# Simple delay circuit, prevent double inputs

#### skate323k137

Joined Sep 21, 2009
4
Hello

this seems like a great and knowledgeable forum. I've searched around, but I think the circuit i need to construct is different from most "delay" circuits. What I need is a essentially a switch that, once pressed, will not be able to close the circuit for another 2-4 seconds. I can't imagine needing more than some capacitors, resistors, maybe a relay or transistor.

some background, all I'm really doing is wiring tape switches to the inputs for a stopwatch. I'm really good at soldering, thats no issue. My friends and I are downhill skateboarders, and the problem is our two axles will either stop or reset all the stopwatches I've found because of the second "press" from our rear wheels. I don't want to interface them [the tape switches] to my computer and use software, because out in the 'wild' i won't have a timing system without power for my computer.

I imagine 3-5 volts would be the momentary voltage, it if matters that much. If i need i can go get a stopwatch, solder in some leads, and check for sure.

the best idea i have so far, is to wire the stopwatch to the NO/COM on relay, and wire the tape switch with its own power source to the coil. then, Would a resistor and a big enough capacitor on the tape switch side do the trick (by taking time for the cap to charge to close the circuit in the relay again), but i know i'm missing something as the tape switch would be an open circuit so the cap wouldn't charge. I really don't construct my own circuits, more repair and modify existing ones. any help is greatly appreciated.

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#### Ron H

Joined Apr 14, 2005
7,014
I'm assuming that the tapeswitch closure starts a stopwatch. How do you stop it?

It seems to me that a better solution than a delay would be to set a latch when the switch closes. Then it would ignore the second closure caused by the rear wheels. Of course, you would need some power (a 9V battery) to run the latch.

#### skate323k137

Joined Sep 21, 2009
4
I've got plenty of tape switches, so one tapeswitch would be placed at top and bottom of the hill. we just run cat3 or cat5 down the hill for the finish line switch. I could wire the tapeswitches to seperate start and stop/reset buttons, or both switches to one start/stop button depending on the stopwatch I buy.

edit, holy cow i might actually understand those things. I'm looking at a SR (bistable?) latch. would I wire start to S and Q, and stop to R and "opposite" Q ?
another edit, it looks like a 2coil latching relay would be cool, if i can find a couple of those I should be set.

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#### BillB3857

Joined Feb 28, 2009
2,536
If you trigger a one-shot MV with the switch and use the output of the MV to start the stop watch, you could adjust the MV time so it would "hold down" the start button on your stop watch as long as it needs to be. Many one-shot circuits are available. A 555 timer could also be used.

#### skate323k137

Joined Sep 21, 2009
4
Thanks guys.

I ended up finding a double coil DPDT latching relay. I didn't even know what a latching relay was until today, but it got the job done for $5. the tape strips activate the set coil, which in turn activates the start/stop button of a stopwatch on one of the relays outputs. A button is wired to the reset coil on the relay, so a timekeeper would have to press that button to open the circuit before the start, and again any time between start and finish. no big deal. I used the other available circuit on the relay to wire an LED to NC, so I've got an indicator that the system is ready for start/finish input and no sensors are shorted. I hooked a nice big pushbutton to reset, and put in a manual start/stop as well in case a racer falls and the lane needs reset. all in all, between battery cases, relay, phono jacks to accept the tape switches, and the stopwatch i was around 30-35 bucks. a steal for what usually costs at least 6 times that much. #### Ron H Joined Apr 14, 2005 7,014 Thanks guys. I ended up finding a double coil DPDT latching relay. I didn't even know what a latching relay was until today, but it got the job done for$5. the tape strips activate the set coil, which in turn activates the start/stop button of a stopwatch on one of the relays outputs. A button is wired to the reset coil on the relay, so a timekeeper would have to press that button to open the circuit before the start, and again any time between start and finish. no big deal. I used the other available circuit on the relay to wire an LED to NC, so I've got an indicator that the system is ready for start/finish input and no sensors are shorted. I hooked a nice big pushbutton to reset, and put in a manual start/stop as well in case a racer falls and the lane needs reset.

all in all, between battery cases, relay, phono jacks to accept the tape switches, and the stopwatch i was around 30-35 bucks. a steal for what usually costs at least 6 times that much.
Excellent! You sound like a resourceful guy (or gal ).