The equipment as originally installed has only one signal that goes to the buzzer. As installed, this buzzer is used to provide notice that the automation is beginning to move and also to provide an audible notice when the system is in an alarm state. What was not part of the original system is a warning/alarm light so in order to provide that to the maintenance folks I need to make use of the buzzer signal to provide that alarm. BUT, I do not want this new alarm light going off everytime the automation starts up and only when the system is in an actual alarm.
So, my intent was to tie the buzzer signal into a time delay relay (ON delay) so that I can filter out the automation start notice and only light the alarm light when the buzzer signal is active for more than, say, 4 seconds. The circuit I first came up with uses an ON delay relay that I set to 4 seconds, but becasue the buzzer signal pulses on/off about once a second the signal to the relay is never on long enough to trigger it to come on even when there is an actual alarm. So, if I condition the buzzer signal before the relay so that it produces a minimum pulse of, say, 2 seconds and is retriggerable then once the system goes into alarm the buzzer signal will pulse on/off once a second but this conditioned signal from the retriggerable will remain on so long as the system is in the alarm state. I can then use the time delay relay to control how long I wait to turn the alarm light on.
So, with my present circuit I'd have to add a 123/423 type multivibrator (retiggerable) but also add a voltage regulator to limit voltage to the 123/423 to 5VDC or 12VDC and I'd also have to add another opto-isolator between the output of the retriggerable and the 24VDC relay to prevent the output seeing 24VDC.
In summary then, the circuit would have...
An opto-isolator with Darlington output that takes the buzzer signal as input and have that feed the retriggerable input. The retriggerable would drive the input of another opto-isolator with Darlington output to drive the time delay relay. I would need either a 7805 or 7812 to regulate the voltage to the retriggerable. The opto's would be used to isolate the 24VDC from the 5VDC or 12VDC.
Now, if there is a retriggerable component that can run from 24VDC supply that would simplify the design a bit. I should also mention that the opto's with Darlington output have a minimum gain of 1500% and I have about 6ma going through the LED giving me a minimum of 90ma at output.
Brian