Scalextric Lap Counter Help

[alexb1406]

Ok, I see what you mean - my last post crossed "in cyberspace!" The base is referenced to ground via the 8.2ohm resistor - should that be connected after the diode? - it seems to work ok as it is, I don't think that is what is causing the false triggering.

 

[colin55]

You need to drive the whole circuit via a battery. Then see if it false-triggers.
Motors produce very high spikes. Try running a screwdriver in place of the car.

 

[alexb1406]

Motors produce very high spikes.

You may have hit the nail on the head there - That could explain why it worked yesterday when I placed a 100uf cap before the rectifier - (effectively filtering the output of the transformer - and any spikes from the motors).

I haven't looked at the transformer output in any detail but it may not be filtered at all - it wouldn't surprise me if it was only half wave rectified. For that reason I wondered whether a car battery may cure the problem, but I didn't really want to do that.

I've got a 1000uf cap now that I will try ahead of the rectifier - if a 100uf worked then I hope the 1000uf will!

 

[duffy]

**broken link removed**

This is a good suggestion - try this 100K pull down resistor Colin shows in the second pic. It will help eliminate a possible source of false triggers.

 

[alexb1406]

Done it!!!

I tried what you both suggested but that didn't work - I can't see though how adding the 100K resistor would make any difference as the 8.2ohm resistor (in parallel) will pull the base down to ground till a car goes over the track section. The overall resistance will be a fraction under 8.2ohms. Can you explain?

Then I tried adding a 1000uf cap with a diode as we discussed last night but that didn't work either - strange I thought as a 100uf cap worked last night without a diode. I then removed the diode leaving the 1000uf cap and that now seems to have cured all false triggers, so for some reason it doesn't like the diode. I have now connected the 1000uf cap directly on the output of the transformer and that works fine. I hope to get away with this one cap instead of having to put one in every lap counter. So it must be something to do with the filtering - or lack of - or more likely as Colin suggested earlier something to do with spikes from the motors.

Anyway, thanks guys, we did it!!!

 

[duffy]

Congrats!

The overall resistance will be a fraction under 8.2ohms. Can you explain?

The 8.2 ohm resistor is going to keep the anode of the diode clamped pretty firmly to ground. But after it triggers the first time, the other side of the junction has nothing pulling below +.7v - the diode is reverse-biased, the B-E drop is +.7V. So, it doesn't really shut off cleanly, just kinda bleeeeeeeds dowwwnnnn through the base. The transistor probably has a gain of 100, with the 100k resistor and the 10V supply that means it only takes 1 micro-amp of current to trigger it. If hunting for something noise sensitive, you could make a worse guess than this one.

 

[alexb1406]

Ok, I understand now thanks, I never thought of that! - I can't remember why I used the diode, is it necessary? I will struggle to place an extra resistor on the board, instead I could remove the diode and put a connecting link in its place. Or connect the 8.2 resistor after the diode? Which would be better?

 

[colin55]

Simply short-out the diode. It is not needed.

 

[duffy]

Yes, I don't think it's needed.

 

[alexb1406]

Great, thanks guys!!!