555 timer circuit help

[groundhangers]

Im currently trying to turn a Standard stopwatch into a race timer for slalom skateboards.
The main problem faced is that a stopwatch uses the same button for start and finnish. The switch that i am using is a tape switch which goes on the ground for you to run over activating the stopwatch, the problem is when you run over it with your front wheels the timer starts but when you run over it with your back wheels the timer stops.

What i need is a simple 555 IC timer circuit so when i run over it with my front wheels it triggers the switch but when i run over the tapeswitch with my back wheels the switch is ignored for a period of 1-5 seconds before retriggering again.
I have looked through the net extenively and havent found anything suitable. I can build from a diagram but unfortunately i dont understand electronics that well to make a diagram.

Any help would be great.
Thanks,
Greg

 

[groundhangers]

Oops must work on 9v
Thanks

 

[analoguealan]

All is needed here is a monostable that is triggered by your front wheel and remains in the triggered state for a predetermined time (set by your design) and justlong enough to ignore the back wheel) before resetting.

I have never used the 555 timer (it is a hobbyist IC with very poor tolerances and not suitable for professional use) but I believe it can be configured to be a monostable.
See for example National Semiconductor web site and view application note for the 555.
The monostable output is logically ORed with a second monostable from your finnish tape. The first monostable starts your timer counter which should be stopped by a signal from your second monostable.

Analogue alan

 

[nbucska]

First : I have to know if the stopper input has to be
connected to GND or to 9 V and for how long to start or stop ?

I don't think you can do it with only two timers( 2 x 555 or 1 x 556 ) but it is certainly possible by adding additional parts :
1.) Some RC to generate short pulses
2.) some combinatorial logic e.g. some gate of the 4000
family.
3.) if you can't find out about the stopper input
some switching device ( MOSFET circuit or REED-relay)
------------------------------------------------------
If you need many, I would suggest using a cheeap PIC
microprocessor with the stopper or even doing the stopper
in software ( display cost may make this uneconomical)





Plesae read FAQ240-1032
My WEB: <

http://geocities.com/nbucska/>

 

[groundhangers]

I found one that is very basic and works perfect.