Double pulse detection circuit

[mbabayan]

Hello,

I'm trying to design a circuit that would produce an output pulse (.5-1 seconds) when double pulse on the input is detected with a time gap of <=.5 seconds or so.
This is supposed to be a gadget that activates a garage opener on the double-tap of the "flash-to-pass" switch on the motorcycle. Devices like this are available commercially, but I'm not paying $100 or so when I can build it
I've came up with the following design in LTSpice, but I'd really appreciate an opinion on it before I proceed with prototyping.

It is based on a timer and a counter - when input pulse is detected, 555 in monostable mode gets triggered, releasing the reset on the counter. If a subsequent pulse is detected withing the output pulse of the monostable, it will switch the counter into subsequent position, thus activating second monostable to produce an output pulse of preset duration. Otherwise, the first monostable will "reset", disabling the counter. Does this make any sense whatsoever?
The circuit along with the simulation plot is attached.

Any feedback is greatly appreciated.
-MB

 

[luben111]

I would suggest you use microcontroller - you can put the whole fucntionality into some small package micro

 

[mbabayan]

Yeah, I know I can do this with MC - but it seems such an overkill for this. Besides, I'm a software engineer by trade - so it's a lot easier for me to do this in software (well, firmware), but where's the challenge ?

 

[luben111]

Believe me, if you try properly to detect the double click with all adaptive timings, to make it very reliable is a big challenge. It looks simple, but if you have to make it perfect it's quite complex task. The challenge is in the firmware quality which will result at the end in reliability. You can also use RTOS inside (like SALVO - Pumpkin: Salvo, the RTOS that runs in tiny places) - you'll see the whole project from completely different angle.
But if you just want to learn how to work with 555 and 74HCxxx - it's a good start.

 

[mbabayan]

I agree that if I needed a precise timing detection, it would have been a much more serious undertaking. All I'm talking here is detection of a double tap of a NO switch withig certain time period.
Would somebody chime in on feasibility of my proposed ckt?

Thanks,
-MB

 

[betwixt]

Go for a micro. The cost of say a PIC10F200 is less than a single NE555, it's the same physical size, needs little or no external components and can do the job in a few lines of code.

Brian.