home *** CD-ROM | disk | FTP | other *** search
- Newsgroups: sci.electronics
- Path: sparky!uunet!usc!sol.ctr.columbia.edu!eff!snorkelwacker.mit.edu!thunder.mcrcim.mcgill.edu!server.cs.uri.edu!reynhout
- From: reynhout@cs.uri.edu (Andrew Reynhout)
- Subject: Simple transistor project (Q)
- Message-ID: <1992Aug13.062418.999@cs.uri.edu>
- Organization: University of Rhode Island, Computer Science Dept.
- Date: Thu, 13 Aug 1992 06:24:18 GMT
- Lines: 62
-
-
- (the first post contained an error in the schematic. I hope the cancel
- catches it before it propagates too far...)
-
- Hello. I've been having some problems with a circuit, and I'm wondering if
- someone out there can help me...
-
- Basically, I need to take a 5v low-current logic signal, and on the
- transition from low to high, cause a relay to close for about half a second.
- This is my attempt (forgive the ASCII graphics...)
-
-
- +----------------------------------+
- | 1/6 7404 |
- | |\ INVERTER |
- logic in >--+----+-----| O-----+ |
- | |/ | |
- R1 | B Q1 | B Q2
- 3.3k ----- 2N2222 ----- 2N2222
- | / \ / \
- GND C / \ E + C / \ E +
- +5v >-----+ ->----|>|---+---+ ->--+----|<|-------+
- D1 | | D2 |
- 1N4148 | | 1N4148 |
- --- C1 | |
- --- ~10uF | relay |
- | +---coil-------+
- | (5v / 80mA) |
- GND |
- GND
-
- I hope that's reasonably clear...My theory follows:
- R1 is to make sure that a logic low is really 0v. (Important to make
- sure that Q2 doesn't conduct until I want it to.)
- When the logic input is low, the inverter output is high, causing Q1 to
- conduct. ~5v should be available at the emitter. It charges C1 through
- D1, but doesn't go through Q2, since the base voltage on Q2 should be zero.
- C1 charges quickly, and after that point, very little current flows thru Q1.
- Then, when the logic goes high, it turns OFF Q1, and turns ON Q2, and
- C1 discharges quickly through Q2, and then through the relay. Since
- Q1 is off, there is no other current available after C1 discharges, and
- the relay should return to its normal state.
- When the logic input goes low again, the process repeats.
-
- I am aware of a possible problem involving a one-diode drop in voltage by
- the time the relay sees anything, but what else is wrong with this circuit??
- It is distinctly possible that I've overlooked something simple, or even
- used the wrong approach to begin with.
- Currently (no pun intended, :) the emitter of Q2 seems to follow logic,
- although at a greater current capacity...but it goes high and stays
- high...this is not particularly useful.
-
- If anyone can help me out or point me in the right direction, I would be
- greatly appreciative. This started out as a seemingly simple project, but
- has turned confusing...
-
- Many thanks,
- Andrew
- --
- Andrew <reynhout@cs.uri.edu> "If you remind me of my dog,
- we'll probably get along" -jane siberry
-
-