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Retriggerable time delay

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I have a request to provide an external/remote alarm light from a piece of equipment my company installed but the only signal source is a buzzer that goes off when the system is in an alarm state but also beeps when the automation begins to move. So, I built a simple circuit with an opto-isolator with Darlington output that power both the buzzer (25ma) and a time delay (on delay) relay (also about 25ma). I used the opto-islolator because I did not want to increase the loading of the drive circuit powering the buzzer. The on delay relay was chosen to filter out the short duration ( under 2 seconds) beeping when the automation starts so that the alarm light only goes on when the buzzer alarms for more than the time delay setting.

Now, I made the mistake of assuming that the beeping was driven by the buzzer itself and the the signal to it was not pulsing -- WRONG. So now I need to add a retriggerable delay of, say 3 or 4 seconds so that when the system goes into alarm the ouput of the retriggerable will remain on and then the on delay relay, set to somthing more than the 3-4 seconds, will operate correctly.

I could use something like the 74HC423 but since the system is powered by 24VDC I'd prefer a retriggerable able to tollerate 24VDC without having to add a 7805/7812.

So, any recommendations on component choice for the higher voltage or should I just use a 123/423 component and add the 7805/7812?




I have read your post, and the first paragraph is clear enough.
However, the subsequent idea/circuits are not completely clear.

It seems that you have a pulsed signal for the automation alarm buzzer, and then you have a steady state signal for the alarm light.

Is there any chance that you might employ a charging capacitor with a resistor shunt to accomplish your design goal?
The idea is that the pulsed signal would charge the capacitor, but the bleed/shunt resistor would discharge this capacitor, except when the signal was steady state, at which point the alarm light would turn on.

Just a rough concept .... whatever you think.


New Member
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.

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