Multimeter inbox 1: The idea

I would like to start by thanking everyone involved in making this design challenge happen. Things like this are great for the electronics community and are one of the reasons I got insterested in electronics. Thank you.

Problem: I don't check my inbox on Element 14 for new messages at regular intervals. This leads to messages either sitting around for a few days or me checking in to find only an empty inbox.

Solution: Repurpose an old analogue rapiest multimeter to display the number of messages in my inbox via the voltage indicator.

/\ fig 1 This is the meter I want to use, along with some objects for scale

/\ fig 2 And here we have a photo of the meters circuitry

Hardware Design: The idea is to drive the display using a motor driver board and PWM. Conveniently the meter has a range of about 12 volts so a standard DC wall wart should be fine for powering this This does mean that the device can't display more than 12 messages, but hopefully I will check in regularly enough for this not to be an issue. Any values greater than 12 can easily just be have to be displayed as 12, or alternatively by a needle alternating between 0 and 12. The wall wart could also power the main board of the unit via a voltage regulator. For the main board I intend to use an Arduino Yun. The meter will conveniently operate on DC voltage mode with no power source so powering it will not be an issue. It also features 2 convenient side mounted switches that can easily be used to turn the unit on and off. It may also be worth adding an LED backlight to the unit that can be driven by the Yun and turned off with the other switch.

/\ fig 3 A quick visual overview of the hardware. Note that for simplicity, some passive components and the switches and the circuitry from the multimeter have been ommited.

Software design: The first stage of software will run on the Linux half of the Yun. Using Python it will regularly load and parse my inbox to find the message count. this will then be passed on to the ATmega. The mega will compare the value it receives to a table of PWM values and then use this to power the motor driver and move the needle to the correct position, or possibly between positions in the case of more than 12 posts, but this will be handled as a special case. The Yun can also drive the backlight with PWM, potentially allowing for either a heartbeat effect or a brightness increasing with message count.

Success Criteria: I will deem the project a success if:

•It can accurately and reliably display the number of messages in my inbox up to 12

•The system can cope with more than 12 messages in a way that does not result in catastrophic failure

•The display can be read in darkness with an LED backlight

•The unit is powered by a single DC wall wart

Challenges in this project:

• Writing the code for the parser

   -Whilst I do have some experience of python, I have only coded something like this once in the past

•Calibrating the pwm

   -I will probably needs a separate program to test pwm values via a serial terminal. I will probably use an arduino Leonardo for this, simply because I feel more confident     using it

•Comunicating between Linux and the Atmega

   -Having never used a Yun, this appears to be one of the challenges specific to the platform

Edit: Apologies for the delay in posting this. I have recently changed my PC and have been having some issues posting. These issues have not yet been resolved, but I am now able to post through another device so updates should now be a little more frequent.