Things used in this project

Hardware components:
Atmel ATTiny85
Zener 3.6V 500mW
Resistor 68 Ohm
USB Male A connector
Resistor 152 - 182
LED, button, etc.

About this project

New: the 328 Pro Trinket Clone!

Chachka is born!

Chachka is an Adafruit Trinket CLONE built from the ATtiny85 microcontroller.  Because it is a current Adafruit product, it has a training guide and its own support forum and lots of sample code.  All of this means that like an Uno clone, the Chachka will be well supported with firmware and libraries and other users.

The Trinket in itself, is not expensive.  But hacking is not about expense, it is about exercising total control over the hack. It is about dominance. Your soldering iron is not just for soldering but it is a lance to strike for freedom for open source hardware: the ultimate hacker treasure. Building your own clone from scratch is a great feeling.

The hardware schematics are here:

The firmware is here:

This is an advanced project and you will need soldering skills as well as software skills and a level of competency necessary to install bootloaders on the ATtiny series of Atmel microcontrollers.  You will need an AVR-ISP programmer or the knowledge of how to configure and use your UNO in the ArduinoISP mode.

Building your Chachka

You will need a piece of scrap perfboard, the width is 0.50 inches x 2.0 inches (approx.)  Approximately 1/3 inch of the perfboard will be under the USB Male "A" connector.  With a hobby knife or rotary tool, the board needs to be shaped so that the metal shell "ears" can be  crimped around the board.  The 4 USB leads from the shell should be positioned on top of the perfboard so that you can wire the zeners and resistors in this area.  Keep the connections neat and short - we will coat the entire area later with quick-set epoxy.  For now, make certain your soldering is done cleanly and is done rapidly to avoid damage to the Zener diodes.

Note: Adafruit uses a 1500 Ohm pull-up resistor.  I specified an 1800 Ohm value.  I have seen values in other articles as high as 2200 Ohm and lower than 1500 Ohm.  Feel free here to deviate and use the exact Adafruit value (or as close as common values will allow) or use the value I specified based on a couple of years of V-USB and tiny85 bench experience.

Note: As the completed Chachka will be inserted and removed numerous times from the PC, you want to take extra care with mechanical construction.  The epoxy coating will generally be adequate to avoid flexing to the degree that solder joints will break away, but good initial mechanical construction will certainly help prolong the life of the Chachka.

Note: If this is your first V-USB project, you may definitely wish to use an 8-pin DIP socket for your tiny85 microcontroller. I have found that the socket, while adding cost, can prevent massive hair loss.  Your option.

Sample files

See text for needed library from Nick Gammon
10K NTC + 10K resistor required
See text for needed library from Nick Gammon


For those experts among you, there has likely been enough information to complete your Chachka. If you need a bit more closure, however, I have a more technical write-up that includes the boards.txt file, fuse settings, and other tidbits up on the Official Arduino Forum:

I will also attach some (known) working files but you will find a wealth of stuff on the Adafruit Trinket official forum:   You will also find a lots of frustrated users. The V-USB technology that is used in the Trinket firmware and the way the Trinket interacts with AVRDUDE on the PC seem to be in constant discussion.  This is another good reason to have your tiny85 in a socket!  If you cannot get the Adafruit download process to work for you, you can always take the chip off the board and use an AVR-ISP programmer to download the HEX file.

When you know everything is working correctly, you can use a nice layer of 5-minute quick set epoxy to reinforce everywhere being careful not to cement the tiny85 in the socket if you elected that method of construction!

In conclusion...

Just a recommendation from me because I believe in hacking but I also believe that Open Source companies need to recover their investments in advancing the art and providing live support and jobs for their employees...

PLEASE support Adafruit Industries and purchase at least ONE (1) Trinket if you plan on doing breadboarding.  The $8 will simply be good business, support Adafruit, and provide YOU with a firm foundation to work through issues you may have; that is, you will be able to use the Adafruit support forum and feel good about yourself. (Or fuss like I have done in the past!)

The attiny85 is a very capable microcontroller.  Adafruit has built a decent set of libraries and support infrastructure for this device which should make the transition easier.  Note however, Digispark is also a tiny85 based device and you may wish to consult my article on how to Hack a Chachka to clone a Digispark! 

Happy hacking fun and keep your soldering irons hot!



Ray Burnette

IT architect - retired AT&T... USAF-Secure Comm , Burroughs, Clemson U. School of EE Research, Southern Bell, BellSouth, SofKinetics, Inc. (President), AT&T IT


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