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Long pulse to short pulse

T

Terry

Jan 1, 1970
0
All,

Off a PIR motion detector I get a 24 volt output and when it detects
someone the pulse drops to zero for about 4 seconds. I want to trigger
a 555 timer to output about a 1/2 second pulse. What is the standard
way to take this long trigger and shorten it? Thanks.

Terry
 
K

Ken Smith

Jan 1, 1970
0
All,

Off a PIR motion detector I get a 24 volt output and when it detects
someone the pulse drops to zero for about 4 seconds. I want to trigger
a 555 timer to output about a 1/2 second pulse. What is the standard
way to take this long trigger and shorten it? Thanks.

Ascii art:


VCC
VCC !
! \
--- /
^ D1 \ R1
! !
------!!--+--+--+---TRIG and THR
C1 !
---
^ D2
!
GND


Warning: If the signal source has a low impedance, put a resistor in
series with C1 that is smaller than R1 but more than about 100 Ohms.
 
J

John Fields

Jan 1, 1970
0
Ascii art:


VCC
VCC !
! \
--- /
^ D1 \ R1
! !
------!!--+--+--+---TRIG and THR
C1 !
---
^ D2
!
GND


Warning: If the signal source has a low impedance, put a resistor in
series with C1 that is smaller than R1 but more than about 100 Ohms.

---____ ___
To TRIG only. THR and DIS are tied together and to the junction of
the timing cap and resistor to make a one-shot.
 
R

Robert Monsen

Jan 1, 1970
0
Terry said:
All,

Off a PIR motion detector I get a 24 volt output and when it detects
someone the pulse drops to zero for about 4 seconds. I want to trigger
a 555 timer to output about a 1/2 second pulse. What is the standard
way to take this long trigger and shorten it? Thanks.

Terry

VCC
.--------.
| |
\ -----
/ ^
R \ / \ D1
/ -----
|| | |
PIR --------||-------o--------o----- 555 Trigger
|| |
C -----
^
/ \ D2
-----
|
GND

created by Andy´s ASCII-Circuit v1.25.250804 www.tech-chat.de

When the PIR sensor goes to 0V, it'll drag the 555 trigger down
momentarily, triggering the 555 monostable. The R*C constant determines
how long. The diodes prevent the 555 trigger from getting an input
outside its voltage rail, although depending on the variant, this may
not make any difference.

When the PIR comes back up, the diode will prevent the trigger from
going above the VCC.

Note that 24V is too high for VCC for most 555s. Look up the datasheet
on the web for the 555 you are using to determine what the voltage
should be.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.
 
K

Ken Smith

Jan 1, 1970
0
---____ ___
To TRIG only. THR and DIS are tied together and to the junction of
the timing cap and resistor to make a one-shot.

His input pulse is longer than the output pulse so, you don't need the
extra parts. The circuit I posted does this:



. . . . . . . . . . . . . . . .
**** . . . . . . . . . . . . . . +24V
. * . . . . . . . . . . . . . .
. * . . . . . . . . . . . . . . Input
. * . . . . . . . . . . . . . .
. ******************************************* 0V
. : . . . . . . . . . . . . . .
. : . . . . . . . . . . . . . .
. : . . . . . . . . . . . . . .
. : . . . . . . . . . . . . . .
. : . . . . . . . . . . . . . .
**** . . . . . . . ******************** Vcc
. * . . . . . . *. . . . . . . .
. * . . . . * . . . . . . . . .
. * . . . .* ............................ 2/3Vcc
. * . . .* .: . . . . . . . . . .
. * . .* .: . . . . . . . . . .
. * . *. . .: . . . . . . . . . . Trig and THR
. * * . . .: . . . . . . . . . .
. * *. . . .: . . . . . . . . . . 1/3Vcc
. ** . . . .: . . . . . . . . . .
. * . . . .: . . . . . . . . . . 0V
. : . . . .: . . . . . . . . . .
. : . . . .: . . . . . . . . . .
. : . . . .: . . . . . . . . . .
. ************** . . . . . . . . . . Vcc
. * . . . .* . . . . . . . . . . Q
. * . . . .* . . . . . . . . . . Output
**** . . . .****************************** 0V
 
T

Terry Pinnell

Jan 1, 1970
0
All,

Off a PIR motion detector I get a 24 volt output and when it detects
someone the pulse drops to zero for about 4 seconds. I want to trigger
a 555 timer to output about a 1/2 second pulse. What is the standard
way to take this long trigger and shorten it? Thanks.

Terry

Didn't you get two replies to this yesterday? Including mine:

It's sensible to stick to one thread while discussing the same
subject.
 
R

Robert Baer

Jan 1, 1970
0
Terry said:
All,

Off a PIR motion detector I get a 24 volt output and when it detects
someone the pulse drops to zero for about 4 seconds. I want to trigger
a 555 timer to output about a 1/2 second pulse. What is the standard
way to take this long trigger and shorten it? Thanks.

Terry

Use a one-transistor one-shot.
 
T

Terry

Jan 1, 1970
0
Terry Pinnell said:
Didn't you get two replies to this yesterday? Including mine:

It's sensible to stick to one thread while discussing the same
subject.

I did post the day before, but although I am working on the same
project, I considered this topic of shortening a trigger signal
different than the question on voltages.

Being new to the group, didn't mean to break etiquette and apologize
if I did. Appreciate the help.

Terry
 
J

Jim Thompson

Jan 1, 1970
0
I did post the day before, but although I am working on the same
project, I considered this topic of shortening a trigger signal
different than the question on voltages.

Being new to the group, didn't mean to break etiquette and apologize
if I did. Appreciate the help.

Terry

Terry, You need to branch out to other parts besides 555s. You need
an 'HC4538 non-retriggerable one shot.

...Jim Thompson
 
F

Fred Bloggs

Jan 1, 1970
0
Terry said:
All,

Off a PIR motion detector I get a 24 volt output and when it detects
someone the pulse drops to zero for about 4 seconds. I want to trigger
a 555 timer to output about a 1/2 second pulse. What is the standard
way to take this long trigger and shorten it? Thanks.

Terry

There is no "standard" way. If you have a bunch of 555's then something
like this is fairly foolproof:

View in a fixed-width font such as Courier.


..
.. 1M
.. +-/\/\-+--+--------------------------+-< 555 VBATT
.. | | | |
.. | | | |
.. | | | |
.. | | | +-------------+ | 1.1xRTxCT
.. | | | | | | +--+
.. | +---+--+-----+ | +-+----+-----+ | |
.. | | RST V+ | | |RST V+ | | |
.. | | | | | | --+ +--
.. 1N914 | | | | RT | |
.. (+)>--|<|--+--|THRESH OUT|-+-/\/\--+---|THRESH OUT|-------->
.. | | | | | |
.. +--|TRIG DIS|---------+---|TRIG |
.. | | | | |
.. | | CT === | |
.. FROM |CONT | | |CONT DIS|
.. MOTION | | | | |
.. DETECTOR | GND | | | GND |
.. +-------+----+ | +-------+----+
.. | | |
.. | | |
.. (-)>------------------+--------------+-----------+
.. -+-
.. GND
..
..
 
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