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mikep

PPM/PCM Plane Locator Voltage Watch

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OK well I decided I will tackle the PCM plane locator.

And while I'm at it I will add a voltage watch to it as well.

So just a bit of brainstorming here to start. Any input/feddback is appreciated.

The uP will be a PIC12F675. It is an 8 pin PDIP with internal oscillator (4Mhz) and 10-bit ADC. Lots of other stuff but that's all I need.

The basic function is this.

The circuit will be powered by the RX battery.

The uP will loop through 2 measurements. The first looking for a pulse width, the second at the battery voltage.

It the pulse width is not there, it will open up a BS170 and start beeping a piezo.

This should work with PPM. If the pulse width is over 2ms, the same sound will be heard through the piezo. This should cover PCM using an empty channel and failsafe options.

The next is to measure the battery voltage.

First we need a reference voltage. I'm thinking 3.5V. I found the AN8035 at Digikey. It's a 3-pin, low dropout, fixed positive output type monolithic

voltage regulators (50ma). This is easy to regulate and by dividing the battery current by 2 using a resistor voltage divider we can get between 0-7V easily. Should be enouh for 6V packs as well as 4.8V packs.

If we get a measurement below 4.8 volts than we start beeping the piezo. A different sound than the plane locator to differentiate what the problem is.

One option is to have a 9V battery on board as well to feed the piezo. This way if you do have a dead battery in flight, at least it won't be drained more by th piezo buzzing. However it adds more weight. What doyou think?

I'll post the schematic, code and PCB layout as I work on it.

Cheers,

Mike

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I had a couple of free hours today and completed the code.

Here is a description. I'm just waiting for the 3.5V reg to come in to finish testing it.

The way it works is that you plug it in to a spare channel on the RX. You setup your failsafe to drive that channel above 2ms. (The top value of most TXs is about 2.150ms).

If failsafe is activated a piezo buzzer will sound and an LED will flash.

It will also do the same for a missing signal (for PPM receivers).

The other thing that it does is that the circuit puts itself to sleep after beeping/flashing to conserve the battery. It's starts off with sleeping for 5 secs. Beeping and going back to sleep. The lower the voltage of the battery gets, the longer it sleeps between beeps/flashes to a maximum of 150 seconds between each cycle.

So this way, if it's way out in a field hard to find, it will try and keep beeping as long as possible by preserving the battery.

Next, it also monitors the battery voltage.

If the battery gets to 4.4 volts or lower it will give some quick beeps and LED flashes every 5 seconds. It will take several samples of the battery voltage to make sure it was not simply a voltage drain due to servos forcing etc.

This function is disabled when the locator is activated to save battery power once again.

I'm also thinking of adding a feature that will quick flash the LED if failsafe was ever activated, and a different flash if the battery get's below a certain voltage.

If it's a quick "glitch" you probably will not notice these things in the air. But it would be nice to know it happend once you are on the ground.

I will make the whole project available on a web page in the next few days.

Basically I don't want to start of this season like last one by watching a plane fly off to oblivion! ;-)

Cheers,

Mike

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Sounds like a very useful gadget. There are a lot of folks interested in LiPO battery protection, so that might be something you might consider adding.

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I started tetsing out the piezo today and found a couple of frequencies that seem to excite it quite a bit.

I made a little tri-note sequence that I'm thinking will be easily distinguishable.

Last part is to test it out a different voltage to see if I can get away without using a voltage regulator.

All the components can handle a good range of V however I'm not sure about the piezo. I will put off that test for another day though because my ears are now ringing and it gave me a headache.

Basically I stepped through hundreds of frequencies until I found some that really get the piezo going. The manufacturer recommended 2400Hz which is a nice buzzing sound but not great for locating a plane.

Simply putting up an update,

Mike

Edited by mikep

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For the past hour the little piezo buzzer is buzzing away in my office. I moved everything from a dev board to a bread board using the receiver battery (the difference between the 2 is no voltage regulator and a few other nifty things that make it easy to program micro-controllers).

So now I will have a good proof of concept.

I basically wanted to see how it will react at different voltages and how long it will work for.

I grabbed a battery from my battery box measured it and it was at 4.8V, not bad considering I haven't charged it since last fall. I set it all up on a PCM receiver, shut off the transmitter and it's buzzing away.

I'll let you know how long it lasts. It put a piece of tape on the piezo and shut the door to my work room so you just barely hear it.

If all goes well I will put up the details on how to make one this week.

Cheers,

Mike

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Hmm... Interesting project. I'm using the same 12F675 for a variation on the R/C Nav-Lights project, and am also considering adding the low battery condition to the project. I was going to use the strobe as an indicator if the battery was getting low.

Have you measured the peak voltage of a 6V pack to see if it gets too close to the 7V limit of the PIC? I noticed that when I charge my 7.2V packs, they peak somewhere around 9V. I guess the regulation thing becomes less of a problem if you are using electric planes (ESC BEC), which I'm assuming you are not. That part you are using from Digikey, the 3.5V regulator, is that a small part, like a transistor size? I was thinking of just using zener diodes on mine, but maybe that part would be better.

Yes, I would agree with the comment about the LiPoly batteries. Jeti makes ESC's that have selectable cutoff voltages for using either NiXX or LiXX batteries. That would be a nice feature.

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A freshly recharged 5-cell pack will give up to 7.5V. The PIC's absolute max is 6.5V, so either use a regulator or zener limiter otherwise the PIC will burn. I personally use 4-cell packs so nothing is needed. But to make my things compatible I put a 5.6V Zener / 100 Ohms resistor to limit the voltage if needed.

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