18F4550 with 74LS138ericJ2015-06-11T08:31:03+00:00 Home › Forums › Microcontrollers › PIC Microcontroller › 18F4550 with 74LS138 This topic contains 2 replies, has 2 voices, and was last updated by ericJ 1 year, 9 months ago. Viewing 3 posts - 1 through 3 (of 3 total) Author Posts June 10, 2015 at 11:25 am #11467 ericJParticipant I am trying to connect 127 switches to the microcontroller using three 74ls138 decoders how will I scan the switch for buttons pessed June 11, 2015 at 8:30 am #11471 Ligo GeorgeKeymaster You may use it like matrix keypad. Use 6 pins of PIC as inputs such that you can connect 24 x 6 = 144 switches. June 20, 2015 at 9:41 am #11507 ericJParticipant I used the program in the tutorials on the 4×4 keypad as an example I modified it to scan the rows using the 74LS138 but I can not get it to work properly. I attached the I used. Please let me know what I am doing wrong. Attachments:You must be logged in to view attached files. Author Posts Viewing 3 posts - 1 through 3 (of 3 total) You must be logged in to reply to this topic.