transistor-tester

Documentation of my modifications to the AVR Transistortester.

A transistor tester is a device where you can insert different kinds of electronic components and have them analyzed. It not only reads transistors and tells you whether it's NPN or PNP, but it tells you the resistance of resistors or capacitance of capacitors, and more. There are many different types, and they are usually very cheap.

I have the GM328 kit from Banggood. It's usually called AY-AT instead, as it's not really a GM328.

Notable hardware:

• ATmega328P with 8MHz crystal
• ST7735 160x128px screen
• Rotary encoder
• 5.5mm x 2.1mm center positive barrel jack for 9V
• Terminals for frequency generator, frequency counter and voltage reader
• ZIF socket for analyzing components
• Holtek HT7550-1 voltage regulator
• WS TL431AA voltage reference

Modifications

To improve the accuracy of the tester there are some simple modifications you can do.

(Image from https://github.com/Upcycle-Electronics/AVR-Transistor-Tester)

See also this post on eevblog.

From the readme of the k-firmware:

"The resistors R1 to R6 are critical for measurements and this 680Ω and 470kΩ resistors should be measurement type resistors (tolerance of 0.1%) to get the full accuracy."

"The additional 2.5V precision voltage reference connected at pin PC4 (ADC4) can be used to check and calibrate the VCC voltage, but is not required. You can use a LM4040-AIZ2.5 (0.1%), a LT1004CZ-2.5 (0.8%) or a LM336-Z2.5 (0.8%) as voltage reference. If you don’t install the precision voltage reference and you don’t add the relay extension, you should install a pull up resistor R16 to PC4 with a higher resistance value (47kΩ). This helps the software to detect the missing voltage reference."

From the readme of the m-firmware:

"The external 2.5V voltage reference should be only enabled if it's at least 10 times more precise than the voltage regulator. Otherwise it would make the results worse. If you're using a MCP1702 with a typical tolerance of 0.4% as voltage regulator you really don't need a 2.5V voltage reference."

The part about R16 in the readme of the k-firmware is confusing since it's not mentioned in the readme of the m-firmware. This post confirms that it's only necessary for the k-firmware:

"For running the m-firmware you can skip step #3 or remove R16 (no need to replace it). When HW_REF25 is disabled (default setting) the firmware will ignore any external voltage reference. The k-firmware handles external references a little different and always checks for a 2.5V reference (no setting to disable that). Therefore the k-firmware needs the resistor mod (step #3) to make it believe that there isn't any reference."

I chose to order 0.1% resistors and just the MCP1702 voltage regulator, skipping the voltage reference. Also added a 16MHz crystal for increased speed:

• 3x TE Connectivity H8470KBZA - Metal Film Resistor 470K 0.1%
• 3x TE Connectivity H8680RBYA - Metal Film Resistor 680R 0.1%
• 1x Microchip MCP1702-5002E/TO - LDO Voltage Regulator 5V 250mA 0.1%
• 1x IQD LFXTAL003240 - 16MHz Quartz

The concrete changes, referenced to the schematics:

• Y1: 16MHz crystal instead of 8Mhz
• R1, R3, R5: 680R 0.1% tolerance instead of 1%
• R2, R4, R6: 470k 0.1% tolerance instead of 1%
• IC2: MCP1702 high precision voltage regulator instead of HT7550 low precision voltage regulator. Pin compatible.
• IC3: TL431A voltage reference is not necessary because of MCP1702.
• R16: Due to missing voltage reference, this 2k2 resistor is not necessary with the m-firmware.

This is how it looks with these changes in place:

The 0.1% resistors (black) are a bit thicker than the default (blue), so they overlap a little bit.

Firmware

There are 2 different firmware choices for the transistor tester. The original is the k-firmware. The tester comes with version 1.12k (quite old), and the newest is 1.13k. Development of the k-firmware is currently on hold. It's been forked into the m-firmware, which is rewritten and with additional features, and still under active development.

The k-firmware source is available here, with precompiled firmware for the AY-AT in the mega328_color_kit directory. There is also a Makefile there with the correct parameters. The source of the m-firmware is available here, but only as tarballs. There are no precompiled versions. Both firmwares can also be found here.

I chose to use the m-firmware, due to it still being actively developed. The newest version at the time of writing is 1.39m.

Configuration

There are 3 configuration files in the firmware that needs to be adjusted. See the file "Clones" (from the firmware tgz) for the basic changes required to build a compatible firmware for the AY-AT. The additional changes I made are documented here. The files are also available in the firmware directory.

config.h

• Disabled HW_REF25 because I don't use the TL431A voltage reference or the recommended LM4040 replacement.
• Disabled SW_IR_RECEIVER because the firmware was too large (105%) and wouldn't fit on the ATmega328P. The support for IR is not important to me so it was ok to disable.
• Enabled UI_AUTOHOLD because continuous test mode was annoying. I'd rather take the time I need to read the result.
• Enabled POWER_OFF_TIMEOUT so it turns off when idle.
• Enabled SW_POWER_OFF so I can turn it off from the menu.

config_328.h

• Enabled LCD_LATE_ON because the screen is very garbled when initialized and this setting makes it look fine.

Makefile

• Changed FREQ to 16 because of my change from 8 to 16MHz crystal.

Building

In addition to the build tools that was already installed, I had to add the following packages (in openSUSE):

• avr-libc
• cross-avr-gcc9

Then make to build.

When build is finished you should have these files, which make up the firmware:

• ComponentTester.eep
• ComponentTester.hex

Flash firmware

The AY-AT does not have an ISP programming header. It's possible to solder pins on the back of the circuit board like this, but I used a TL866II Plus universal programmer together with the minipro open source software for Linux. See https://github.com/blurpy/minipro for more about how to use.

To flash the firmware we need 3 files. The 2 firmware-files from above, and ComponentTester.cfg. The last file contains the configuration of the fuses of the ATmega328P. The fuse configuration is extracted from the Makefile. To see what the fuses mean you can use this online calculator.

Commands:

• Erase chip: minipro -p "ATMEGA328P@DIP28" -E
• Write eeprom: minipro -p "ATMEGA328P@DIP28" -c data -w ComponentTester.eep -e
• Write flash: minipro -p "ATMEGA328P@DIP28" -c code -w ComponentTester.hex -e
• Write fuses: minipro -p "ATMEGA328P@DIP28" -c config -w ComponentTester.cfg -e

That should be it. This is the finished result:

Troubleshooting

I initially didn't pay any attention to the output after the compile step:

AVR Memory Usage
----------------
Device: atmega328

Program:   34784 bytes (106.2% Full)

It worked fine to flash the firmware, however when trying to start the tester it would quickly flash white on the screen and then turn off. I thought it was a hardware issue at first, but when testing the original firmware it worked. After disabling enough features in this firmware to get below 100% memory it worked fine as well.

Calibration

It's recommended to follow the steps described in the readme to run a self test and adjustment. When asked to short the probes I used short breadboard wire between test point 1 and test point 2, and between test point 2 and test point 3 in the ZIF socket. Remember to save from the menu afterwards.