Pulse stretching?

My 24vdc input will be fed from an opto isolator from 120vac.
The opto output has just a half-wave rectified DC output.
I reckon the plc input will probably switch off/on with the 60 hz supply cycle.
I need the input to sustain when ac input is there, but get off as rapidly as possible when it stops.
The plc is a Toshiba T1.
Would a single shot timer with a duration > than 15mS do this?

It's possible that the isolator may have a filter circuit and you won't have the pulsations. Check it out.

Thanks for that. Reading the makers specification, it looks like they did nothing in the way of filtering.
Assuming that's so , how would you deal with it to get sustained 'on' while the ac was present but get off in the shortest possible time when it goes away?

I would enable the 3/4 cycle time timer with a NC of the input. Use the Not Done of the timer as you ON indication. When the signal is off for more than 3/4 of a cycle then you will have your filtered OFF indication.

Sounds good

You can often set a filter in the software settings of a PLC to automatically filter out mains frequency on particular inputs. On the Siemens S7-224 that I am working on at the moment I go to System Block/Input Filters. I haven't used a Toshiba PLC so can't offer much help there.

Bryan

Thanks all. The currently working solution is to put an off delay timer known in Toshiba as TOF, with a duration of 25mS on this input.
This copes with the raw half wave rectified output of the opto ( a Transtronics opto120.)
With this particular timer TOF the output goes true when the input rises to the threshold. It remains true for up to 25 mS after the input falls below the threshold.
If the ac is still present, the timer register is reset to zero on every rising edge of the input ac & never turns off.
When the ac fails the timer output will go false within a maximum of 25mS.