Electronics > Beginners
A one time timer
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larsdenmark:
@Benta - You are probably right about the CD4538. However, the world is short on tutorials on how to use the CD4538 or why one should use it.
Perhaps you could share a few (more) words about how and when to use the CD4538?

A quick search finds that the 555 is a lot cheaper than the CD4538 and the 555 is a lot easier to get.
Zero999:

--- Quote from: alsetalokin4017 on May 29, 2018, 11:11:54 pm ---Real Engineers may use a discrete monostable, but many ordinary people just use what they have available in their junqueboxes.



--- End quote ---
What? You don't have a couple of transistors handy? A transistor would most be required to drive a motor anyway, as unless it's a very small motor, the 555 won't be able to output enough current.

My previous post was a bit tongue in cheek: of course, use whatever is available.

And no one spotted my mistake? With high current loads, a resistor is needed to limit the base current to Q1. In the usual configuration it's not needed, since the load on the opposite transistor is normally small, so won't exceed the base current rating of the transistor for long enough to case any damage, but in this case the motor will allow a huge current to flow. R5 limits it to around 44mA, which may be a little over the BC549's rating, but the surge won't last for long enough to cause any damage.
tautech:

--- Quote from: Benta on May 29, 2018, 06:25:34 pm ---Read my reply #2 again. A 4538 will give you inverted input, non-inverted input, inverted output, non-inverted output, retriggering, non-retriggering and is probably cheaper. Why are you f*cking around with a 555?

Sorry, I had to get that out of my system, but it's a mystery to me why all designs are centered on a 555.

--- End quote ---
Agreed.
There are other worthy alternatives too 1/2 of a 4013 or 4047.
Sure none have the drive capability of a 555 but for power switching a silicon switch is needed anyway. Logic level MOSFETs are a good match for 4000 IC's.
hydrolisk1792:
Build a 555 one shot "monostable" throw a general purpose transistor on the output, even put an led there to see of the circuit is on from the board.  Then throw a relay on that transistor, don't forget the flyback diode. Then hook the motor to the relay, and that is that. Bob's your uncle!
Hope this helps.
Benta:

--- Quote from: larsdenmark on May 30, 2018, 08:04:26 am ---@Benta - You are probably right about the CD4538. However, the world is short on tutorials on how to use the CD4538 or why one should use it.
Perhaps you could share a few (more) words about how and when to use the CD4538?

A quick search finds that the 555 is a lot cheaper than the CD4538 and the 555 is a lot easier to get.

--- End quote ---

The 4538 or 74HC4538 is a good choice whenever you need an edge triggered monostable multivibrator (MMV). Note the qualification: if you want a level triggered input (eg., when adding a certain extra time to a variable length input pulse) the 555 is better suited.

There are no tutorials on the 4538, simply because it is super-simple to use and the datasheet gives all the needed information. Choose a resistor and a capacitor for the desired pulse length and you're done.
As a bonus, you get two MMVs in the same package.

Concerning "a lot cheaper": Mouser shows 40 cents for the MC14538 and 30 cents for the NE555. Considering the monstrous capacitor you'll need for making a 5-minute delay with the 555, I'm certain that a 4538 solution will be much cheaper in the end.


Edit: the 7555 (CMOS version of the 555) costs 45 cents. And at such long time delays you're forced to use that version. So much for "cheaper".
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