Simple time delay circuit

[Saulius]

Hello

I'm rather interested in electronic, though i'm kinda green at it. So i have this problem i need to solve at work:
         --I have 20mW DC signal from a electronic force gauge when sertain force is aplied;
         --With that signal i need to switch a relay quickly on and off and after one second (1s) delay turn the same relay on and off again (this would stop the machine at certain force and keep the applied force for 1 second then remove the force(pun intended));
Im looking for simple quick to build solution.
Does anyone have any suggestions. Any ideas would be greately appreciated.

I thought of using some sort of an opamp and a 555 timer, but don't quite know the detailed execution for these.

Thank you

[Falcon69]

You can use an Astable 555 Timer circuit for it I believe. I am working on something similar.

I have a post a few lines down showing my schematic using a relay to switch things on and off, similar to what you are doing.

You can use a direct signal to a relay to switch it immediately off/on as you want, then use a delay timer to switch it back on after the 1 second. Just daisy chain the input of the relay to switch it on/and off again using the timer.

You can also buy one premade off ebay and do it that way. They are cheap, about $2.

http://www.ebay.com/itm/New-1pcs-DC-12V-Pull-Delay-Timer-Switch-Adjustable-Module-0-to-10-Second-DT-/252135253965?hash=item3ab46ea7cd:g:OmQAAOSwwbdWJ5U9

But, depending on your amp load needed, you may need to build your own. Those cheap ebay ones only appear to handle about 10A.

[Seekonk]

Did you intend to say 20mV?  Probably have to amplify that a little for most solutions. You think a 555 is the greatest. I'd rather stick my nits in a vice than use one, but you go and have your fun.  They are sensitive to supply voltage and will false trigger.   Learn to use some micro.  I used to be a pot turner a long time ago.  Digital is just so nice.

[alsetalokin4017]

Did you intend to say 20mV?  Probably have to amplify that a little for most solutions. You think a 555 is the greatest. I'd rather stick my nits in a vice than use one, but you go and have your fun.  They are sensitive to supply voltage and will false trigger.   Learn to use some micro.  I used to be a pot turner a long time ago.  Digital is just so nice.

Well.... while I use 555s where they are appropriate, in this case I have to agree. This is trivial to implement with Arduino and a couple of mosfets. Probably don't even actually need electromechanical relays, just use the mosfets.

You might need an amplifier stage if the input signal is really only 20 mV above ground.

I wish we knew more about the actual input signal from the force gauge, and also what is being controlled by the relays (so as to decide whether mosfets can be used instead).

[Saulius]

Did you intend to say 20mV?  Probably have to amplify that a little for most solutions. You think a 555 is the greatest. I'd rather stick my nits in a vice than use one, but you go and have your fun.  They are sensitive to supply voltage and will false trigger.   Learn to use some micro.  I used to be a pot turner a long time ago.  Digital is just so nice.

Yes I meant 20mV thanks for correction. I concider every option, didital is also one of them, although this can have it's own debugging problems on is't own, so ill chose the quickest solution there is cause i have only 2 days left to through in a prototype, and my programing is not that great at the moment:(.

Thanks for suggestion.

[Saulius]

Did you intend to say 20mV?  Probably have to amplify that a little for most solutions. You think a 555 is the greatest. I'd rather stick my nits in a vice than use one, but you go and have your fun.  They are sensitive to supply voltage and will false trigger.   Learn to use some micro.  I used to be a pot turner a long time ago.  Digital is just so nice.

Well.... while I use 555s where they are appropriate, in this case I have to agree. This is trivial to implement with Arduino and a couple of mosfets. Probably don't even actually need electromechanical relays, just use the mosfets.

You might need an amplifier stage if the input signal is really only 20 mV above ground.

I wish we knew more about the actual input signal from the force gauge, and also what is being controlled by the relays (so as to decide whether mosfets can be used instead).

The signal from the gauge is at 0V until the force threshold is reached, after the threshold is reached the signal ramps up to 20mV continuosely until the force goes down below the threshold, than it goes back to zero again.

The relay is used to bypass the manual togle switch inside the machine.

[Paul Moir]

Well.... while I use 555s where they are appropriate, in this case I have to agree.

As someone who does some industrial one-offs, I'll present an opposing case. 

1)  Often development effort is the critical cost, and on a project with no other software, this adds another large step compared with calculating a single R and C.  Just setting up the development environment or writing the documentation consumes a lot of money.
2)  You want to use parts that can be replaced.  Anyone can replace a 555 even 30 years from now, but a programmed micro is more difficult.  I usually provide extra programmed parts, but that's not the best either.
3)  Minor, but a micro introduces unique failure modes that sometimes must be considered.

However, if there is even a hint of some extra functionality being added, go with a micro.  (Or the time delay is long, or it needs to be accurate, and probably 6 other "or"s.)

[MarkS]

3)  Minor, but a micro introduces unique failure modes that sometimes must be considered.

The first two I can deal with, but this concerns me. What failure modes? My plan was to use one of Atmel's 6 or 8 pin MCUs for the power on and manual reset for the retro 68000 computer I'm building. It would eliminate a load of extra components, basically bringing it down to the MCU, a switch and a decoupling cap.

[Paul Moir]

Oh things like bit flips causing the program to crash.  Not a big deal in situations where you can "reboot" the device easily enough but problematic in other situations where it's hard or not expected to be a remedy.
If you want to discuss it further bring up one of your topics and I'll keep an eye out.  I think we're both getting pretty off topic here. 

[MarkS]

I think we're both getting pretty off topic here. 

Good point. Not trying to get off topic, but I was coming to this thread to post that he should consider using a MCU and then I saw your post. Had to ask.