Dirty Power On

I think my high altitude balloon is consuming a lot of (my) resources. But well… another hack related to the same project.

My primary communication channel is over UHF, modulating in FSK. But I cant rely in just one communication channel. So I think in a cellphone. A typical implementation is to send a SMS over the phone with the coordinates. But that means to deal with some obscure and undocumented commands and interfaces. Not to mention: to have the right cellphone.

So I have another idea: why not to call the cellphone and receive the FSK modulation over the line?. For that I needed to things:

  • A way to power up the phone when I energize it
  • A way to answer the call

Yes, I could use a μC, but I think in a simpler way. To power up the phone I used a capacitor in parallel with the Power key. So, when I energize the phone * the capacitor will be a short until it charges. Playing with the capacitor value I can change the “button pressing time”. And a resistor in parallel will discharge the capacitor for a next power up.

And to answer the call automatically I use a NPN transistor to short the Answer button, and the base that transistor connected to the vibrator output,  so when I call, I close the circuit and answer automatically.


*You may be wonder why instead of energized the phone not to press the power button. Well, during the fly I’ll power up the phone several times (mostly to save energy, because in at some highs there is no signal and when that happens the phone increase the tx power, consuming a lot on energy). So, with this configuration when I energize the phone I’m sure it is power on, and when I de-energize it I’m sure it’s off and not consuming energy. In short: I can control the phone without any feedback, and I can be pretty sure it is in the state I want it.

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