Tech Directions - February 2008 - (Page 23)

+ i+ - You can spiff up the appearance of the BJT checker by applying labels for the switches, as well as to mark the 5pin socket polarity locations. This will make the BJT checker more functional and more professional looking. You can use labels with adhesive backs, your word processor, scissors, and a razor knife to create these decals. Clear tape over white paper with hand-printed letters also works quite well. NPN PNP Using the BJT Checker Whether using the BJT checker constructed on your proto-board or the one in the enclosure, the testing procedure will be the same. Gather some BJTs. Select a NPN BJT. You will need to know the BJT’s pin-out. Make sure that the BJT’s leads are straight. Insert the BJT into the 5-pin test socket. The BJT’s collector must be plugged into the [+], the base into the [I], and the emitter into the [-]. The two LEDs should be off. Press the black push-button marked NPN. Both LEDs should flash, then become very dim. This would indicate a good BJT. Repeat the procedure using a PNP BJT. After inserting the PNP BJT properly, you will see similar results. Get several BJTs of the same type and test them. You will note a difference in the LED’s flash rate. This, of course, is because each BJT has a slightly different beta. A 0.25-second flash rate indicates a beta of about 100 while a 1.0-second flash rate is a beta of about 400. 5 holes, 0.1" apart 0.4" 1.4" 2" 1.1" 0.6" 1.2" 1.2" 3.2" Fig. 3—Panel layout Final Thoughts 11. Referring to Fig. 1 and Photo 3, solder one end of the capacitor to the middle pin of the 5-pin socket. Then solder the other lead of the capacitor to the wire connecting between the two push-button switches. 12. Solder the 10 MΩ resistor across the capacitor. Push the newly wired components down so they resemble the components shown in Photo 3. 13. Get the battery clip. Referring to Fig. 1, solder the red wire to the NPN switch and the black wire to the PNP switch. Inspect your work. The completed circuit should resemble the unit of Photo 3. Make corrections if necessary. 14. While the BJT checker is still apart, attach a new 9 V battery to the battery clip. Insert either a NPN or a PNP BJT into the test socket. If it works—great! If not, fix it. If necessary, refer to the proto-board circuit that you still have intact. 15. Once you know that everything works properly, carefully place the attached battery in the plastic box. Put the piece of sponge over the battery. This will keep the battery from moving around inside the box with the lid in place. 16. Tighten the two hex nuts that secure the pushbutton switches to the perfboard. Place the perfboard on top of the box. Insert the four mounting screws through the mounting holes of the perfboard and tighten the screws. This handy BJT checker can be of great use to both teachers and students. With a little experience, you will learn that you can discern different BJTs without knowing their pin-outs. You can enhance the operation of your BJT checker by getting three different-colored wires about two feet Photo 3— Component placement long. Solder pins on one end small enough to insert into the BJT’s 5-pin socket, and solder clips on the opposite ends. This will allow you to test larger BJT sizes such as TO220s, TO5s, and TO3s. If you discover anything interesting, keep me posted via e-mail (Charman@sbcglobal.net). Good luck and happy checking! www.techdirections.com ELECTRICITY/ELECTRONICS 23 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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