Sequential taillights, 556 circuit, and dirty power :(

Let me start by saying I know just enough about electronics to be dangerous. I can read a schematic and am good with a soldering iron. I have put together two sequential taillight circuits for my 65 Mustang. The circuits works well when I have them inside my house hooked up to a 12vDC power supply. When I install them in the car however the voltage jumps around so much do to the lights come on and a variation in alternator speed that it is making the sequencing of the lights erratic. I believe the voltage variance is between 8 and 14 volts. The circuit uses less than 0.2A. What I am looking for is a way to keep the 12 volts constant. My first thought was to use a 12V zener diode to limit the power. There are two problems with this, one is I am not sure how to use a zener diode to limit voltage, and two what do I do about the power dropping below 12 vDC. I think I could install a capacitor to help maintain voltage, but I am not sure where to put it or what uf to use. The catch to this is that when the turn signal flasher turns off that is what cuts power to the 556 timing circuit and resets it so the cap cant be to big. HELP!! If you need more info let me know.

best way is to get a 5 volt regulator and run the 556 of the 5v regulator. This will take any voltage higher then 5 and convert it down to 5v. then redesign your circuit for 5v. If you can't get 5v relays, use a transistor to drive the relay coils.

Thanks for the idea, but I already have the 556 circuits put together for 12VDC. The power dosn't have to be perfect, but if there is a quick cheep way to fix it that would be wonderful.

Daze said:

Thanks for the idea, but I already have the 556 circuits put together for 12VDC. The power dosn't have to be perfect, but if there is a quick cheep way to fix it that would be wonderful.

Click to expand...

12v regulator

Yes,but LDO.

After seeing the post refering to a 5v regulator I thought about using a 12v regulator. The problem is one, what do I do about the voltage dropping below 12v and two the 12V regulators I have access to require 14-35 volts input. 14 is the near the max in my spike

This is the cause of my LDO recommendation. The LDO can work with 300mV voltage drop.

forgive my ignorance, but what is an LDO

Daze said:

forgive my ignorance, but what is an LDO

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'Low Drop Out' - a regulator which only needs slightly more going in than it can give out.

The first change I would make would be to insert a bypass capacitor between pins 7 and 14. Use a 100 nF ceramic. However, this may not be enough to suppress the noise from the alternator and other devices. You may also need an inductor (try 100 mH) in series with the supply, ie. connected from the 12 Volt supply to pin 14 of the IC (with the bypass cap across 14 and 7 as I said above).

A further improvement would be to insert the LDO that was suggested. This will provide a dynamic impedance to further reject the noise.

The difficulty with a 12 Volt LDO is that the supply voltage will always have to be greater than 12.3 Volt otherwise the LDO will drop out.
But if you only ever want to use this circuit when the motor is running, this should not be an issue.

Len

What do you mean by "insert a bypass capacitor between pins 7 and 14” Are you telling me to leave every thing the same and run the cap between pins 7 & 14. Also again forgive my ignorance uf= microfarad pf= pekofarad what is an nf and how does it relate to the others? Thanks again for your help.

Daze said:

What do you mean by "insert a bypass capacitor between pins 7 and 14” Are you telling me to leave every thing the same and run the cap between pins 7 & 14. Also again forgive my ignorance uf= microfarad pf= pekofarad what is an nf and how does it relate to the others? Thanks again for your help.

Click to expand...

Yes, leave every thing the same and connect the cap between pins 7 & 14.

nF is nano Farad. 1000 nF = 1 uF. nano is a standard metric multiplier. It means 10 to the power of minus 9.

For example, one nano second is one billionth of a second.