Measuring the time between two electrical pulses.

[Daniel Watkin]

Hello. This is my first post here. Name is Daniel and have been interested in joining for a while but never had a good reason to do so, I'm completely new to this and know nothing, but would like to learn, if that enough?. I wasn't sure where to put this please tell me if its in the wrong place. So this is what I need, I'm assuming this is doable, but needs to be done in a cost effective way, otherwise its just worth buying what I need. So here is the story. I race radio control cars as a hobby, we would like to know how fast our cars go, but can't really afford a speed gun. I have two IR beam break detectors that set off a buzzer when broken. I was thinking that the buzzers could be removed, and linked to something else that times the space between when the two beams are broken,when placed a known set distance apart and then work it out from there. Is that even possible? What would I need to time this with? is there a cheaper way to do it? I could sound like a complete idiot right now and not know haha. Anyway, thanks for reading if you got this far, any advice appreciated!

Daniel

 

[Harald Kapp]

Welcome Daniel to our forum.

Of course you can build a timer circuit that is startet by the first detector and stopped by the second detector. SInce you admit having no experience. th eonly way that makes sense would be buying a kit for such a stopwatch.
With some knowledge, you may be able to modifiy an off the shelf stopwatch.

A little thinking outside the box: Can you use a PC or laptop at your racetrack? If so, you can connect detector 1 to e.g. the left audio input channel and detector 2 to the right audio input channel. When starting the race, record the two signals. After the race you can measure the time interval between the two signals using any audio editor - or find someone who can write a little piece of software that does the measuring and displays the time directly. Googling "PC sound card as timer" I found this interesting piece. Obviously I' not the first to stumble across this idea.

Regards,
Harald

 

[Supercap2F]

What I would do is get a stopwatch with start and stop buttons. Put a optocoupler in pace of one of the buzzers (making sure that I did not exceed any voltage or current ratings) and use the transistor side of it in place of one of the start/stop buttons.

Dan

 

[shrtrnd]

Harald's idea is pretty ingenious if you can do it. I'm in the same boat as Harald in that without some kind of knowledge of electronic circuit building, you'd have to use something off-the-shelf.
There are probably plenty of simple circuits you could build to use your IR detectors (If you do indeed have two, not one IR beam transmitter and detector pair), to start and stop a timing circuit.
We'll be interested in your response to Harald's idea, to see if there's anything else helpful someone here can add.

 

[KrisBlueNZ]

I like Harald's idea too. Stopwatches don't normally resolve to better than 1/100th of a second and for such short times, they aren't usually very accurate (I did a big survey of stopwatches and discovered that most of them are pretty badly implemented). The sound card idea is a good one if you want to minimise design time. Alternatively, a few counters with seven segment displays (e.g. CD4026) clocked at a known frequency enabled by a latch that's set and reset by the broken beam detectors would do the job.

 

[JimW]

If your laptop has a parallel port, you might try a simple logic analyzer program. Just a cable and some free software. Use only two bits to connect to the IR sensors. Trigger on the first input. Read the time between them. Accuracy to 1 usec. Too much math to figure out how this would relate to MPH accuracy.

JimW

 

[daddles]

Another idea is to use a frequency counter in totalize mode. The input to the counter is a signal with sufficient frequency to give you the time resolution you want (say, on the order of 10 kHz). The first IR detector would turn the oscillator on and the counter would start counting. The second detector would turn the oscillator off. Read the count off the counter and scale it to get the elapsed time.

 

[Daniel Watkin]

There is some great ideas in here, thank you. I like the sound card idea, seems very easy to impliment in a reasonably short amount of time, and something I didn't think of at all. I'll have to go away and see if I can get that working first I think, before I try too over comlicate things for myself. Thanks for the great idea Harold!

 

[shumifan50]

Another idea is to use a video camera and a smartphone or a PC. Download a timer program on the smartphone that has the desired resolution. Then video the track with both start and end positions in view on the video and also make sure that the smartphone screen is in view and focus on the video. Later review the video and take the reading off the smartphone when the car crossed the start position in the video and again when the car crosses the end position. Thereafter it is just maths to get to speed.
This can be done with various variations: video the smartphone, with the timer running on the smartphone and video the track with the smartphone.