How to flash an Arduino over the Air with Bluetooth

As mentioned in the previous article i wanted to figure out how to connect the Arduino over Bluetooth to my Mac, including flashing the device.
I managed to get the serial console over the air with the HM-13 module as described before, but unfortunately flashing seemed to be a bit of an issue.
This is because the Arduino needs a clean reset signal which usually comes out of the DTR-Line of the module. But: most BT-Modules (and FTDI-Cables etc.) do not make this port available.
But i found three solutions that work nevertheless.
1. easiest and most straightforward, but also most expensive: buy the EZ-Link module from Adafruit. It has the DTR pin (don’t forget to use a 100nF ceramic capacitor “104” to connect to the reset pin) and everything works well just out of the box. This module also has an automatic baud-rate detection and some other useful features like voltage regulation both for power and the TX/RX Pins. The other modules below require 3.3 voltage for power and also for the logic, so be careful if you plan to work with a 5v controller like the UNO.
2. the RN-42 BT Module comes with a DTR-Pin as well, but unfortunately it has a reverse signal on that pin – when it should be high it is actually low and vice-versa. If you want to use it nevertheless you have ot apply an invert circuit or buy an inverter module. You can find a nice explanation on this and the module in general in this video. Although i believe from the video that this setup can work, it did not for me – the DTR-PIN just did not generate any signal…
3. You can also use the above mentioned HM-13 module from Seeed – although it does not have the DTR PIn available. I used a nice little hack described here to make it work. Just reuse the status-led pin to trigger the reset signal! To do this you have to use a short AT-Command on the module “AT+PIO11” which will tell the module to shut off the LED when unconnected and turn it on while connected. This again has to be inverted to be useful as a reset-signal, but if you do so it really works (other modules like the HC-05 for instance even make this available via another AT-command, so no inversion is needed here). The good thing with this solution: the HM-13 is a dual-mode BT-Module which can work as classic BT to simulate a serial connection _and_ as BLE Module at the same time!