@▷ Collector-Base-Emitter Pin Identifier of Transistors | Diagram for Schematic

Collector-Base-Emitter Pin Identifier of Transistors

Part List
Circuit Project Description

The circuit has been designed for transistors to determine whether the pin is emitter, base or collector as well as the type if NPN or PNP polarity, and can also be used in testing diodes.

Terminology 4011 – a quad 2-input NAND gate integrated circuit, generally characterized by small fluctuation in voltage supply, very high impedance, outputs that can sink and source, one output can drive up to 50 inputs, high speed gate propagation time, high frequency, and low power consumption4066 – a digitally controlled quad  analog switch utilizing advanced silicon-gate CMOS technology with features such as individual switch controls, matched switch characteristics, pin and function compatibility, low quiescent current, low ON resistance, wide analog input voltage range, and 15 ns typical switch enable time Circuit Explanation

A ring-of-three oscillator is formed by IC1A, IC1B, & IC1C to produce a 3-phase waveform with 350 Hz frequency which is applied to the device under test through the LEDs. For one third of a cycle, each pair of device terminals is enabled to be forward, reverse, and unbiased. The corresponding red LED will be turned ON by the current flowing into the device while the green LED will be turned ON by the current flowing out. This will assume the polarity of a transistor and the position of the Base lead.

Pressing the SPST push button P1 will switch R7-9 at once in parallel to R4-6 using IC2A, B & C which will allow the Base lead of the device to be driven by a higher current. This operation can also be used for testing low-gain power transistors. The low power plastic encapsulated devices are used mostly for pin identification and the pin layout of TO3 metal encased power device can be assumed easily.

During the procedure of testing, the health of the transistor is check by pressing P1. The two terminals will cause both illuminated LEDs while the remaining will show a single LED illuminated. With reference to the Base connector, a red illumination illustrates a NPN transistor while a green illumination illustrates a PNP transistor. There are cases where the pin identification is not reliable due to weak illumination especially in testing low gain high power transistors. This can be remedied by pressing both P1 and P2.

In testing diodes, it can be placed to two of the three sockets and only one red LED and one green LED will illuminate upon pressing P1. The red LED signal is illuminating with the identification of an Anode pin while the green LED will determine the diode pin to be the Cathode.

Some problems may arise especially when testing Darlington type transistors because the Collector and Emitter pins will be displayed as inverted but the Base pin and polarity will be shown correctly by the pin identifier as with common transistors. This is due to the incorporation of a reverse-connected diode across Emitter and Collector on the same chip by the Darlington power transistor. Testing Darlington transistors will result to bright illumination of two LEDs related to Emitter and Collector pins.


The transistor pin identifier easily determines the polarity and function of a transistor as well as good or bad health. It can also be used to test diodes, FETs, small thyristors, SCRs, and triacs connected between C and E sockets. The circuit was designed for a minimum amount of control manipulation and quick testing of most electronic devices.

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