Use a dual timer, one as 40 Hz osc, the other as 2.3 ms
one-shot. (By the way, the square wave is 12.5 ms high,
same low. )

What are the isolation diode and the VCO for ?

<nbucska@pcperipherals.com>

556 is dual 555 timer ic and agree with previous post.
vco uses varactor diodes to trim frequency determining capacitance

What are your rise / fall time requirements?

I need to build a 555 timer that works in the 7.8 HZ; in witch the oscillator itself does not over heat.

Hi TIMER:

7.8 Hz +/- ??????  Wave form ?

<nbucska@pcperipherals.com>

try to stay away from the 555 in automotive applications!  these devices do very poorly over temperature.  Also, they are extremely sloppy.

melone,
Your statement is not what I have found. Could you elaborate on its shortcomings?

There are many possible solutions to this! As always performance and price matters greatly. Some thoughts:

  Being a microcontroller fanatic I would have used a cheap micro if I needed accuracy and well defined rise times, and get some "intelligence" for free, like startup properties etc.
  A nice low cost solution could be an op-amp in oscillator configuration, with 2 diodes giving control of hi/lo durations.
  Even simpler would be using a cmos 40106 hex schmitt trigger inverter, 1C, 2R and 2 diodes. Slightly better would be adding an MMV stage (just use inverter no 2 of the 6 available). This would not produce the best performance in the world, but probably compete for the cheapest solution (there are 4-5 inverters "free" for other purposes). Bear in mind that the resulting timing will vary from different manufacturers and/or production batches.

  Schematics should be readily available, if needed just let me know.