Re: Problem with 555 Monostable Circuit

On Mon, 13 Mar 2006 23:46:07 GMT, "Rob" <N@xxxxxxxxxx> wrote:


"John Fields" <jfields@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:q9bb12pgl9cdcqv2q6inh12ao61s5tb8qm@xxxxxxxxxx
On Mon, 13 Mar 2006 16:20:10 GMT, "Rob" <N@xxxxxxxxxx> wrote:

I am trying to create a long high pulse (around 10 minutes), then falling
edge. Then have the process start over.

---
How long after the falling edge do you want it to start over?
---

I did not find any astabel circuits that could produce a very long pulse
without additional ICs. I did find some long pulse monostable circuits,
but
am having trouble getting the timing to be consistent.

In my sample circuit I have decreased the RC values just for testing.
Sometimes the pulses are consistent, other times they appear to be at
fairly
random intervals.

For testing I am using 22K for the resistor, and 33uf for the cap
My actual values will be 1.6M and 33uf

My pinouts look like this:

Pins 1,2,5 are grounded
Pins 1,2,5 also go to the - on my cap
Pins 4,8 +5V
Pins 6,7 go through my resistor to +5
Pins 6,7 also go to the + on the cap
Pin 3 is my output

(sorry I tried to draw a schematic, but it did not come out too well)


Why is the timing in my circuit not stable?

Because your wiring is wrong. For a monostable triggered manually
by S1, here's what your circuit should look like. View in Courier:


+V---+---------+-----+--------+
| | | |8
[10K] [10K] [22k] +---+---+
| | Rt| 2|_ Vcc |3
+-[0.1µF]-+-----|---O|T OUT|----OUT>
| | 6| _|4
| O +----|TH R|----+V
| O S1 | 7|_ 7555|5
| +---O|D C|----+
| +| | GND | |
| [33µF] +---+---+ [0.1µF]
| Ct| |1 |
GND>-+---------------+--------+--------+

The cap on pin 5 is optional, and you could just as easily let pin 5
float. Do _not_ ground it, that's part of the problem you're having
now.

The length of the output pulse is defined by:


t = 1.4 Rt Ct = 1.4 * 2.2E4R * 3.3E-5F ~ 1 second


With Rt equal to 1.6 megohms, the width of the output pulse will be
about 74 seconds, not quite the 600 seconds you're looking for. ;)

--
John Fields
Professional Circuit Designer



John,

Thanks for your help. The circuit is running better now. How would I get the
circuit to trigger a reset and start over again, after the output goes low
(re trigger itself)?

BTW: after the falling edge (which will trigger something else) the output
can go high almost instantly.

---
There are two ways. The first is:

+V
+--------|------+
| |8 |
[22k] +---+---+ |
Rt| 2|_ Vcc |3 |
+---O|T OUT|--+----OUT>
| 6| _|4
+----|TH R|----+V
| 7|_ 7555|5
| O|D C|
+| | GND |
[33µF] +---+---+
Ct| |1
+--------+
|
GND

and the second:

+V
|
+------------+
| |8
[Ra] +---+---+
| 7|_ Vcc |3
+-------O|D OUT|------OUT>
| 6| _|4
[Rb] +---|TH R|----+V
| | 2|__ 7555|5
+----+--O|TR C|
+| | GND |
[33µF] +---+---+
Ct| |1
+------------+
|
GND

The first way will get you a 50% duty cycle square wave out if you
use a CMOS 555 (7555) and the second way will allow you to adjust
the duty cycle of the output by adjusting Ra and Rb.

Rather than going through it all here, there are a lot of excellent
references on the web.

Go to:

http://www.uoguelph.ca/~antoon/gadgets/555/555.html

and if that doesn't work for you, Google "555 timer" for something
that surely will.

One caveat with using 555's for long timing intervals is that the
leakage current of the capacitor can become a problem if the timing
resistance can't supply enough current to charge the cap. For best
results you should use a low leakage capacitor like:

http://www.mouser.com/catalog/specsheets/llrl.pdf


--
John Fields
Professional Circuit Designer
.

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