The purpose of this circuit is to test NPN and PNP transistors and to identify their pin layouts, ie ECB, EBC. I find myself testing a lot of transistors to determine their pin layout and type and as such find that building the test circuit on a breadboard has become a hassle so I wanted an easy circuit that would be more permanent and would allow me to test transistors.
The transistor type NPN/PNP determines the polarity of the connections. The orientation of the transistor is the pin layout. I define the pin layout when looking at the top of the transistor with the flat side facing toward you. That is probably not the official way to classify them but it makes more sense to me when I am placing them in a circuit.
- EBC stands for Emitter Base Collector. When I use this acronym I am saying that the pin to the left is the emitter, the middle pin is the base, and the right pin is the collector.
- ECB similar to the above acronym stands for Emitter Collector Base. The emitter pin is again on the left but the middle pin is the collector and the right pin is the base.
I have gone through about 100 transistors that were pulled from old electronics and have never found any other orientations so i have found that there is no need to test for anything but these two orientations.
Step 1: Parts
You will need:
- 1x Radio Shack Printed Prototype Board 276-150
- 2x 8 pin IC Socket
- 4x tactile switch
- 4x 10k ohm resistor
- 4x 470 ohm resistor
- 4x 3mm LEDs
- Solid core wire
- CR2032 battery and holder
- NPN and PNP transistors to test
Step 2: Place Battery
Step 3: Place the 8 Pin IC Holders
Solder the IC sockets in place. While soldering the sockets in place bridge the gap between the following pins and the nearest voltage rail. Starting from the left, pins 1 and pins 6 top and bottom. See the pictures for a better explanation.
To bridge the gap solder the IC socket in and then place a bit of solder on the voltage rail closest to the pin. Now touch both the IC socket pin and the voltage rail with the soldering iron tip and apply more solder until the connection is made. Go slow so that you do not bridge pins that you would not want to connect. Remember you are only connecting 4 pins!
NOTE: I would show a picture of the pins bridged but I did not bridge them until I was building each test circuit. Take a look at the final solder layout in step 6 of this Instructable if you are still having trouble.
Step 4: Place the Resistors
Place the 10K ohm resistor in a horizontal orientation so that it connects to pin 2 on the IC socket and extends to the last pin on the left of the board. Place the 470 ohm resistor on the 3rd pin of the IC socket in a vertical orientation. Solder the resistors in, if you are using new resistors or have resistors with long leads do not trim them yet. We will use these leads for future connections.
Step 5: Place the Switch and LED
The switch will provide positive power to the 10K ohm resistor. Place the switch with one pin farthest to the left so that it aligns vertically with the resistor. If you have a switch with 4 pins make sure you place it so that the connection will be broken between the left and right set of pins. Solder the switch in place. Now you will need to bridge the gap between the 10K resistor and the switch. If you had long legs on your resistor bend one now so that it touches the switch connection. Solder and then trim this connection. If you did not have long legs on you resistor you will need to bridge the gap in a similar way that we did with the IC sockets and the voltage rails.
Now place the LED in the circuit between the 470 ohm resistor and the right pin of the switch. Place the LED so that the anode, +, connection is connected to the pin on the switch. Solder the LED in place. Using the LED leads bend the anode so that it touches the switch pin to create a connection between the two. This is where the positive power will split off. Bend the cathode of the LED so that it connects with the 470 ohm resistor. Solder these two connections.
Now lets provide positive power. Run a wire from the positive voltage rail to the second pin from the bottom. Now run a wire from the wire we just placed to the connection between the LED and switch. Solder all connections.
Now place a NPN transistor in the socket as show and push the button. If the transistor works the LED should light up. Not working? Do you have the correct Transistor? Is your circuit connections correct?
Step 6: Finish It Up
The PNP circuits are mirror images of their similar NPN circuits. Just remember to connect the switch and LED to negative voltage. Also remember to place the LED correctly so the the Anode connects with the resistor.
To finish it up add some labels so you never forget where the different type of transistors go.
Step 7: How To Use It
Situation 1: You know the transistor type and orientation.
If you know the transistor type and orientation place it in the correct testing circuit. Hold the tester in the landscape position with the battery on the right side. Place the transistor in the correct circuit for its type and orientation with the flat side facing you. With the transistor firmly in the socket press the tactile button. If the LED lights up the transistor passes the test.
Situation 2: The transistor type and orientation is unknown.
Start with the tester in a landscape position with the battery to the right. Place the transistor in the bottom left circuit, the circuit that tests if it is an NPN with an EBC orientation. Make sure to place the transistor in the socket with the flat side facing you. With the transistor in the socket observer the LEDs status without pressing the button, don't touch it just yet.
- Is the LED on? If so then the transistor is a NPN type but does not have the EBC orientation. Remove it from the socket and place it in the socket to the right to test if it has the orientation of ECB.
- Is the LED off? Press the button and observe the LED status. If the LED turns on they you have found out three things about this unknown transistor. One it is of type NPN. Two if has an orientation of EBC and three it works.
If the LED remains off with or without pressing the button place it in the PNP testing circuit and try to determine the orientation.
The main thing to take away from this situation is that if a transistor is placed in the correct transistor type testing circuit but the wrong orientation the LED will illuminate. This holds true for both NPN and PNP test circuits. By observing this you will know if you have guessed the correct transistor type but chose the wrong orientation. Knowing this can save you a lot of time when you are testing a pile of unknown transistors.
NOTE: In handling this tester you may have noticed something odd happen if you touch the back side of the circuit, specifically bridge the switch with your finger. If you do this with the correct transistor in the socket the LED will light up without pressing the button. Don't worry nothing is wrong with your circuit or the transistor, in fact this is good. Your finger is conducting a small amount of current which is picked up by the transistor. The transistor, doing one of the things it is designed to do, picks up this small current and amplifies it by letting current pass through to the LED. So while handling the tester try to keep your fingers from brushing the back of the tester to prevent false readings.