555 self triggering.

[alec_t]

Would the fact that I replaced the 555 chip with a CMOS version make the trigger line more sensitive

We don't know how much current your relay draws, but a CMOS 555 might well struggle to drive it. That could cause malfunctioning if, for example, it resulted in the power supply being pulled down.

 

[JimB]

can you explain how this R/C filter works. Is it something like getting the unwanted spike to charge up the capacitor instead of finding it's way into the trigger line on pin 2?

That is a simplified explanation.

I prefer to think of it in two ways:

Time domain - a step change in voltage at the input side of the resistor will cause the voltage at the junction of the resistor and capacitor to change quite slowly.

Frequency domain - sharp edges and spikes are made of high frequencies.
The resistor and capacitor make a low pass filter, high frequencies are attenuated and the spikes are reduced or eliminated all together.

Does that help to explain it?

JimB

 

[cowboybob]

...Would the fact that I replaced the 555 chip with a CMOS version make the trigger line more sensitive, or am I barking up the wrong tree here?

Swap the LM555 back in and see.

 

[mark.w]

The on board relay consumes about 35mA. Is that too much for a CMOS device? I've had a quick look at a few data sheets and that would seem to suggest that it's well within the chip's capabilities.(up to 100mA?)
I have to say here that the circuit works perfectly if the noise glitch is disregarded....the relay pulls in without fail every time.

JimB
I really should have remembered that from ONC & HNC studies done about 25 years ago!! Thanks for the explanation.

cowboybob
I'll try swapping the original 555 back in and see if it improves things.

Thanks to all who are replying here. This must seem like a trivial, dumb problem to you all, but your help is truly appreciated.

 

[cowboybob]

... This must seem like a trivial, dumb problem to you all, but your help is truly appreciated.

Neither trivial nor dumb, I assure you. Frustrating? Yes, especially without the proper tools.

On that note, it's still worthwhile to put your analog volt meter on the S1 points at the PCB (with the stalk switch(s) hooked up). While observing the meter (should show battery level), switch on the HIDs. What I'm looking for is a very quick downward dip (or simply a change) with a return to its previous position. The meter can't respond fast enough (due to damping) to show the true voltage drop (if any) but a dip will be meaningful, .i.e., a large enough dip could cause a false trigger.

Don't give up on us just yet .

 

[mark.w]

Frustrating, yes...giving up?...not yet!

I think my neighbours must think I'm a bit nuts, as just lately, I've been hopping out to the car, sitting in the driver's seat furiously fiddling about with stuff out of the line of sight under the dash, only to see the same ritual unfold before them the following day!!

As said before, I won't be able to make any progress on this until the weekend, which is frustrating in itself as I'm keen to try the suggestions offered on here.
Watch this space!

 

[eTech]

mark.w

When the circuit "false triggers", do the lights flash for the same time duration as if you had pushed the button?

eT

 

[mark.w]

Yes. Both 555 pcb's trigger just as if I'm hitting both buttons at the same time. So what I actually get is both indicators blinking for the duration I set the time delay to (three flashes), just like I had put the hazard flashers on.

This is why I've had to disable it for the time being.....driver's behind must have been wondering why I put the hazards on briefly.....

 

[mark.w]

Ok my friends, back again...
Well it seems like we have some success at last. I put on the 1k resistor on the trigger line as per Jim B's suggestion and together with the 100nF cap it seems to have considerably damped down the tendency of the circuit to self trigger. I guess it does it 10/15% of the time now. If I now replace the CMOS 555 with the one originally supplied, it's near perfect! I've only had one false trigger in this condition, so as far as I'm concerned....it's fixed.

Can I say a hearty thanks to all that took the trouble to offer advice on this little problem, it's very much appreciated.

 

[4pyros]

I've only had one false trigger in this condition

even one false trigger is bad

 

[mark.w]

Yes, I fully agree....but when the previous trials only gave me a 100% fail rate, this feels like I've won the lottery!

 

[JimB]

This sounds like the basis of success!

Can I now suggest that you try increasing the value of the 100nF capacitors, either just double up on the caps, or try a 220 or 330nF.

JimB

 

[mark.w]

Thanks for the suggestion Jim.
It'll start getting messy if I try to parallel up some more caps on that little board, so changing to a larger value will be the next objective. I shall be enjoying the fruits of my (our?) labour first though before diving in again!