Pulse Stretching

[Sceadwian]

Colin the VOH transition voltage of TTL is from about 2 volts and goes up to 5 volts. 3.5V should give a clean TTL signal.

 

[Hayato]

Colin,
TTL Level High is anything from 2.7V to 5V, if you loose the tolerance can range from 2.2V to 5V.

 

[Sceadwian]

I wonder what Tesla thinks.

 

[ericgibbs]

hi Saiello,

This CMOS555 1uS to 200uS pulse stretcher works OK.

Dont waste your time trying that supposed transistor pulse stetcher posted earlier, it will not give the result you want.

 

[xiptron]

After all this point stretching, no one seems to have realised that Saiello never said he was limited to 5V to power the stretching circuit, he only stated that the photodiode produced an output pulse of 5v amplitude.
He may well have an independant supply voltage available e.g. 12 or 24v from which to power the subsequent circuit. Therefore perhaps the bulk of this discussion has been a waste of space, though nonetheless informative lol.

 

[ericgibbs]

After all this point stretching, no one seems to have realised that Saiello never said he was limited to 5V to power the stretching circuit, he only stated that the photodiode produced an output pulse of 5v amplitude.
He may well have an independant supply voltage available e.g. 12 or 24v from which to power the subsequent circuit. Therefore perhaps the bulk of this discussion has been a waste of space, though nonetheless informative lol.

hi,
Appreciate that, the 555 cct, as you know can run from 5V thru 15V.

As you can guess, we have to make some assumptions on the Forum in order to get some feedback from the OP.

 

[saiello]

Holy smoke, never thought my little post would generate so much 'debate'..! I was only looking for a very simple way to modify an existing circuit to improve reliability/applicability (see attached). Input to the photodiode is via laser light reflected off the flats of a nut on a rotating body. Works great up to a physically tested 80,000rpm ( designed for +125,000rpm ). Ordinarily, given the particular R1C1 combination on the LM2917, at anything above around 15,000rpm the LM2917 won't process the input signal due to the very short pulses from the photodiode. Hence the use of the 4018 that serves to latch the short pulses ( and provide input flexibility in terms of a general divider options ). This scheme only begins to work if there are two reflections per revolution, combined with a divide by two setting on the 4018, to give 1 pulse per revolution input to the 2917. Unfortunatley, two or more reflections are harder to set up but more importantly could potentially be a problem in that if one reflection/pulse goes 'missing' due to dirt or change in ambient light then you would get a false reading. Much better to have a single pulse per revoultion so that I get either a true reading or nothing at all. Hence the idea of stretching the one pulse per revolution to a width that would be acceptable to the LM2917 and bypassing the 4018 altogether. 125,000rpm equates to around 500uS per revolution, and assuming that the reflected light hits the photodiode for 1/360th of this time then we are looking at the very most a 1.5uS pulse when the LM2917 needs at least a 150uS pulse width. If I can't get this improvement with nothing more than the addition of a few simple passive components then I'll just stick to what already works..! Thanks for all of your inputs!

 

[xiptron]

I'm not all that impressed by the 555. I have built several instances of an astable to obtain a short duration pulse using this beast, only to find that they simply refused to work. On other occasions exactly the same circuit performs fine. Could it be that the 555 is so mass produced, that faulty ones find their way to market?

 

[Tesla23]

Holy smoke, never thought my little post would generate so much 'debate'..!

I'm fairly new here, but from what I've seen this is fairly typical of the kids around feeding time.

If I can't get this improvement with nothing more than the addition of a few simple passive components then I'll just stick to what already works..!

Well under these constraints, two things come to mind, neither particularly asthetically pleasing, but may get you out of trouble.

You could look to modify the simple comparator to act as a monostable. Something like an op amp monostable. Yours is all inverted relative to that example, but the idea of adding a time constant to the hysteresis to act as a temporary latch may work.

Alternatively, you could modify the 4018 to act as a monostable by using one of the jumper settings to feed Q1 back to the reset pin via an RC delay.

 

[saiello]

After a bit of a think, I have a solution that will work without the addition of any extra circuitry, I just have to change the specification/expectations of the circuit. To explain: at the moment I have these input options:

1) 1:1 meaning direct input to the LM2917 of 1 pulse per revolution, bypassing the 4018. This is the 'problem' input and will only work at top RPMs if the reflected light dwells on the photodiode for at least ¼ of a revolution, so reflection off the face of a nut is out and therefore some means of preparing the rotating body so that a ¼ of it's surface is shiny to reflect the laser light has to be carried out.

2) All the other input options 2:1( 2 pulses per rev ), 6:1, 8:1, 10:1, all go through the 4018 that latches the short duration pulses from the face of the nut and also provides the 1 input pulse per rev required by the LM2917 as it stands in my circuit.

Given the limitation of the 4018 in terms of options as a divider, my initial choice of user input options ( 2:1, 6:1, 8:1, 10:1 ) was arbitratry in that I simply chose those that I thought may be the most useful, i.e. to make my circuit the easiest to apply to a number of different types rotating bodies ( in particular the 6:1 option ).

The solution to my N.o. 1) input 'problem' is simply to change the input options and have them all go through the 4018. Now, a 1 pulse per revolution input is always fed through the 2:1 input of the 4018 giving effectively a ½ pulse per revolution feed into the LM2917. All I have to do now is double the output voltage of the LM2917 by changing the R1C1 combination. This scheme now affects the variety of user input options and limits them to 2:1, 3:1, 4:1 and 5:1 by using the 4:1, 6:1, 8:1 and 10:1 divider options respectively of the 4018. The most useful of these will always be the 2:1 option and to a lesser extent the 3:1 input option, so this change of scheme is something I can happily live with..!