IIRC, the 74HC14 has a buffered inverter stage, and this won't work. You need an unbuffered inverter. I'm not sure if there are unbuffered 74HC14s like there are the 74HCU04's, or you could go ahead and use the 74LS14.... Cheers, Ishaan ----- Original Message ----- From: "Chris Emerson" To: Sent: Saturday, December 06, 2003 5:30 PM Subject: [EE:] 74HC14 timer circuit, capacitors > I'm confused about a circuit I've been experimenting with. The circuit > is at http://www.nosreme.org/tmp/timer1b.png, using a 74HC14 hex Schmitt > trigger inverter. > > U1A (buffered by U1B) produces a low duty cycle square wave, which > slowly charges C2 through D2 and R3. So far so good. The voltage on C2 > starts to rise with the expected exponential curve, but then stops at > about 2.6V (Vcc is 5V from a 7805), and I don't understand why. I would > expect it to rise close to 5V minus a diode drop. > > I've tried the following: > > 1. Disconnecting U1C, in case input leakage was causing problems - no > effect on C2. > > 2. Changing C2 does change the time constant (I've tried 0.1uF and > 100uF) as expected, but doesn't change the final voltage. > > 3. Reducing R3 does increase the final voltage on C2, but not by very > much. With R3 around 20-30k C2 gets up to about 3V. > > I'm stumped. Can anyone enlighten me? > > The overall intention is a 4-5 minute timer (turn on, and a few minutes > later it beeps), using the first IC I happened to pick up. I already > know it's a terrible circuit. :-) > > Chris > > P.S. Thanks to everyone who helped with my previous question about DC > motors; the thread is still in my mailbox waiting for me to have time to > do something about it! > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body