This post is still a work in progress

Our lab coursework in college requires everyone to build a simple project on any microcontroller and which uses interfacing of some I/O devices like a relay, Tx-Rx, temperature sensor etc.

I started thinking about different project ideas to implement and eventually landed on the Persistence of Vision Display using an ATTINY85. Though a simple display is already implemented here , I wanted to take it further – to drive more than 5 LEDs and also if possible to take in an analog or digital sensor input. There are multiple methods to get more I/O pins – one could use an I/O expander (not possible in this case as size and power is a big constraint), multiplexing or Charlieplexing.I couldn’t entertain the idea of using another IC for multiplexing due to the aforementioned constraints. So I decided Charlieplexing was the way out.

Design Constraints

Using the following Dot Matrix ASCII table I found here I figured out that I needed at least 7 rows of LEDs to display most characters.

  1. Minimum no. of LEDs required = 7


2. No. of I/O pins on the microcontroller


With only a total of 8 pins out of which – 1 VCC, 1 GND and 1 Reset pin set back the useful total to 5. 5 I/O lines. That’s it! Going by the normal method of one LED per line, this could only drive a total of 5 LEDs. So I started searching if I could use the other RESET pin as an I/O port and it turns out – its possible, but then there is a catch – Setting the RESET pin as an I/O pin involves reprogramming the internal fuses and one extra I/O line comes at a hefty price – No more ISP programming with an Arduino is possible. Then altering the microcontroller code will require a high voltage programmer. Though my curiosity led me to a workaround, making a whole new device for a project beats the whole purpose of it.Hence I concluded that I’m stuck with 5 I/O pins.

No. of available I/O pins – 5

3. Charlieplexing

Using the equation No. of pins = [(1+sqrt(1+4L))/2] where L is the no. LEDs. In this case with L=7, we get no. of pins required as 3.192. We cannot do any rounding here and come to the conclusion that only 3 pins are required. 3.192 => we need at least 3.192 pins that is we need more than 3 pins => 4 pins for 7 LEDs

4. Calculating your current requirements

LED = 7*5mA = 35mA

ATTINY85 <= 2mA

The total comes to 37mA.

Most button cells (CR2032) have about 220mAh. So 220/37 = 5.945 hrs of battery life which is not that bad. But unfortunately, a bit of digging up led me to this page and according to the author at such high consumption (relatively) the battery voltage falls of rapidly and if it goes below 2.7V the m

so probably use series connection of button cells?


Components Required:

  1. ATTINY85
  2. LEDs 5mm ( preferably red due to their low current consumption)
  3. Button cell CR2032
  4. Motor and it’s power source
  5. Arduino Uno – just to program the IC

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