Introduction: Component Tester in a Keychain

About: A New York City based creative technologist who loves designing pretty PCBs and playing around with LEDs ❤️. I am currently pursuing my master's degree in design and technology from Parsons School of Design.

Being an electronics engineer, I always wanted to have a portable component tester, which could test every electronic component out there. In 2016, I built myself a Component Tester based on AVR TransistorTester by Markus F. and Karl-Heinz Kübbeler. This summer, I thought, can it be made pocket-sized? Since my last version was quite big and difficult to carry around.

I started redesigning the PCB with SMD components and an OLED display since it is small, lightweight, and consumes less power. I wanted to retain banana jacks since they offer testing device a robust look and makes it more compatible. Say, I can use SMD tweezer probes for testing SMD components or say I can use alligator clips or anything else. I am no longer limited to plug in my part to the tester to test it.

After spending a few hours fiddling around the PCB layout, I managed to bring it down to as small as 58mm x 32mm (2.28in x 1.26in). Pretty tiny, right? To make it a fit as a keychain, I added a punch hole on the bottom right corner of the PCB so you can put in a keychain ring or a fancy lanyard. The tester boots up as soon as you power it up via USB C.

Once you connect the component, press the button next to the display to begin testing. Since it has only one button, it makes it easier to use and makes it look less complicated as an electronic tester, which may appear very complex to use at first to many users. Also, to navigate to the menu the user just needs to double press the button once a component has been tested or a "No component screen is shown".

I used 0805 package components for the tester since they are the smallest components one can solder by hand and can be eyes directly without a magnifying glass. I used solder paste along with a hot air reflow tool to solder in the components.


PCBWay offered to step in and helped me to support this project. They also offered their PCB services for the build. I got PCBs in premium matte-finished soldermask which just upgrades the overall build quality of the tester. They offer 10 custom PCBs for as low as $5 with a wide array of choices such as soldermask colors, surface finishes, and much more. The turnout time for PCBs was amazingly fast. I got my PCBs delivered in 3 days from the day I placed an order. They also examine each PCB design manually before manufacturing so you do not receive any defective PCBs. I recommend trying their PCB service if you need one.


Bill of Material on GitHub (click the link to get a detailed BOM for the project)

Tools and Supplies

  • Hot Air Rework Station
  • Magnifying Glass / Electronic Microscope
  • Soldering Iron
  • Solder Paste and Solder
  • Tweezers
  • USBasp

Step 1: Getting Your PCBs Fabricated

The first step in building the keychain component tester is getting the PCBs fabricated for it.

You can find the Gerber Files for PCB manufacturing under the files section or on the project's GitHub repo. I used PCBWay's Prototyping Service for getting my boards fabricated. They offer great quality boards for a very low and affordable price. They also offer a variety of solder mask options to try. I got my boards manufactured with Matte Black solder mask which makes the PCBs look very premium.

Alternatively, you can click the link to order PCBs directly from PCBWAY. You can also add an SMD Stencil from them for just $10.

Step 2: Let the Soldering Begin...!!

The list of components in the supplies section above provides detailed information on components you need to order to get started with the assembly process. The components have a manufacturer's reference number so you can hunt the parts hassle-free.

I usually start by soldering resistors first followed by capacitors and then moving onto bigger components like microcontroller, buttons, OLED Display, etc. It makes life a bit easier.


Step 3: Let's Go Bananas

To add the banana sockets, lightly tin the base of PCB where banana socket goes into and slide in the socket as shown. Once you slide-in put a bit of solder between the little space between hex nut and the base. This will ensure a strong connection and give you better and more accurate testing results.

Step 4: Uploading the Firmware

Hardware Setup

Using a USBasp programmer connect it to the ICSP header on the board. Refer the pinout for connections (Bottom Side of PCB).

Software Installation

⚠️ REQUIRED: You must have AVRDUDE installed in your system in order for following instructions to work.

  1. Download firmware files from GitHub.
  2. In Terminal/Command Prompt, navigate to folder's location with the above files and execute the following commands-

// Flashing .hex and .eep files to MCU

avrdude -c usbasp -B 20  -p m328p -P usb -U flash:w:./TransistorTester.hex:a -U eeprom:w:./TransistorTester.eep:a
// Setting fuses for MCU

avrdude -c usbasp -B 200  -p m328p -P usb  -U lfuse:w:0xe2:m -U hfuse:w:0xd9:m -U efuse:w:0xfc:m

Step 5: Calibration

In case your tester says "Not Calibrated"

You will need two capacitors one with values >100nF and another one with 4-35nF.

  1. To begin calibration, short the three probes together.
  2. When it says "Selftest?" , Long Press the button for 2 seconds.
  3. Follow the instruction on screen. When prompted for capacitor >100nF. Plug it between pins 1 and 3. Following it will prompt for a capacitor with a value between 4nF and 35nF. Plug it in.
  4. The Tester will prompt as "Test Completed".

You have successfully calibrated the tester 🙌

Step 6: HALLELUJAH! You Made One :D

TAA DAA!! You just made yourself a keychain component tester 🤩

Power the tester via USB-C🔋and Plug in 🔌 the components you wanna test.

Cheers 🥂

You can also buy a fully assembled component tester from my Tindie Shop

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