555 as "on delay" timer?

D

Don A. Gilmore

Guest
Hi guys:

Is there a way to hook up a 555 timer to function as an "on delay" type
timer? In other words, when you hold the trigger line it waits for a
specific time, then activates the output. I have schematics that show how
to wire it in "one shot" mode, but that doesn't do me any good.

It would also be nice if I could trigger it with a logic "high" rather than
a "low". Is this possible? Maybe another chip besides a 555?

Thanks for any replies.
--
Don A. Gilmore
Mechanical Engineer
Kansas City
 
"Don A. Gilmore" <eromlignodNOSPM@kc.rr.com> wrote in message
news:2mssp0Fqj6ukU1@uni-berlin.de...
Hi guys:

Is there a way to hook up a 555 timer to function as an "on delay"
type
timer? In other words, when you hold the trigger line it waits for
a
specific time, then activates the output. I have schematics that
show how
to wire it in "one shot" mode, but that doesn't do me any good.

It would also be nice if I could trigger it with a logic "high"
rather than
a "low". Is this possible? Maybe another chip besides a 555?

Thanks for any replies.
--
Don A. Gilmore
Mechanical Engineer
Kansas City
Click to expand...
You can make a transition into a pulse with a series capacitor, along
with a resistor to ground (or Vcc, depending on which way you want to
go.) There was a thread on this recently (subject "Looking for a
circuit to").

Inverting the logic value can be done with an inverter switch, or with
a discrete transistor. The inverter chip is possibly easier to work
out.

Here is one simple circuit you might want to try (view with
mono-spaced font)



5V .-------------------------.
| | |
.-. .-. |
| | 1k | | 10k -
| | | | ^
'-' '-' |
| | |
| || | ___ |
o------||-----o----|___|--o---- Output
| || 100k 5-0-5 Pulse
| 1uF to 555 trg input
10k |
___ |/
Input -|___|--| NPN
0-5V |>
|
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

I haven't built this recently, so breadboard it first. The
resistor/capacitor values may need adjustment for your 555.

Regards,
Bob Monsen
 
On Thu, 29 Jul 2004 13:11:05 -0500, "Don A. Gilmore"
<eromlignodNOSPM@kc.rr.com> wrote:

Hi guys:

Is there a way to hook up a 555 timer to function as an "on delay" type
timer? In other words, when you hold the trigger line it waits for a
specific time, then activates the output. I have schematics that show how
to wire it in "one shot" mode, but that doesn't do me any good.

It would also be nice if I could trigger it with a logic "high" rather than
a "low". Is this possible? Maybe another chip besides a 555?
Click to expand...
---
An inverter or a single transistor wired as an inverter and coupled to
the 555's trigger input will give you the necessary low-going input as
long as you observe the chip's input pulse time requirements. OTOH, a
74XX123 or a 4538 will trigger on either low or high-going edges and
eliminate the 555's trigger pulsewidth timing hassle.

I'm assuming from your description that what you want is a circuit
with an output that goes high a certain amount of time after its input
goes high and stays high, and that the output will be terminated (will
be forced low) any time the input goes low.

Ive posted a circuit for something that'll do that on abse under
"Delayed-ON timer".

I've shown 7555's and an HC00 as the NAND, but there's no reason you
shouldn't be able to substitute 4538's (and get rid of the coupling
caps and pullups) and a 4011 if you want to use a 12V supply, as long
as you can supply the current you need to from the RS latch's output.

--
John Fields
 
"John Fields" <jfields@austininstruments.com> wrote in message
news:diiig0h1lcpc4kdssfcb3hfa6snarng7t8@4ax.com...
An inverter or a single transistor wired as an inverter and coupled to
the 555's trigger input will give you the necessary low-going input as
long as you observe the chip's input pulse time requirements. OTOH, a
74XX123 or a 4538 will trigger on either low or high-going edges and
eliminate the 555's trigger pulsewidth timing hassle.

I'm assuming from your description that what you want is a circuit
with an output that goes high a certain amount of time after its input
goes high and stays high, and that the output will be terminated (will
be forced low) any time the input goes low.

Ive posted a circuit for something that'll do that on abse under
"Delayed-ON timer".

I've shown 7555's and an HC00 as the NAND, but there's no reason you
shouldn't be able to substitute 4538's (and get rid of the coupling
caps and pullups) and a 4011 if you want to use a 12V supply, as long
as you can supply the current you need to from the RS latch's output.
Click to expand...
Thanks, John. I think I'll try the CD4538 since it's not so picky about the
input pulse and is good for 12V.

I have another question. Unfortunately, in my application, the 12V trigger
signal for the timer is also the 12V supply! In other words, when I apply
12V to the circuit I want the timer to start timing immediately. Will this
cause problems?

Thanks again to everyone for the help.

--
Don A. Gilmore
Mechanical Engineer
Kansas City
 
On Fri, 30 Jul 2004 10:43:29 -0500, "Don A. Gilmore"
<eromlignodNOSPM@kc.rr.com> wrote:

"John Fields" <jfields@austininstruments.com> wrote in message
news:diiig0h1lcpc4kdssfcb3hfa6snarng7t8@4ax.com...
An inverter or a single transistor wired as an inverter and coupled to
the 555's trigger input will give you the necessary low-going input as
long as you observe the chip's input pulse time requirements. OTOH, a
74XX123 or a 4538 will trigger on either low or high-going edges and
eliminate the 555's trigger pulsewidth timing hassle.

I'm assuming from your description that what you want is a circuit
with an output that goes high a certain amount of time after its input
goes high and stays high, and that the output will be terminated (will
be forced low) any time the input goes low.

Ive posted a circuit for something that'll do that on abse under
"Delayed-ON timer".

I've shown 7555's and an HC00 as the NAND, but there's no reason you
shouldn't be able to substitute 4538's (and get rid of the coupling
caps and pullups) and a 4011 if you want to use a 12V supply, as long
as you can supply the current you need to from the RS latch's output.

Thanks, John. I think I'll try the CD4538 since it's not so picky about the
input pulse and is good for 12V.

I have another question. Unfortunately, in my application, the 12V trigger
signal for the timer is also the 12V supply! In other words, when I apply
12V to the circuit I want the timer to start timing immediately. Will this
cause problems?
Click to expand...
---
Not if you use the 555's. If you use the 4538's you'll have to assure
that an input changes state in the proper direction _after_ POR goes
high in order to get them to start timing out. The 555's have a
couple of comparators in the front end, followed by a latch, so all
the trigger input needs to see is a level less than about 1/3Vcc in
order to start things going, then the trigger input needs to rise to
2.3Vcc before timeout in order to let the output fall when it's
supposed to. PLUS, some 4538's have the down side that you've got to
Click to expand...
put a fairly hefty Schottky across the timing resistor in order to
keep the charged-up timing cap from hurting the chip if the +12 gets
shorted to ground or there are spikes on it. Fairchild's 4538BC's are
an exception, plus their active period is simply T = RC, so that's a
good trhing.

All in all though, for your application, if it was me I'd go with the
7555's (or a single 7556) and a 4013.

BUT, it might be much simpler than that, depending on what your load
looks like. What's the big picture?

---





--
John Fields
 
Robert C Monsen wrote:

"Don A. Gilmore" <eromlignodNOSPM@kc.rr.com> wrote in message
news:2mssp0Fqj6ukU1@uni-berlin.de...
Hi guys:

Is there a way to hook up a 555 timer to function as an "on delay"
type
timer? In other words, when you hold the trigger line it waits for
a
specific time, then activates the output. I have schematics that
show how
to wire it in "one shot" mode, but that doesn't do me any good.

It would also be nice if I could trigger it with a logic "high"
rather than
a "low". Is this possible? Maybe another chip besides a 555?

Thanks for any replies.
--
Don A. Gilmore
Mechanical Engineer
Kansas City




You can make a transition into a pulse with a series capacitor, along
with a resistor to ground (or Vcc, depending on which way you want to
go.) There was a thread on this recently (subject "Looking for a
circuit to").

Inverting the logic value can be done with an inverter switch, or with
a discrete transistor. The inverter chip is possibly easier to work
out.

Here is one simple circuit you might want to try (view with
mono-spaced font)



5V .-------------------------.
| | |
.-. .-. |
| | 1k | | 10k -
| | | | ^
'-' '-' |
| | |
| || | ___ |
o------||-----o----|___|--o---- Output
| || 100k 5-0-5 Pulse
| 1uF to 555 trg input
10k |
___ |/
Input -|___|--| NPN
0-5V |
|
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

I haven't built this recently, so breadboard it first. The
resistor/capacitor values may need adjustment for your 555.

Regards,
Bob Monsen
Click to expand...
A Microfarad? I've done that with anywhere from .01 to .1, but
I think 1U is too big. I'd also dramatically decrease the value
of the 100K, to maybe 2K2 (or 2K4, since I have about 1K of them
on hand. :)

Other than that, I've done this more than once.

Cheers!
Rich
 
On Sat, 31 Jul 2004 18:54:16 GMT, Rich Grise <null@example.net> wrote:


5V .-------------------------.
| | |
.-. .-. |
| | 1k | | 10k -
| | | | ^
'-' '-' |
| | |
| || | ___ |
o------||-----o----|___|--o---- Output
| || 100k 5-0-5 Pulse
| 1uF to 555 trg input
10k |
___ |/
Input -|___|--| NPN
0-5V |
|
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

I haven't built this recently, so breadboard it first. The
resistor/capacitor values may need adjustment for your 555.

Regards,
Bob Monsen

A Microfarad? I've done that with anywhere from .01 to .1, but
I think 1U is too big. I'd also dramatically decrease the value
of the 100K, to maybe 2K2 (or 2K4, since I have about 1K of them
on hand. :)
Click to expand...
---
Dramatically decreased to zero ohms works very well, as does 0.1ľF for
bipolar 555's and 0.01ľF for CMOS. Whether the diode is needed is up
for grabs, too, but for a penny or two it's a good heartburn
preventative.

--
John Fields
 
Hi Don,

Try this website. It's not mine, but I use it a fair amount.

http://home.cogeco.ca/~rpaisley4/LM555.html#7

John


Don A. Gilmore wrote:

Hi guys:

Is there a way to hook up a 555 timer to function as an "on delay" type
timer? In other words, when you hold the trigger line it waits for a
Click to expand...
 
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