Confused by monostable setup

  • Thread starter Somebody Else's Wossname
  • Start date
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Somebody Else's Wossname

Guest
Hi guys.
Once again, I am trying to get my head around an odd layout, and having
trouble.

The only experience I have with a monostable circuit is using a 555
back in school. However, I'm not sure if that's the right tool for the
job here, as as far as I know you need a low to act as the trigger.

If somebody could please recommend a solution to this task, or (if
you've really got lots of time) to supply me with a simple circuit
diagram, would be much appreciated.

I have 2 power supplies from external circuits - one provides 5V DC and
is always on, the other provides 12V DC and is switched externally. I
also have several DPST relays connected to various external components
(the circuits controlled by the relays should have no electrical
connection to this one, so what they are is probably irrelevant).

When the 12V input is 'on', both relays should be closed. When the
power goes off, all the relays except R1 should open. R1 should open
after 200 seconds, unless the power comes back on during that time (in
which case the whole thing starts again).

Can anyone suggest how I would go about setting this up?
 
"Somebody Else's Wossname" <andrew@affordable-anime.co.uk> wrote in message
news:1125079404.173140.108900@g14g2000cwa.googlegroups.com...
Hi guys.
Once again, I am trying to get my head around an odd layout, and having
trouble.

The only experience I have with a monostable circuit is using a 555
back in school. However, I'm not sure if that's the right tool for the
job here, as as far as I know you need a low to act as the trigger.

If somebody could please recommend a solution to this task, or (if
you've really got lots of time) to supply me with a simple circuit
diagram, would be much appreciated.

I have 2 power supplies from external circuits - one provides 5V DC and
is always on, the other provides 12V DC and is switched externally. I
also have several DPST relays connected to various external components
(the circuits controlled by the relays should have no electrical
connection to this one, so what they are is probably irrelevant).

When the 12V input is 'on', both relays should be closed. When the
power goes off, all the relays except R1 should open. R1 should open
after 200 seconds, unless the power comes back on during that time (in
which case the whole thing starts again).

Can anyone suggest how I would go about setting this up?

A counter that gets reset when the 12V is ON. Start counting when the 12V
Click to expand...
goes OFF. When the count gets to 200 seconds, it freezes, and R1 opens. If
the 12V comes back before 200 seconds, the counter resets and R1 stays on.
Easiest way to do this is with an 8 bit counter. Look for the most
significant bit to go to a 1. You need a frequency of 200/128 = 1.56 Hertz.
You could have an astable running at that frequency, or take 60 Hz and
divide it by 94 (probably 100 would be OK).

Tam
 
What you want is a retriggerable monostable. (12V going high again
should "abort" the 200 second countdown.)

The 555 in its simplest configuration is non-retriggerable. If you
look in the datasheet, you will see
example circuits where there's a PNP transistor to make
it be retriggerable.

Other one-shots are retriggerable without such
shennanigans. For example 74HC123. The HC123 has other nice
features like choice of inputs for selectable trigger polarity etc.

Tim.
 
Somebody Else's Wossname wrote:
Hi guys.
Once again, I am trying to get my head around an odd layout, and having
trouble.

The only experience I have with a monostable circuit is using a 555
back in school. However, I'm not sure if that's the right tool for the
job here, as as far as I know you need a low to act as the trigger.

If somebody could please recommend a solution to this task, or (if
you've really got lots of time) to supply me with a simple circuit
diagram, would be much appreciated.

I have 2 power supplies from external circuits - one provides 5V DC and
is always on, the other provides 12V DC and is switched externally. I
also have several DPST relays connected to various external components
(the circuits controlled by the relays should have no electrical
connection to this one, so what they are is probably irrelevant).

When the 12V input is 'on', both relays should be closed. When the
power goes off, all the relays except R1 should open. R1 should open
after 200 seconds, unless the power comes back on during that time (in
which case the whole thing starts again).

Can anyone suggest how I would go about setting this up?
Click to expand...
Are these 12V relays or 5V relays?
 
Somebody Else's Wossname wrote:
Hi guys.
Once again, I am trying to get my head around an odd layout, and having
trouble.

The only experience I have with a monostable circuit is using a 555
back in school. However, I'm not sure if that's the right tool for the
job here, as as far as I know you need a low to act as the trigger.

If somebody could please recommend a solution to this task, or (if
you've really got lots of time) to supply me with a simple circuit
diagram, would be much appreciated.

I have 2 power supplies from external circuits - one provides 5V DC and
is always on, the other provides 12V DC and is switched externally. I
also have several DPST relays connected to various external components
(the circuits controlled by the relays should have no electrical
connection to this one, so what they are is probably irrelevant).

When the 12V input is 'on', both relays should be closed. When the
power goes off, all the relays except R1 should open. R1 should open
after 200 seconds, unless the power comes back on during that time (in
which case the whole thing starts again).

Can anyone suggest how I would go about setting this up?
Click to expand...
All the relays except RY1 Are powered from the 12 V supply.
When the supply drops, they drop. When the 12V supply comes
on, they energize. That leaves RY1, which you want to
drop out 200 seconds after the 12V supply drops out. It is a
5V relay. Here's a circuit that may work for you. It will
drop out RY1 after an adjustable delay.

+12 ---[R1]---+----+ +---- +5
| | |
| | [RY1]
| | |
[ZD] | c/
| +-----+---| TIP 120
| | | e\
| [VR1] [C1] |
| | | |
Gnd (12V) ----+----+-----+-----+--- Gnd (5V)

Zd - 5V 1 watt
R1 - 330 1 watt
C1 - .1 farad 5.5 v super cap
VR1 - 50 K
RY1 - 5 volt relay

When the 12 volt supply drops, C1 begins discharging
through VR1 and the transistor. When the voltage across
C1 falls to about 1.2 volts, the transistor stops conducting,
and the relay de-energizes. If the 12 volts supply turns back
on before then, the transistor is held on by the supply,
and C1 re-charges.

Ed
 
ehsjr wrote:
Somebody Else's Wossname wrote:
Hi guys.
Once again, I am trying to get my head around an odd layout, and having
trouble.


All the relays except RY1 Are powered from the 12 V supply.
When the supply drops, they drop. When the 12V supply comes
on, they energize. That leaves RY1, which you want to
drop out 200 seconds after the 12V supply drops out. It is a
5V relay. Here's a circuit that may work for you. It will
drop out RY1 after an adjustable delay.

+12 ---[R1]---+----+ +---- +5
| | |
| | [RY1]
| | |
[ZD] | c/
| +-----+---| TIP 120
| | | e\
| [VR1] [C1] |
| | | |
Gnd (12V) ----+----+-----+-----+--- Gnd (5V)

Zd - 5V 1 watt
R1 - 330 1 watt
C1 - .1 farad 5.5 v super cap
VR1 - 50 K
RY1 - 5 volt relay

When the 12 volt supply drops, C1 begins discharging
through VR1 and the transistor. When the voltage across
C1 falls to about 1.2 volts, the transistor stops conducting,
and the relay de-energizes. If the 12 volts supply turns back
on before then, the transistor is held on by the supply,
and C1 re-charges.

Ed
Click to expand...
Wow - what a nice, simple, elegant solution. No IC's...........

Andrew VK3BFA
 
ehsjr wrote:

Ooops - forgot the "backwards" diode (1N400x) across RY1

+12 ---[R1]---+----+ +-----+---- +5
| | k| |
| | [D1] [RY1]
| | | |
| | +-----+
| | |
[ZD] | c/
| +-----+---| TIP 120
| | | e\
| [VR1] [C1] |
| | | |
Gnd (12V) ----+----+-----+-----+--- Gnd (5V)
 
Andrew VK3BFA wrote:
ehsjr wrote:

Somebody Else's Wossname wrote:

Hi guys.
Once again, I am trying to get my head around an odd layout, and having
trouble.


All the relays except RY1 Are powered from the 12 V supply.
When the supply drops, they drop. When the 12V supply comes
on, they energize. That leaves RY1, which you want to
drop out 200 seconds after the 12V supply drops out. It is a
5V relay. Here's a circuit that may work for you. It will
drop out RY1 after an adjustable delay.

+12 ---[R1]---+----+ +---- +5
| | |
| | [RY1]
| | |
[ZD] | c/
| +-----+---| TIP 120
| | | e\
| [VR1] [C1] |
| | | |
Gnd (12V) ----+----+-----+-----+--- Gnd (5V)

Zd - 5V 1 watt
R1 - 330 1 watt
C1 - .1 farad 5.5 v super cap
VR1 - 50 K
RY1 - 5 volt relay

When the 12 volt supply drops, C1 begins discharging
through VR1 and the transistor. When the voltage across
C1 falls to about 1.2 volts, the transistor stops conducting,
and the relay de-energizes. If the 12 volts supply turns back
on before then, the transistor is held on by the supply,
and C1 re-charges.

Ed


Wow - what a nice, simple, elegant solution. No IC's...........

Andrew VK3BFA
Click to expand...
Thanks! Ed
 
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