Any ideas?

D

Dingus

Guest
Hi,

I have a circuit in which I need to enable 4518 counters for as
long as a signal is present on an input to the circuit. The input is
240VAC via a 4N25 optocoupler.

The clock input to the 4518's is a 1kHz signal - the idea is to count
pulses as long as the 240VAC signal is present, a few milliseconds.

The problem I have is this:
I need a fast rise/fall time clean DC signal applied to the enable input
to ensure an accurate count.

The signal I get from the above scenario is pulsed DC from the 4N25
(50Hz) If I attempt to smooth it, the smoothing leads to a slowing
up of the rise/fall times of the turning ON/OFF of the AC for obvious
reasons.

Any ideas how to get a good clean go/no-go DC signal from an
AC source??

Cheers,

Dingus.
 
"Dingus" <baas@inindabababa.zu> wrote in message
news:E2nPc.8094$N77.401393@news.xtra.co.nz...
Hi,

I have a circuit in which I need to enable 4518 counters for as
long as a signal is present on an input to the circuit. The input is
240VAC via a 4N25 optocoupler.

The clock input to the 4518's is a 1kHz signal - the idea is to count
pulses as long as the 240VAC signal is present, a few milliseconds.

The problem I have is this:
I need a fast rise/fall time clean DC signal applied to the enable input
to ensure an accurate count.

The signal I get from the above scenario is pulsed DC from the 4N25
(50Hz) If I attempt to smooth it, the smoothing leads to a slowing
up of the rise/fall times of the turning ON/OFF of the AC for obvious
reasons.

Any ideas how to get a good clean go/no-go DC signal from an
AC source??
Click to expand...
If you are using 50Hz AC, one cycle will last 20mS. I'd suggest that if you
are expecting the AC to last only a few mS, then you are in trouble getting
an accurate reading. How range of time do you anticipate "a few
milliseconds" to be?

You can expect around 20mS (one cycle) of uncertainty in your measurement as
you will have to wait to see if the next cycle gets there.

Mark
 
"Dingus" <baas@inindabababa.zu> wrote in message
news:E2nPc.8094$N77.401393@news.xtra.co.nz...
Hi,

I have a circuit in which I need to enable 4518 counters for as
long as a signal is present on an input to the circuit. The input is
240VAC via a 4N25 optocoupler.

The clock input to the 4518's is a 1kHz signal - the idea is to count
pulses as long as the 240VAC signal is present, a few milliseconds.

The problem I have is this:
I need a fast rise/fall time clean DC signal applied to the enable input
to ensure an accurate count.

The signal I get from the above scenario is pulsed DC from the 4N25
(50Hz) If I attempt to smooth it, the smoothing leads to a slowing
up of the rise/fall times of the turning ON/OFF of the AC for obvious
reasons.

Any ideas how to get a good clean go/no-go DC signal from an
AC source??

Cheers,

Dingus.
Click to expand...
Use a schmitt trigger.
 
Yea Mark, thats the problem - I need to measure between say 80 to 300mS.
If I stay with enabling using the pulsed AC input as 20mS then I would loose
quite a few input clock pulses as they are coming in at 1kHz.

I'm thinking of toggling a 4013 multivibrator with the 20mS signal smoothed
only enough to keep the 4013 in a toggled state for enough time to keep the
counter enabled.
Failing that I might have to look at plain smoothing of the
input to provide a good clean DC level to enable the counter and compensate
the derived turn-off overrun by reducing the input frequency to (1kHz - nnn)
etc.

Dingus.

"Mark Little" <mark@au.id.little> wrote in message
news:410e1909$1@duster.adelaide.on.net...
"Dingus" <baas@inindabababa.zu> wrote in message
news:E2nPc.8094$N77.401393@news.xtra.co.nz...
Hi,

I have a circuit in which I need to enable 4518 counters for as
long as a signal is present on an input to the circuit. The input is
240VAC via a 4N25 optocoupler.

The clock input to the 4518's is a 1kHz signal - the idea is to count
pulses as long as the 240VAC signal is present, a few milliseconds.

The problem I have is this:
I need a fast rise/fall time clean DC signal applied to the enable input
to ensure an accurate count.

The signal I get from the above scenario is pulsed DC from the 4N25
(50Hz) If I attempt to smooth it, the smoothing leads to a slowing
up of the rise/fall times of the turning ON/OFF of the AC for obvious
reasons.

Any ideas how to get a good clean go/no-go DC signal from an
AC source??

If you are using 50Hz AC, one cycle will last 20mS. I'd suggest that if
you
are expecting the AC to last only a few mS, then you are in trouble
getting
an accurate reading. How range of time do you anticipate "a few
milliseconds" to be?

You can expect around 20mS (one cycle) of uncertainty in your measurement
as
you will have to wait to see if the next cycle gets there.

Mark
Click to expand...
 
The schmit trigger's output will mimic the ripple input. Need to be able to
latch
on first pulse and release on last pulse!

Thanks for reply.


"Andrew" <a.pearson@no.spam.aasiascales.com.au> wrote in message
news:410ea852@dnews.tpgi.com.au...
"Dingus" <baas@inindabababa.zu> wrote in message
news:E2nPc.8094$N77.401393@news.xtra.co.nz...
Hi,

I have a circuit in which I need to enable 4518 counters for as
long as a signal is present on an input to the circuit. The input is
240VAC via a 4N25 optocoupler.

The clock input to the 4518's is a 1kHz signal - the idea is to count
pulses as long as the 240VAC signal is present, a few milliseconds.

The problem I have is this:
I need a fast rise/fall time clean DC signal applied to the enable input
to ensure an accurate count.

The signal I get from the above scenario is pulsed DC from the 4N25
(50Hz) If I attempt to smooth it, the smoothing leads to a slowing
up of the rise/fall times of the turning ON/OFF of the AC for obvious
reasons.

Any ideas how to get a good clean go/no-go DC signal from an
AC source??

Cheers,

Dingus.

Use a schmitt trigger.
Click to expand...
 
Dingus wrote:

Hi,

I have a circuit in which I need to enable 4518 counters for as
long as a signal is present on an input to the circuit. The input is
240VAC via a 4N25 optocoupler.

The clock input to the 4518's is a 1kHz signal - the idea is to count
pulses as long as the 240VAC signal is present, a few milliseconds.

The problem I have is this:
I need a fast rise/fall time clean DC signal applied to the enable input
to ensure an accurate count.

The signal I get from the above scenario is pulsed DC from the 4N25
(50Hz) If I attempt to smooth it, the smoothing leads to a slowing
up of the rise/fall times of the turning ON/OFF of the AC for obvious
reasons.

Any ideas how to get a good clean go/no-go DC signal from an
AC source??

Cheers,

Dingus.


Use a zero crossing detector to get a pulse twice every cycle, then a
Click to expand...
missing pulse detector, say a 555 set to 12mS. Then 2mS after the pulse
doesn't turn up, you get your trigger pulse.

http://www.elecdesign.com/Files/29/6225/Figure_01.gif
http://www.uoguelph.ca/~antoon/circ/mispulse.htm

Glenn
 
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