# Simple One Shot circuit needed

Discussion in 'Electronic Design' started by [email protected], Sep 22, 2007.

1. ### Guest

I need a simple circuit that when triggered will send out a pulse that
will last about a minute and then go off. The
circuit will stay off until it is manually reset. The input will be a low
voltage 5-12VDC that will flow thru a switch that is normally closed. When
the switch is opened the circuit is activated and the pulse will be created.
This circuit has to be extremely stable. There cannot be any false
triggering. It would probably be best if it
operated under battery power.

Robert Barch

2. ### HowhurleyGuest

Robert,

How accurate does the duration of the pulse need to be?

When you say extremely stable, does it have any safety connotations to
it?

What voltage battery power?

3. ### Hal MurrayGuest

I'd probably start by doing it in software - one of the tiny PICs or AVRs.

They come with internal oscillators. If that's not accurate enough
you can use an external crystal.

4. ### Robert BaerGuest

The simplist circuit is a one-transistor oneshot, but time stability
for a miinute is not good.
For reasonable stability, use a few 555s setting the first one at
(say) one second, the next at (say) 10 seconds and the last at 60 seconds.
The first one to be capacitivly coupled to the second, and the second
to be capacitivly coupled to the third in a similar manner.
Use a scope to adjust the timing so that the Nth pulse from the
preceeding 555 "overrides" the RC timing.
Division this way to 15 at most is reliable.

5. ### Guest

There are no safety connotations
The voltage of the battery is open just as long as it will power the
circuit and generate the pulse. You can select
anything you want just make it easy
The pulse really doesn't have to be very accurate. Lets make it variable
with a pulse of one minute plus or minus five
seconds either way.
Robert

6. ### ehsjrGuest

You can use a 555 monostable, like this:

12V+ ------------------+------+---+
| | |
| ---------
| | 8 4 |
+---|7 |
| | |
[270K] | 555 |
| | 3|---> Out
+---|6 |
o | | |
In >---o-o---+---[.1uF]-- | --|2 |
SW | | | 5 1 |
| |+ ---------
[10K] [220uF] | |
| | [.1uF] |
| | | |
Gnd ---------+------------+------+---+

When the switch is closed with + on the input, the output
at pin 3 will be -. When the switch opens, the timer will
run and pin 3 will go + for about 65 seconds, then return
to minus. The ground for your input signal and the ground
for the 555 must be connected together.

Ed

7. ### Fred BloggsGuest

Things get complicated when you ask for a one-time trigger with manual
reset and immunity against false triggers. You can do something like
shown below but I would recommend adding a POR (power on reset) IC with
open drain output connected to RST node shown with '*'. Left 555 is
latched OUT high when IN switch opens, and this is used to pull right
555 out of RST while simultaneously triggering its output for specified
duration. The PN3904 tied into manual RST momentary switch prevents
false triggers by disabling RST action until input switch is re-closed.
View in a fixed-width font such as Courier.

..
..
.. V+
.. --------------------+-----------+------------+----------+------------
.. | | | |
.. | | | |
.. NC | .----------------------------------.
.. IN-o->o-. | | | | | |
.. SW | +---------555 | [1K] [RT] +---------555 |
.. | | VCC | | | | | VCC | |
.. 10K | | | | | | | |
.. | | | | | | | | |
.. +-----|TRIG OUT|--+--|--[10K]-+--------|TRIG OUT|---->
.. | | | | | | | | |
.. 10K .-|DIS | | | +-1K-|DIS | |
.. PN3904| | | | | | | | | |
.. __ | +-|THRESH RST|-----+ * | +----|THRESH RST|-'
.. | \| | | | | | | | | |
.. | |-+ | | GND | | 0.1 |CT | GND |
.. | <| | | +-----------+ | === === +-----------+
.. | | 10K | | | U| | |
.. ----+---+---+-------+--------------------+---+----------+-------------
.. | | | |
.. | RST --- MOM | ---
.. '------------o o--------------' ///
.. SW NO
.. OUTPUT PULSE WIDTH
..
.. 1.1 X RT X CT
..

8. ### MooseFETGuest

If you are going for a minute, you are likely to want something like a
CD4060. Making time constants a minute long is not an easy matter if
you want any sort of accuracy in the timing.

The CD4060 will fairly easily make a pulse that goes high one minute
after the reset is removed.

The handling of the switch debouncing can also be done with CMOS
circuits. You will need to put a resistor in series to protect the
CMOS against any voltage spikes.