voltage controlled one shot timer

[MikeHS]

Hello,
Need to design or come up with a 10 to 20 second delay timer that auto-resets each time a +12 to 24VDC is applied.
The timer source voltage (Vcc) can be a separate input, say +5V, or the same +12 to 24V. The one shot output needs to be a positive pulse to drive an SCR gate terminal...but only after 10 to 20 seconds of STEADY 12 to 24V input. Short bursts of +12 to 24VDC need to be ignored.

I would appreciate any help, ideas or suggestions.

Thanks for thoughts.
Mike....mwh87120@yahoo.com

 

[MacGyverS2000]

A cheap processor (e.g., PIC)...

OR

A couple of 555 timers...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[DRWeig]

Look at the IDEC FL1 series.

Good on ya,

Goober Dave

 

[VEBill]

Conceptually - look-up a 'missing pulse detector' cook-book circuit for a 555 timer. But you'll probably need to sort out the polarity of the inputs and outputs.

 

[MikeHS]

Thank you for the replys thus far. I am attempting to avoid any programming and prefer a KISS solution, if possible. Low parts count is part of the KISS solution, IMH0.02O. Regarding the 555s... sounds interesting. Any more ideas would be appreciated.

Mike

 

[IRstuff]

I think you need to relook at which solution is actually the more complicated one. A single PIC with a couple of resistors will do the job you've described.

You would need at least the 555 and a PLD to implement the state machine required to handle logic. This would be in addition to the resistors and capacitors needed by the 555 and the crystal oscillator needed to clock the state machine. And to implement the 10 second timer would cost you another 555, potentially, although one could implement a countdown time in the PLD, but you'd need to get one that could handle a sufficiently large count, while still being able to ignore the short pulses.

TTFN

FAQ731-376

 

[VEBill]

Google this: 555 event failure alarm Mims. You should find the Google Books outrageous copyright violation of Mims' Fig 2-19. Focus on just the 555 and associate parts, ignore all those silly gates on the output for now.

The transistor and switch on C1 need to be replaced with a diode (and maybe a resistor) to discharge C1 when the input voltage disappears (needs to be pulled low, not just open circuit). Basically you need to reset C1 to zero volts when the input goes low. Easy.

Next issue is that the 555 output is the wrong polarity (open collector IIRC). Perhaps use a simple PNP transistor circuit on the output. You should probably use a good capacitor (1uF or so) on the SCR gate to filter noise and glitches.

Double check the maximum voltages for your chosen 555. 24 is probably too high. Inputs and power supply might need to be knocked down.

All above is just conceptual. A suggestion.

 

[MikeHS]

Hello again and thanks for replys...

Let me add one comment to my intial posting, just as a clarification.

only after 10 to 20 seconds of STEADY 12 to 24V input. Short bursts of +12 to 24VDC need to be ignored. ... New Part... Also, the total absence of the 12 to 24VDC input needs to be ignored. The timer delay to begin ONLY with a STEADY 12-24V on the input or "sense" terminal.

Sincerely,
Mike

 

[IRstuff]

Is it ever noisy? Will there be 10V pulses or steady state?

TTFN

FAQ731-376

 

[VEBill]

Seeing your latest post, I realize that I didn't explicitly state that the input signal (used to charge C1 on Mims 555 diagram) can be separated from the PS if the circumstances demand it.

And the noise and other issues mentioned by IRStuff need to be carefully considered. There is obviously an infinite range of possible signals that would defeat a simple 555-based circuit. In that case, then you would have no other choice but to use a computerized solution where you could describe the acceptable and unacceptable pass/fail criteria in as much excrutiating detail as you wish.

 

[MikeHS]

While nothing is totally noiseless... I do not see the application here as being overly burden by noise. I'm still favoring a 555 type or similar discrete part solution.
I haven't looked at a Mims solution yet but hopefully it will work. There is no need to reinvent the wheel. I'll try to locate a Mims book. I know that RS use to carry a lot of his articles. Thanks again for the ideas.

Mike