On Thu, 26 Mar 1998 11:46:08 -0500 Bill Kennedy writes: >Dear Piclisteners, > Does any one know where a DTMF keypad that connects a row and >column >and ground together might be located? I've been told that this is the >only thing that will work with a TP5089 encoder chip I'm trying use. There is a way to use a conventional keyboard that just connects the row and column together. Use transistors to invert the row or column signals. I think the 5089 normally pulls the inputs up and detects when they are connected to ground. In this case, use NPN transistors with the emitters to ground, bases to the column leads from the keyboard, and collectors to the column pins on the chip. The row pins connect directly to the keyboard. When a key is pressed, current flows out of a row pin on the chip, through the key switch, and into the base of the corresponding column transistor. The transistor turns on and pulls the column pin to ground. Thus two pins are pulled down, and the proper tone pair is selected. Good design practice is to add suitable base-emitter resistors to ensure the transistors stay off when no keys are pressed. Some later chips had a mode to use a conventional keyboard. It may be simpler to just find another chip. _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]