555 timer or VCO

[kspiroff]

I need to create a 40 Hz square wave, "ON" = +5V, time high = 2.3 miliseconds for an automotive application.

I'm looking for recomendations & advice on a 555 timer using an isolation diode or a voltage controlled regulator for this application.

I'm familiar with the 555, but have no experience with the VCO.

 

[nbucska]

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>

 

[DON43]

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

 

[melone]

What are your rise / fall time requirements?

 

[timer]

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

 

[nbucska]

Hi TIMER:

7.8 Hz +/- ?????? Wave form ? <nbucska@pcperipherals.com>

 

[melone]

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

 

[pebe]

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

 

[yesway]

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 &quot;intelligence&quot; 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 &quot;free&quot; 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.