time delay relay alternative?

Question

Hello,

I'm trying to set up a relay that will ignore short spikes in the control signal. Specifically, I'd like to activate the relay when a signal is at least 10 seconds long, but ignore anything shorter. I know I can do this with a delay on make relay, but it seems overkill for what I want to do. Are there any simpler alternatives? I don't need to adjust the delay time - a fixed delay would be fine.

jw0752 · Accepted Answer

Hi Craigt, The 555 is just a chip and the delay is selectable by the choice of R1 and C1 in my schematic. There are no plug in modules at a reasonable price though I am sure there are some multi function 24 V time delay relays available for industrial applications. The 24 volt input that you mention would require us to put some sort of simple voltage regulator on the 555 as it is limited to 15VDC max. It does, at this point, sound like your application would be fairly easy to adjust to but if you have no experience you may want to get an engineer or a technician to help with the design. Any ideas or design ideas that I put out on this thread are free for you to use but If you are going to use them for any critical process you must get verification from an engineer that they adequate for what you want to do and will work in your industrial setting. Anyone who is going to put a design together will need all the parameters of the application so they know the voltage going in and how big a load you are controlling as well as any other important aspects. You will also have to decide how you want the system to respond to different inputs. For example the circuit that I have drawn will respond to an 11 second input pulse with 1 second of relay actuation. Is this what you are looking for? There are often many solutions for a particular problem but the resolution will start with a good picture of your operational parameters and a clear description of your goals. Keep us posted how you are progressing. John

jw0752 · Answer

Hi Craigt; I had some extra time today so I bread boarded a circuit to delay the actuation of a relay for 10 seconds from the time power is applied to the circuit. Here are a couple pictures and a schematic. If you click on the images they will enlarge. I had to actually use 2 resistors on the breadboard to get the required 80K to make the unit trigger at 10 Seconds. John

jw0752 · Answer

Hi Craigt, We just talked about something similiar for Nick in another thread. The best way to do this would be to use an Op Amp or a 555 timer. You will have to pick a resistance and a capacitor with the RC constant that will meet the 10 second time you have selected. You did not mention the signal level that you are using, If it isn't digital you will need to convert it so that the digital signal can be properly read by the input of the 555. When the capacitor gets sufficient charge the 555 or op amp will turn on and this can trigger your relay. There a many sources for circuits like this available on the web. Here is just one example.

http://www.talkingelectronics.com/projects/50%20-%20555%20Circuits/50%20-%20555%20Circuits.html

John

time delay relay alternative?

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