Tech Directions - February 2009 - (Page 21) drawing, solder the battery clip into the circuit, red wire to pin 8 and black wire to the outside pin of the PTT switch. The 555-timer checker’s +9 V LED 1 4 R1 1K 7 R2 1M 6 LED 2 C1 0.1 μF 2 1 8 R3 3 R4 source LEDs. This will help make the 555-timer checker more functional and 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. Refer 470 Ω to Photo 2 to aid in the positioning of the labels. 470 Ω Photo 3 the left LED will light. If the 555-timer checker’s LEDs toggle, the 555-timer is good. If no toggling occurs, or if only one LED glows, the 555-timer is not good. Using the 555-Timer Checker Whether you are using the 555-timer checker constructed on your proto-board or the one in the enclosure, the testing procedure is the same. Gather up some 555-timers of the 8-pin package style. You do not need to know the 555-timers pin-out. Make sure that the 555-timers’ leads are straight. Insert a 555-timer into the 8-pin test socket. Insure that pin-1 of the IC is inserted into pin-1 of the 8-pin DIP socket. Press and hold the push-button switch marked PTT. The right LED will light first, then Final Thoughts Whether you are a student or an instructor, this handy 555-timer checker can be of great use. It can allow you to check 555-timers of the 8-pin DIP type package or the 8-pin round-can TO99 package. You can check ICs at the proto-board stage, or simply test 555-timers to determine whether they are functional. If you discover anything interesting, keep me posted via e-mail (charman@sbcglobal.net). Good luck and happy checking! PTT Fig. 4— Schematic wired circuit should resemble the unit shown in Photo 3. 9. While the 555-timer checker is still apart, attach a new 9 V battery to the battery clip. Insert a good 555-timer into the test socket and press the PTT button. If the LEDs toggle, great! If not, find out why and fix the problem. Insure that no soldered wires touch any other points. If necessary, refer to the proto-board circuit that is still hooked up and functional. 10. Once you have satisfied yourself that everything works properly, carefully place the attached battery in the plastic box. Put a piece of sponge over the battery to keep the battery from moving around inside the box once the lid is in place. Tighten the hex nut that secures the push-button switch 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. You can spiff up the appearance of your 555-timer checker by applying labels to indicate what the unit is, identify the PTT switch, mark the 8-pin socket, and label the sink and Parts List Item 01 02 03 04 05 06 07 08 09 10 11 12 13 Quantity 1 1 1 package 1 or 1 package 2 1 package 1 package 1 package 1 1 package 1 roll 1 1 Description, Part Number, Supplier Enclosure, 270-283, Radio Shack Perf-board, 276-1395 Push-button switch, NO, 275-1547, Radio Shack 8-pin DIP socket, WWICS-8, All Electronics 8-pin DIP socket, 276-1995,Radio Shack LED, green, 276-022, Radio Shack R1, 1 kΩ, 271-1301, Radio Shack R2, 1 mΩ, 271-1356, Radio Shack R3, R4, 470 Ω, 271-1317, Radio Shack C1, 0.1 μF capacitor, 272-135, Radio Shack Battery clips, 270-325, Radio Shack #24 AWG bare wire, 278-1341, Radio Shack 9 V battery, any store Small sponge, any store Radio Shack—1-800-843-7422; All Electronics—1-800-826-5432 www.techdirections.com ELECTRONICS 21 http://www.techdirections.com
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