Design Improvements on Versatile Turn Signal Flasher

We had turn signal flashers that that would misbehave from time to time, varying wildly, even though the math said it should work properly.  I investigated.

It turned out that the supply to the main timing IC, a bipolar 555 timer, was reliant on the drop across the turn signal flasher… which itself varied as the turn signal flasher turned “on” and “off”!  It was shorting out its own supply, essentially!  Oh boy.

I developed a circuit that would allow the 555 timer to still drive the monster output transistor, the TO-3 cased 2N5301, but hold the IC’s supply during the “on” time.  There were two problems with this – the circuit consumed too much current to be reasonably held up during the “on” time, and the output turned out to be referred to the wrong rail, in order to drive the output transistor using the same mechanism as before.

I addressed the current consumption by switching from the bipolar 555 timer to the then-relatively-new Intersil CMOS ICM7555.  What a joy that it ran on almost no current!

Then, I developed a 2-transistor output drive to replace the 1-transistor stage.  It worked well, until we ran it over temperature.  It was then that I learned one of the fundamental lessons that I’ve taken through my entire career: do not rely on one transistor to “short out” second transistor’s base drive over temperature!  It turned out that the Vce(sat) of the first transistor could exceed the Vbe(th) of the second transistor, sometimes, at low temperatures.

I found that you can short out the second transistor’s base drive, but then you need to put a resistor to the second transistor’s base, just to make it that much harder for the second transistor to turn on with leakage.

Well, that was a lesson learned, but then I just redesigned the stages to avoid that problem altogether.  Once bitten, twice shy!

I did do some preliminary work on using a MOSFET output on the flasher, but I couldn’t make the short-circuit protection work, and decided to stick with the bipolar output.  Later, a colleague, James White, took up that effort and successfully developed the “FET flasher”, which was another step forward turn signal flasher design.