Time measurement between two points

Question

i need to measure the time between 2 points. I can not use a simple watch because i have no free hands for this. (mountainbike - downhill)

My first idea was to set two GPS points and measure the the time between those points, but I think it will be to inaccurate.

my second idea was to set one bluetooth device to each Point and just measure the distance from me to the bluetooth device , check the shortest distance and save the time.

But than I realized that I would need 3 bluetooth devices to make a distance calculation, which is kinda annoying.

Any idea how i can measure the time between two points with an accuarcy <1sec ? I want to control this via smartphone.

edit:

Okay i have another idea. Maybe i could set one bluetooth device to each point. I set the signal strenth to <=1m. Than i could easily start and stop the time until i receive the bluetooth signal on my android. Do you think this would be the best approach ?

Answer 1

It seems you're trying to take total and split times for a course. Definitively, forget of using bluetooth for other things that are not constant transmissions, with slow connection time... (I speak from experience). Of course you want to do it in the cheap, because there already professional solutions for that (e.g.). From my point of view, if you want something accurate and been able to integrate it with you phone, you'll need the following:

1. DIY photocell sensors. Maybe involving simple chip (e.g. msp430), phototransistor-diode pair, cheap 433mhz transmitter and batteries.
2. DIY 433mhz receiver for your phone. There already many boards that supports android open accessory library, (e.g. 49$ from sparkfun), so you only have to connect it to a 433mhz receiver and to your phone.
3. Custom code programming in your android phone. To be able to receive the signals from the split signal senders and keep track of all times.

It seems to be a pretty decent project for under $250-200(if you self made it :P).