I need to build a switch that breaks when a mountain bike crosses it?

Just about to run a downhill mountain bike race. Need a switch to help record times at the bottom of the downhill. Has anyone got any ideas?

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You might try making one of those rubber hose detector that are often used for recording road traffic. Plug one end of the hose and stretch it across the track. The other end is connected to a pressure sensor that is triggered when the tire compresses the air inside the hose. I'm guessing the ones used for cars can get away with a simple diaphragm switch due to the larger displacement of car tires. You might need to use a very small diameter piston to be sensitive enough to a bike. A softer tube may also be necessary. I'm not sure if even this would work, but using a solid state pressure sensor connected to a microcontroller should definitely be sensitive enough. Another thought: filling the hose with water may improve the response.
You have a jump ramp, right? Here's what you do. You need a 'jump ramp' type setup for the bicycles to run over. For the switch, find an old microwave with the good door switches that have the metal levers on them or just buy them. There should be at least 3, some N/O (normally open) and some N/C (normally closed). N/O makes a circuit when the bike runs over and N/C turns off. Use either one depending on what you're attaching to it. You only need one switch. For the ramp, probably a piece of 2x4 is good for the brace and 2 pieces of plywood for the top and bottom. The pieces of plywood have to be the same size. The other pieces are just a spring and a piano hinge. The bottom piece of plywood just lays flat on the ground. On one end of it will be the piano hinge and on the other the piece of 2x4. Attach the spring in between, close to the 2x4. Nail, or better yet, screw this together. Then on the top goes the other piece of plywood. Attach the other end of the hinge to this piece, but don't attach the other end. You now have sort of a wood sandwich. On one side you have a 2x4 and a spring to hold up the top 'bun' so it doesn't touch the 2x4. On the other side is a piano hinge to connect the 'buns'. The spring goes between the bottom and top pieces of plywood so there's a gap between the top and the 2x4. Then when someone rides over the ramp, it compresses the spring a little bit and they touch. Now to install the switch, push down on the top piece to compress the spring so that it touches the top of the 2x4. Then mount the switch to the 2x4 so that it closes when there's weight on the top. Then your wires go in place of the button on your chronograph to stop it. I'll try and draw a picture.
Switch Ramp.jpg
Rishnai8 years ago
Why use a laser? Use two copper straps held apart by and glued to a strip of soft foam that has a channel cut down the middle.
Rishnai Rishnai8 years ago
*In case anyone else is looking at this question for ideas.
Mrmtbinthenaki (author) 8 years ago
Thanks for the help dudes.I"m more a track builder than a tech wiz. I was thinking of a laser pointer we hane got a hand held stopwatch with a printer.It has a cord that has a button trigger which I was thinhing of pulling apart and putting a laser trigger on it.. Any ideas on what I would need for this?
Iknow someone who made a set of timer gates for a race of some sort. they had a laser pointing on to a photo transister (I think), they then got one at the top and one at the bottom and conected them to a clock chip. the timing was started/stoped by the two beams being brocken. hope that helps.
UziMonkey8 years ago
IR sensors don't always work so well in the sun, lasers won't have that problem. You can build one for cheap with a cheap laser pointer, a photoresistor or phototransistor and a ballpoint pen for a tube around the sensor. You can have this trigger whatever timing device you're using.
How about an IR beam device that triggers a relay. If you aim it at about chest height, there should be enough interruption to flip the relay and stop the clock