BlueFlyVario_TTL version 8 released

I have spent many months working on the BlueFlyVario_TTL. This post announces a limited release. Anyone can purchase one, but it is ‘limited’ because you will need some electronics skill to make it work the way you want. I will release another version of this in the next few months with extra components, but I am still not sure what is the best configuration.

I made a deliberate decision to produce a simpler version of the BlueFlyVario, rather than pursue the standard approach taken by many developers to make something more complex. This is a lot harder than it sounds.

I initially decided to pursue something like the TTL in an effort to produce an audio only vario module with the same awesome audio which the standard BlueFlyVario provides. Wide interest in modifying the Kobo eReader pushed me towards exposing a TTL serial interface, but with only one port available on the simple eReaders I had to multiplex GPS information using the second serial port on the micro controller. Development of this lead to the change in the BlueFlyVario version 8 which I released in the previous blog post.

Description

Compared to the standard BlueFlyVario with Bluetooth, the baseline BlueFlyVario_TTL is simply just a populated PCB which is programmed with the BlueFlyVario firmware. There is no Bluetooth module, battery or case. The key specifications are summarized in the image below:

• 1: Second serial interface. A serial port of the PIC microcontroller (UART1) is exposed with V+, Rx, Tx and GND (3.3v). By default it is configured at 9600 Baud to accept NMEA sentences from widely available GPS modules. Sentences starting with $ and ending with /n (carriage return character) are stored then immediately multiplexed with the information being sent out the primary TTL Interface. XCSoar is smart enough to interpret the multiplexed stream via a single device.
• 2: Pressure sensor. The MS5611 pressure sensor is a super accurate little temperature compensated module with a digital interface. The surrounding components filter the power supply. When shipped this component is covered with neoprene to avoid light sensitivity (but do not press on it when active to avoid touch sensitivity). 
• 3: Microcontroller. The PIC24F16KA301 is running at 8MHz. This is fast enough to provide advanced Kalman filtering of the pressure measurements (which is reads from the pressure sensor at 50 times a second). At this speed the micro controller can also handle reading and sending data over the two serial ports. 
• 4: Speaker interface. Super keen experimental users might want to integrate the audio signal into a headset. 
• 5: Solder jumper. If you want to disable the integrated transducer clear this solder jumper.
• 6: Status LED. This lights up with every beep. 
• 7: TTL Interface. The primary interface (which is actually UART 2 on the microcontroller) is used to provide power to the device and send and receive data via Tx and Rx (at 57600 kps). See the hardware settings manual for detailed information about the interface.
• 8: Programming Pads. I program the firmware using a PICKIT 3 and this pads. Short pads 2 and 4 when turning on to reset the hardware settings (see the Hardware Settings Manual). 
• 9: Voltage Regulator.  A Microchip TC1015 LDO voltage regulator outputs 3.0V to run the micro controller and pressure sensor. It works with input power between 3.3V and 6V. Below about 3.2 V it fails to provide a stable voltage to the pressure sensor. Standard 3.3V in from other low voltage TTL devices (such as the Kobo) work well. 
• 10: Audio. An integrated small electromagnetic transducer provides the kind of pleasant sounding vario that is normally reserved for bigger devices. 

Use

To make the TTL work you will need to do the following:

• Provide power. I have deliberately not included a battery and switch. There are so many options and everyone has a slightly different need. You might choose to power it with a 1S LiPo, 3 x AAA alkaline batteries, power from USB, or power it from a Kobo. Whatever you choose you need to do ensure you meet the following specs.
• Voltage: The TTL needs between 3.3V and 6V at the V+ pin on the main TTL interface end (do not provide power to the other end).
• Current: The device consumes 10mA when not making noise, and around 50-60mA when sounding. Assuming a duty cycle of silence:noise of about 4:1 the average current consumption is about 20mA.
• Provide a programming interface. A USB to Serial connector provides the ideal programming interface. This is used to configure the hardware settings if you want to change the defaults. You can get cheap USB to TTL serial converters on ebay, at hobby king or electronics shops like Sparkfun. I am considering including one in future releases of the TTL. 
• Provide a data interface for external applications. If you want the vario data to be sent to an external application you can use the aforementioned USB to Serial convertor (for devices that support such a converter). Other devices (such as the Kobo) can be wired directly to the Tx and Rx pins. 

I envisage that the TTL can be used in many ways once it has been configured via the programming interface. Some examples:

• Audio only mode. In this mode you only need to provide power and it will work. Something like a 3 x AAA case with integrated switch should meet the needs of most people. I am considering including one of these in future releases with the baseline configuration.
• Integrated Helmet vario. The board is small enough to be integrated into a helmet. In this configuration a small RC 1s LiPo of around 600mA provides heaps of juice. A switch in series allows it to be turned on and off in flight. 
• Integration via USB to a Linux device. Some users in sailplanes might want to integrate the vario directly into a Linux device via a USB to serial interface. I understand a few people are going to try to encase the vario and connect it to the TE probe. I am keen to get feedback. 
• Integration with a Kobo. The picture below shows how I have integrated it with the Kobo on my workbench. In this configuration the Kobo is providing power to both the BlueFlyVario_TTL and the attached GPS module. This type of GPS module, based on the NEO-6M chip, is widely available and the default settings should work for most users. 

Schematics

The images below show the schematic and PCB layout for the BlueFlyVario_TTL version 8 prototype.