiate static procedures and blew the 2N7000. If the transistor is shorted, the piezo would always be on. It's important to follow static procedures when working with semiconductor components. Grounded wrist strap, soldering iron tip grounded, ...all that good stuff. Yes, it is important.

If that's not it, it's possible it might be another situation.

The TL431 does have a rather high OFF voltage. If you get a 2N7000 with a low gate threshold voltage (it's possible if you check the minimum specs, could be as low as 0.8 volts) that's not "typical" value of 2.1 volts...then you might have a problem. Basically the piezo would always be ON.

I've built a bunch of MicroScreams and yet to run into this scenario, but technically it could happen. I checked all the ones I've built, and had plenty of margin to prevent this issue.

My experience is most parts come in as "typical", but cheap component manufactures give themselves an out specifying wide tolerances. If the 2N7000 is not shorted, try replacing it in hopes of getting one with a higher gate threshold voltage.

May 08, 2004, 08:23 AM
Registered User

Unsoldered the gate and manually shorted out Q1 and it works as expected.
So it is not the 2n7000 causing the problem

As I am unfamiliar with the TL431 at what volt should it short or go open to make the circuit work?
I take it it is biased on via Divider R1 and R2 and once the Ref exceeds a low voltage (2.495) it is triggered off and R3 pulls high the Gate.

If this is correct then it may be the TL431 that just won't pull R3 voltage down to ground. I'll try replace on Monday when I can get more.

May 08, 2004, 10:56 PM
Trying to defy gravity...
jimsky's Avatar
Do a web search on the TL431 and download the datasheet, it will help explain things.

I had stated before, the TL431 has a high OFF voltage. When it's OFF it does not go to "zero volts" or ground, not even close. This voltage may turn a 2N7000 "ON" if it has a low gate threshold voltage. This means it's possible the 2N7000 would always be ON.

If a BJT (even lower ON voltage) was used for Q1, something (more parts) would have to be done to get rid of the high OFF voltage of the TL431. The logic level FET addresses this without the need for extra components. But like I said it's possible the tolerances can stack up against you.

May 20, 2004, 12:20 PM
Trying to defy gravity...
jimsky's Avatar
The MicroScream alarm posted in this thread works quite well. I have one in all my planes, it provides real peace of mind when flying. All my personally built units in my planes are ultra lightweight "dead bug" style builds, using magnet wire to connect to the battery. A couple of installations have the piezo buzzer remote from the electronics, optimizing the location of both the electronics and the piezo buzzer.

I posted the schematic so people handy with a soldering iron could build it for themselves for the cost of the parts. For those who would prefer to buy a complete and tested MicroScream low voltage lipo battery alarm; they can be purchased at:


These units are fabricated using a more formal printed circuit board build technique. There are great for people who what the peace of mind having a battery alarm, but don't want to build circuits. Presently there are 2 cell and 3 cell versions being offered. Attached are a couple of photos of the finished alarm.

May 22, 2004, 10:58 PM
5,200 Led Lighted Bike-Bob P.
**neons**'s Avatar
Hi Jimsky,
Just purchased a unit. Can you tell me the best way to connect to the battery or whatever way that you suggest. I am using Sermos -Anderson connectors if that means anything. I plan to attach it to different planes as I fly them so it is portable. I use the Jeti ESC's that has a red cover and is not setable voltage for 3s lipos. Nice idea. I just happened to find this tonight.
Thanks **Neons**
May 22, 2004, 11:12 PM
Trying to defy gravity...
jimsky's Avatar
Well the MicroScream monitors the battery voltage, so it needs to be connected to the right to the battery wiring. Red wire to battery positive, Black wire to battery negative. That doesn't leave a lot of options on how to connect it. It has to be on one side the battery buss or the other...battery connector side, or ESC side.

The only way I see to make it "portable" would be to add wires & JST connector to one of these locations, and put the mating JST connector on the MicroScream. And then do this to your other planes. This way you can simply move the MicroScream from plane to plane using the JST connector to "tap" into the battery line.


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