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555 timer based flasher "noise" problem

I built a simple 555 based flasher to operate in an antique car (6 volt, positive ground) for a turn signal system set for about 1 flash every second. The "dash" indicator is an LED and all turn signals on the fenders are led arrays. It works fine on the bench with 6 volt batteries, but when I hooked it up to the car electrical system using the chassis for the positive connection and the engine running, the flashing was totally "high frequency". When I power it directly from the AA batteries, with the positive lead provided by the chassis to each LED array on the fenders, the flash is almost the design frequency, but the time between flashes is erratic. I even get interference if I don't use the chassis for one lead to the fender lights. The car has a new 6 v positive ground alternator.

I have tried a few basic capacitor filters (e.g., 0.1 and 0.01uf, polarized electrolytic) across the power feed at the circuit board, and non electrolytic on pin 5 of the 555, with no success.

I suspect it is ignition noise. Any suggestions on a filter design for the flasher circuit? Shield the 555 with metal foil? other?

Reply to
oldcarman
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I think you need way more decoupling than 100nF. consider 1000uF and some suitable series resistance. The max drive on a 555 is 200mA so an Ohm in series with the supply wont loose any voltage but give the capacitor a chance of doing something useful.

Having a positive earth wont help much either! so think carefully about where return currents are flowing and where decoupled noise currents will go.

Bob

Reply to
Bob Minchin

a guy in aus.electronics had a similar problem.

he tried that too. replacing the 555 with a different brand worked for him.

maybe try this:

0V ----[10R]--+------+-------+ | | | to LEDs | | | .--- | 10n| ---- |/ 10u===== === 555 ------| NPN | | ---- |\| | | | ~| -6V-----------+------+-------+---------+

also what size resistors are you using to set the speed? Bye. Jasen

Reply to
jasen

Thanks for the reply. I will check that.

I am using 4.7K and 1 meg with a 1uf cap. for the time constant.

Reply to
oldcarman

try 47 ohms 10K and 100uF it should run about the same speed and be less effected by electrical noise. mount the capacitor as close to the chip as possible

--

Bye.
   Jasen
Reply to
jasen
1000 uF electro. plus 0.1 uF on 555 pin 1 and 8 solves the problem provided I stay with AA battery supply, not car system.

Thanks to all who replied!

Reply to
oldcarman

WHAT??!! You're not using the ORIGINAL-type turn-signal lamps, on an antique car??!!

Ha. Just kidding.

- Tom Gootee

"He who lives in a glass house should not invite he who is without sin."

Reply to
tomg

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