Hi folks,
I am not all that au-fait with electronics. I can read a circuit diagram and follow the explanation of how the circuits (to an extent) operate and can sometimes recognise how a certain sub-circuit may fit in with what I want it to do. However, the fine detail of circuit design goes right over my head.

My particular problem at the moment is how to produce a pulse of variable length independent of the length of a trigger pulse. To be specific, I want to use the voltage to a fuel injector to trigger a second fuel injector but with variable pulse length to suit other parameters. I have searched many sites looking at 555 monostables and various others using two 555s but they all seem to require the trigger to be shorter than the output pulse. I will need to experiment, but it is likely that the required output pulse may be shorter or longer than the input pulse dependant on engine temperature, speed, etc. I would sincerely appreciate any help given.

Pete

 

If you use capacitive coupling, you can essentially define your trigger width. The leading edge of your original trigger pulse is coupled through the cap and as the cap charges, the output voltage drops off. You only see the edge. You would also need to add a reverse polarity diode to protect the trigger input of the 555 from the negative going spike that is produced when your original trigger pulse goes low.

 

Thank you for that Bill. I think I understand what you are saying. But will the capacitor have any effect on the circuit from which the pulse originates. I do not wish to cause problems with the ECU.

 

For what you want to accomplish, the cap will need to charge very quickly and the load will be controlled by the load resistor following the cap. Check here....
http://forum.allaboutcircuits.com/showthread.php?t=14018
for a quick tutorial on triggering a 555.

 

"For what you want to accomplish, the cap will need to charge very quickly and the load will be controlled by the load resistor following the cap. Check here....
http://forum.allaboutcircuits.com/sh...ad.php?t=14018
for a quick tutorial on triggering a 555"

.Thank you Bill, I found that very informative. I will have to alter the timing RC though as the pulse width range of the vehicle is from 2mS - 15mS at full throttle. I came across this circuit :-http://picasaweb.google.co.uk/10626...N-ZmNqM2YCZjgE&feat=email#5503426269158360914 during my previous searches and wonder whether the variable pulse (op-amp) section can be adapted to the circuit in your tutorial. Also, is it possible to replace R4 (pot) in the op-amp circuit with a variable (1-5V) voltage from the throttle sensor of my car to vary the pulse width?

 

Also, is it possible to replace R4 (pot) in the op-amp circuit with a variable (1-5V) voltage from the throttle sensor of my car to vary the pulse width?

The circuit in your example is using a voltage range of 4 to 8 volts to control the pulse width. Breadboarding the circuit and experimenting would be an interesting project. I did find the circuit you referred to quite interesting. Seems as though I have seen a circuit that will control pulse width from the 555 directly by use of diodes around the pot. One section controls the on time, the other controls the off time.

 

I would like to breadboard this circuit, but how would I test it? Especially with the timings that I would need. I have an oscilloscope but am still learning how to use it properly.

Incidentally, would I be in the range of 1 to 4 Volts on the output if I used a 5V supply for the 555?

 

Again, bread boarding and looking at the results is the best way for you to learn, not only about the 555, but how to use your 'scope. As for the 1 to 4 volts to control your project, a simple Op Amp used to scale the signal range could be used.

 

I built a "pulse strecher" years ago for a porshe 914. It would charge a capacitor from a fixed current source during the time the injector pulse was high, and then discharge the cap from a variable current source back down. A comparator turned on an output circuit to drive the injector. this allowed an increased fuel delivery proportional to computer's output- the proportion could be varied by a potentiometer