A whole range of devices is available under the name “M-Tester” which can automatically detect electronic components such as transistors, diodes, resistors, capacitors, etc. and test their properties. Hence the name: “M-Tester” is the abbreviation for “Multi-function Tester”.

All products of this type go back to the article “AVR Transistortester” at mikrocontroller.net, which first described a transistor tester based on an ATmega8/168/328 and an LCD with 2×16 characters in 2012. The latest version of these devices, such as the “M-Tester T7” or “M-Tester TC1”, offers a color display with a graphic display of the components and an infrared sensor with which the function of remote controls can also be checked.


If you want to get such a device for yourself, I can recommend the model “Joy-IT LCR-T7” based on my own experience. This device is based on an ATmega 324PA and uses a lithium battery, so that you don’t have to deal with 9V block batteries. Charging is done via micro USB.

The components to be tested are inserted into the ZIF socket, each of which offers three connections in multiple repetition. In addition, there are connections “K” and “A” for measuring Zener diodes. All other components can be connected to the inputs as you like, but you just have to make sure that you use different contacts, e.g. 1 and 3 for a resistor or 1, 2 and 3 for a transistor.

Cables with clamps are also included in the scope of delivery if the components to be tested cannot be plugged directly into the socket. I replaced the plug with a triple combination and connected the individual wires with shrink tubing to make handling easier.

The display has a diagonal of 1.8″ with a resolution of 160×128 pixels.


The following components can be tested:

  • Transistors (bipolar and FETs)
  • Thyristors and Triacs (IGT < 6 mA)
  • Diodes and zener diodes (0,01 – 4,5 V, break down voltage up to 30V)
  • Resistors (0,01 Ω – 50 MΩ)
  • Capacitors (25 pF – 100 mF)
  • Inductivity of coils (0,01 mH – 20 H)
  • Voltage of primary cells and rechargable batteries (0,1 – 4,5 V)
  • Infrared remote controls

To take a measurement, press the power button once. The current battery voltage is then first briefly displayed and then the component used or connected via cable is tested. The component is then shown visually on the display with the exact pin assignment and the measured values.

Of course, the accuracy of the measurements is not comparable to expensive measuring devices for professional use, but the device is still very helpful for easily identifying components in terms of their properties. In my direct comparison with a BM257s multimeter, the deviations in the determined values of resistors or capacitors were sometimes less than 1%.

To test infrared remote controls, leave the ZIF socket empty and point the remote controls at the “IR” sensor after powering on. As soon as you press a key, a corresponding message with the recognized codes should appear on the tester’s display:


For a self-test, connect inputs 1, 2 and 3 (a suitable jumper is usually supplied) and switch on the device. The self-test then prompts you to disconnect as it progresses and then continues as soon as you disconnect again. At the end u.a. the firmware version is also displayed, which is 2.12k on my copy. An update of the firmware is not officially planned.

Rechargable battery

The built-in battery with a capacity of 350 mAh is charged via a micro USB socket. A two-color LED indicates the charging status with red and green. As soon as the LED is green, the battery is sufficiently charged.

Although it is permanently installed, the device can be easily opened with four screws. In my copy, the battery is fixed with double-sided adhesive tape and connected internally with a plug connection and can be easily replaced if necessary. If necessary, the tester also works with an external power supply via USB.

Update 2023-01-15: Modifications

Switch for the battery

For a long time I noticed an effect that made me think that the battery was defective: even if the tester is not used, the battery is largely discharged within only 2-3 weeks. The reason for this behavior is that the tester actually does not switch off completely, but only the display. As a result, a small current flows even when the display is switched off.

The solution is simple and very effective: a switch that allows the device to be completely switched off. This should also allow the device to be stored for a few months without having to immediately recharge the battery.

The switch breaks the connection between the battery and the circuit board. I insulated the soldering points with shrink tubing to avoid short circuits. Since there must be a connection for charging, I have also attached a corresponding label.

ISP connecor for modified firmware

On https://github.com/madires/Transistortester-Warehouse and https://github.com/kubi48/TransistorTester-source you find sources of the firmware and an overview about the different models.

This model allows to add an ISP connector. For the connection I used a STK500 compatible USB programmer, the “mySmartUSB Light” which recognized the hardware without any issue. Important: before connecting with a programmer you should disconnect the battery!

But before I change anything in the firmware, I wanted to backup the original firmware first – but that wasn’t possible. Both the myAVR tool and avrdude only reported an empty flash memory. This means that the lock bits were probably set accordingly. Since I haven’t found a source for the original firmware yet to reprogramm the controller completely, I’ll leave it as it is for now.

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