NE555 / timer question

74LS123 or similar. Or better yet, in the 21st century we use 8 pin microcontrollers.

For :

+----------------+ | | | | | +---------

RC + bipolar transistor RC + mosfet RC + gate (schmitt) RC + comparator RC + regulator with shutdown Isource + C variations of above. oscilllator (1) + counter + gates oscillator + shift register + gates one shot IC or quivalent uC

Offbeat Ideas Thermistor + transistor PC port + software

(1)Maybe use line frequency from transformer. Clock radio style.

D from BC

Use a monostable mode config..

Since a low signal is needed to start the trigger, Use a PNP transistor to pull down the timing cap to common while the input trigger signal is being held down. this keeps the cap from charging and holds the timer from expiring. when you release the input from the trigger so won't the PNP transistor you put in there to keep the cap from charging. when this happens, the cap will then start to charge and force the 555 output off when it reaches 2/3 of the Vcc voltage.

Look at the monstable example there and add the pnp transistor.. Collector to common , emitter to the cap (pin 7/6),. use something like a 1k ohm resistor to bias the transistor from the same signal used to trigger the input.

P.S. this is a retriggerable configuration, meaning, if the input signal gets seen before the timer expires, the pnp transistor will discharge the cap and start the time off delay again with out any notice on the output.. if that is ok then you're all set. I think for the most part, this is what you want.

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

Oh man, a micro for something this mundane? You can do that with 1/6th of a 74HC14, one resistor, one FET and one cap. Comes to under 10 cents.

R to VCC and input, C to input and GND, FET across cap, done.

--
Regards, Joerg

http://www.analogconsultants.com/

You can also do that with a 555. (If I understand right that he wants output to be low for the first 5 seconds, then go high and stay there.)

C from Vcc to input (which is pins 2 and 6 tied together), R from input to ground; pins 4 and 8 to Vcc; ground pin to ground; pin 3 is output.

If memory serves me right!

Most people will never be able to learn what an R, C, FET, VCC, or GND is. Remember that 50% of all people are below average. You're expecting too much from them, Joerg.

Bob

How about a 12AU7 dual triode? Why use semiconductors for something so mundane? Maybe it's availability and learning something relevant? With the uC, you can do it with no external parts, and much more flexibility. The upfront work pays off when all of a sudden he wants a 1 second pause between turning off and on again, etc...

How much power needs the output signal? One solution would be something like this:

R4 is needed to discharge the capacitor (Joerg, how do you do this with one resistor?), which means after turning off the power, the full delay is available again after some 20 seconds power down. With Rpoti adjusted to about 20k I've managed to get a 5 second delay (not only in ltSpice, but on my breadboard, too :-)

--
Frank Buss, fb@frank-buss.de
http://www.frank-buss.de, http://www.it4-systems.de

On a sunny day (Fri, 23 Nov 2007 21:05:20 -0800 (PST)) it happened snipped-for-privacy@netzero.com wrote in :

HAS ANYBODY MENTIONED SMALL ELECTIC MOTOR WITH CAMSHAFT AND CONTACTS YET?

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What will start the 5 second timer?

I'd do it with a nuvistor ;-)

Learning something relevant is exactly the problem but not in the way you decribed it: IME young grads often do not even know how to create a one-shot out of simple parts anymore. All they can do is write code, VHDL and wield PSPICE. Soldering iron? Nope. Analog parts? Yuck. Mention tapped inductors and they might think it's some kind of newfangled cholesterol medication.

Yes, it may only be one part. But what does that part cost in large quantities? And what does the semi-discrete solution cost? How does the little uC fair in brown-out? I can count anymore the times I found uCs as the culprit. "Well, the datasheet says it has some kind of POR" ... "Ok, let's gradually decrease VCC and then raise it again" ... "Oh drat!"

Then out of the blue that nice little uC goes unobtanium and, of course, like usual there ain't no 2nd source.

--
Regards, Joerg

http://www.analogconsultants.com/

50% of people are below the median but not necessarily the average- could be way more or way less.

Nonsense, a heater and bimetal...although 5 seconds may be pushing the lower boundary of obtainable.

Nah, use the chain of a coocoo clock. Tick-tock-tick-tock .. click :-)

Since we got it and I repaired it (one of those honey-do projects...) that thing never failed, despite the fact that it looks rather flimsy. Ok, the air brake wheel is still busted because they made it from plastic. Arrgh! My gut feel is that some tourists are being ripped off quite a bit.

--
Regards, Joerg

http://www.analogconsultants.com/

I haven't checked for very slow blowing fuses. I wonder if this goofy circuit can go low in 5 seconds..

V+ V+ | | R R | /-----Vout +-------| | >

fuse | | | gnd gnd

D from BC