Drag race Xmas tree project continued.

Ghetto style: A piece of plywood,

4 nails, A strip of spring steel, and the mechanics from a microwave dish spinner. Use inverter between battery and motor.

Isn't all that matters is premature start and distance time.?? The tree lights can be doing random timing. When the Go light lights...it means go and that can happen at any time. Atomic timing not required.

Unless it's standard that the 'Go' light is predicted by rhythm. Tap ...tap ...tap ... press gas! I suppose without rhythm and the race is part machine performance and part eye/foot response time.

This is a professional contract between the race organisers and me. An amateurish lash up has to be avoided as far as possible.

That's just it. It's entirely based on rhythm. Pros time their reaction and skill at anticipating the moment of 'Go' to the millisecond - literally. This is partly why I intend to change the lights to LEDs after this race. I'm using a crystal oscillator for the countdown timer and false starts will be detected to within a millisecond.

Anyway, my concern is not about possible loss of timing precision. It's about cross-coupling causing unwanted switching and cable reactance causing instability and slow turn-on, with consequent over-dissipation of the MOSFETs.

On a sunny day (Sun, 29 Nov 2009 04:57:20 +0530) it happened "pimpom" wrote in :

Why not use optocouplers at the MOSFET side?

opto coupler

--- R1 -- ---------------------------+ | | | --- |/ | \\ / / -| BULB --- |\\/ | | | | |--- _________| |_____________| |

??? Huh I dunno but this doesn't seem like the type of project to have that type of problem. Wouldn't the only cable going to the tree be a pair of wires to a single 'start sequence' button? Battery and all control electronics are on the tree.

No. It's not just a visual countdown system. The lights are linked with four optical sensors to detect and indicate correct positioning of the racers before the start and also unambiguously indicate a false start.

In addition to driving the lights, the control unit has to drive four pulsed IR LEDs and receive inputs from four IR receivers. The IR emitters have to be located directly in between the racers' starting position, the receivers on each side of the tracks at least 15 ft from the middle. The lights have to be some distance in front of the racers. All of this means a lot of wires going in and out of the main box. The start and reset buttons can be anywhere within a reasonable distance.

I would use a low value R at gate-source on the FETs, something close to the characteristic impedance of the cable, if you know that. Something around 200-500 ohms would seem appropriate. Delays caused by cable capacitance & inductance should not be an issue for your timing resolution. I did a project for a Christmas light show years ago and had success driving gates from 300 feet. I think you're going to be far removed from any source of external interference, so anything that gets you will probably be of your own making. I'd also suggest a TVS on the gates.

On your problem of timing... could you not measure the on-time of the headlights and build in a delay so the timing start trigger occurs at a point where the lights become visible? I know you said you've already finished the design so maybe this suggestion is not feasible now.

Thanks for the reply. It's reassuring to know about your successful experience. The distance I'll be working with will be about a tenth of yours. Switching speed is the one factor that's not crucial here - even a 100 µsec rise time shouldn't be a problem as far as timing resolution is concerned. As there won't be time for a major redesign, I was being extra careful in case someone could anticipate a problem I hadn't thought of.

That's a thought, and the design is not set in stone. But with the limited time remaining before the race, I'd like to avoid introducing anything but minor changes unless something makes it imperative. I'm working on the pcb design right now. For this particular event, I have to work with materials that I can quickly collect. For later races, I'm thinking of using triacs to switch readymade mains-voltage LED lamps, coupled to the control circuitry via optoisolators.

"Wingsy" schreef in bericht news:291120090525008823% snipped-for-privacy@nowhere.com...

I'd be carefull with that assumption. He's got high performance race cars idling just a couple of feet away from his electronics. Their design criteria might not include EMC guidelines!

Antoon

Not worth the candle, by using the same type bulb it will be very nearly equal over all the bulbs and not disturb the rhythm.

That might turn into a step backwards. Look into just how mains supply LED lamps work, there is a little SMPS in there.