Most modern garage doors come with a light beam and detector to stop motion when someone is in the way. If you could get one of those, then maybe you could adapt it to your timer application. You may have to sacrifice the retro-reflector idea, since it takes twice as much power or sensitivity to use as compared to a emitter/detector pair. You don't need cables if the emitter and detector are each powered by their own battery.
I'm looking to buy or build a low cost timing set for our club events. Basically a retro reflective start beam (to avoid cables across the horses path) which a horse will ride through to start the timer, followed by a similar beam at the end of the jumping course to stop the timer. Timing range would be up to around five minutes with resolution of 0.1 seconds. Power supply to be via a 12volt car battery as the events are out in the field, no mains available. I've been pretty unsuccesful googling for such a device but can anyone point me at either a source for the unit or a design which I can build. (I have a fair bit of electronic construction experience)
Presumably because of the problems of deep discharge. I think the term leisure battery is used for the type you describe? The thinking behind my wordinig of car battery is that I have one which came off a wrecker and is in use sometimes to power temporary electric fencing (with regular recharging). Long term I will have to get an appropriate battery :-)
Thanks - I hadn't thought of electric eye having already looked for IR beam, blacklight beam and a few other variants. I'll see what comes up. In terms of the timer/display if there isn't a ready made cheap unit available, I had thought of using a crystal derived clock with dividers down to 0.1 seconds pulses then sequential CMOS 4017 decade counters with tenths, seconds, tens of seconds resetting at six and so on up to tens of minutes. I doubt if there will ever be a case of reaching 9minutes 59.9 seconds but if it does I'll arrange a count inhibit and alarm signal at that point. A manual reset will be applied before every competitor is called into the course
I think there are some up front design considerations that need to be incorporated in looking for the solution.
You need 4 batteries (2 if you use reflective beams) for a no cable solution, plus an RF link from one of the beam detectors to the clock (for either start or stop). If you can run a cable between the start and stop lines, it greatly simplifies things - no RF link needed, 1 battery (or 2 if you go non-reflective on the beams).
Assuming you can do it with a single battery, the car battery is fine, but nevertheless wrong. The current requirements for the electronics should be small, well under an amp total, with properly designed electronics. A car starting battery wouldn't have a problem powering the setup for a couple of days. But gel cells are more practical for this application.
You also need to consider the mechanical setup of the beams. "There is no way a horse will ever run into the start line post to which the the beam is affixed." Hello, Mr. Murphy! The point is that if something happens during the event to cause either beam to be misaligned, you need to be able to adjust it quickly without delaying the event extensively.
Some input from those who have done this sort of thing might be real helpful. To me, the electronics isn't the problem - it's the overall design that is worrysome.
My initial concept was for one battery with cables to the start and stop "gates". Each gate would be reflective so no cables across the horses path. The gates are at the entry and exit points to a roped off area so the cables (extra low voltage) will be run along the "fence line" out of harms way. The long cable (stop gate) would normally be maximum of around 60 metres in length
I appreciate the battery I already have is not ideal but it will be in use anyway for the pa system and for its remaining life it will serve. When it finally dies it will be replaced with a more suitable type.
My concept would be to have a simple sighting tube on the tx/rx head to point it more or less at the reflector and the beam arriving back at the head unit would be monitored by an led on the head to confirm its presence. The professional (hired) setups I've seen but not studied at shows are normally mounted on tripods which rarely get knocked over or disturbed but I take your point they need to be easily and rapidly put back into alignment.
Having seen the sort of thing at shows it seems that commercial (but expensive) sets work on the same general principles as I have in mind. I have recently found that the security industry make break beam sets which run on 12 volt dc supplies and can be found on ebay for a few pounds. This will take care of the start/stop gates, now I have to sort out the timer. I'm minded to go down the CMOS 4017 road to do this for the counting, using a crystal oscillator and divider so I have a 10Hz clock pulse. I havent got a mains supply in reach or the power could come from that and I could divide down the 50Hz for clock pulse. A 0.1 second counter overflowing into a 1 second counter, then a tens of second counter with a reset pulse derived from the 6 outlet for 60 seconds producing a 1 minute pulse to count and so on. Reaching 9 minutes 59.9 seconds would inhibit the count and show overflow but typical rounds are over within 3 or 4 minutes. All counts resettable to zero by a manual reset for next competitor. I can make an etched pcb using copperclad board with ferric chloride and an led display using 7 segment blocks should not be difficult too Does this sound ok to you or can you throw in any other suggestions? (Ideally I need to be producing a working unit for next season which starts in March)
Sounds like you've already given a good deal of considertaion to the issues involved. I'll mention some other points below.
I'd like to see the clock isolated to its own battery. That is an easy way to keep all kinds of "ugliness" (electrical noise) from disrupting the clock. The 60 meter cable should not be a problem. You can use opto isolators on the sensing cables to keep noise out of the clock.
On the battery: from the electronic perspective, a car starting battery is fine for this application - it is a non-issue. The current draw will be way too low** for any consideration of needing a deep-cycle battery. The reason to go to a gel cell is for practical portability - gel cells can't spill, while a car starting battery can.
**= I don't know what the burgular alarm beams draw - I assume well under 1 amp.
The burgular alarm beams may cause you a problem. Murphy says that you will have your sensor aimed into direct sunlight at some point, and your beams need to work reliably in sun as well as shade. In addition, they need to function reliably over a wide degree of temperature.
The FPDK 26N5303/S14 made by Baumer Electric would seem to satisfy your requirements.
type retro-reflective sensor (rrs) light source pulsed red diode actual range Sb 10 m nominal range Sn 11 m polarization filter yes soiled lens indicator / alignment aid flashing light indicator light indicator LED yellow sensitivity adjustment Pot, 270° wave length 660 nm response time / release time < 1 ms voltage supply range +Vs 10 ... 30 VDC current consumption max. 55 mA current consumption typ. 45 mA voltage drop Vd < 1,8 VDC output function light / dark operate output circuit NPN output current < 200 mA short circuit protection yes reverse polarity protection yes width / diameter 25 mm height / length 80 mm depth 58 mm type rectangular housing material plastic (ASA) connection types connector operating temperature -25 ... +65 °C protection classes IP 67
Coupled with a bicycle reflector on the opposite side this would be more than adequate. Your track is probably no wider than 5 meters and a larger reflector (60mm) and the fact that this sensor has an LED to faciliate beam alignment you would be able to line the emitter-detector in a few minutes. This device has low power consumption and fits into your 12V DC power supply range very nicely. Being solid state, it also has fast on/off times.