Tech Directions - February 2008 - (Page 21)

Press to Test Shop-Built BJT Checker Is Easy to Use By Charles Harman charman@sbcglobal.net HETHER you are a student or an instructor in an electronics lab, having the means to check the operation of a bipolar junction transistor (BJT) at the proto-board stage is a blessing. Most students do not have the experience or knowledge that it takes to recognize whether or not a BJT is operating. With this handy BJT checker, a student or the instructor can simply remove the component from the proto-board, insert the component in the BJT checker, push the pressto-test button, and observe the BJT checker’s LEDs (light emitting diodes). It’s that simple. W How It Works The BJT checker allows the user to dynamically check a NPN or a PNP type of BJT, a current-controlled device. To facilitate accurate testing, the user should know the BJT’s pinout configuration. The BJT checker’s circuit allows a emitter resistor, and a series LED. BJT under test to not conduct when the BJT is correctly inserted into the checker’s test socket, then conduct Procedure when the appropriate NPN or PNP I suggest that you first build the switch is pressed. A 5-pin test socket BJT checker on your proto-board so accommodates most BJT case types that it resembles the BJT checker (TO18, TO39, TO92, etc.). When the shown in Photo 1. This will allow user correctly inserts a BJT in the you to see and physically hook up checker’s test socket, no LED should light. When the user presses the appropriate NPN or PNP switch, the base-emitter junction of the BJT will become forward biased and the two LEDs of the BJT checker will flash. The flash duration of the LEDs is a function of the BJT’s beta. A fast flash indicates a low beta, and a longer flash indicates a high beta. This action is the result of charging a capacitor through the BJT’s baseemitter junction, an Photo 1—BJT checker on proto-board the necessary components of the circuit, as well as better understand the BJT checker’s theory of operation. All of the parts needed to build the BJT checker are available at Radio Shack and are listed on the parts list. 1. Refering to the wiring diagram drawing (Fig. 1) and the schematic Charles Harman taught electronics at ITT Technical Institute in both Houston and Garland, TX, before retiring. He is currently a proctor for the Electronics Technician Association. Qty. 1 1 1 bag 1 bag 1 1 bag 1 bag 1 bag 1 1 roll 1 Parts List Description Enclosure Perfboard PB switches Red LEDs 1μF capacitor 1 kΩ resistors 10 M resistors Battery clips 14-pin WW socket #24 AWG solid wire Sponge (cut to 1" × 2" × 3/4") Part Number Radio Shack 270-283 Radio Shack 276-1395 Radio Shack 275-1547 Radio Shack 276-330 Radio Shack 272-996 Radio Shack 271-1321 Radio Shack 271-1365 Radio Shack 270-325 Radio Shack 900-7243 Radio Shack 278-1341 www.techdirections.com ELECTRICITY/ELECTRONICS 21 http://www.techdirections.com

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Tech Directions - February 2008 Technically Speaking Contents Direct from Washington The News Report Technology’s Past Technology Today Mastering Computers How Engineers Engineer—Lessons from My First Big Engineering Project Tips on Creating Complex Geometry Using Solid Modeling Software Press to Test—Shop-Built BJT Checker Is Easy Cable Television Technician International Technology Education Association Conference—Teaching TIDE with Pride Professional Development Opportunities Product Central Free for the Asking More than Fun

Tech Directions - February 2008

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