Do you recognize this circuit?

[OlivierNum]

Hi,



I have some problem identifying this circuit. What it does is when I apply a frequency of 50% duty cycle on the input, I can modify the duty cycle changing the R3 (or the cap value).



Do you know the name of this circuit?



Thanks

[Paul Price]

You can also change the duty cycle by changing R72 or R1. I would call this circuit a duty-cycle modifier circuit.

[Rufus]

What I would call it is a completely shit drawing.

After re-drawing it it becomes recognizable as some kind of pulse stretcher. It delays rising edges by about 100us and falling edges by about 2ms.

[OlivierNum]

Yeah thanks for the compliment... 

Care to upload an awesome drawing of yours?

[c4757p]

Yeah thanks for the compliment... 

Care to upload an awesome drawing of yours?

Well, the way you draw a circuit is important for being able to recognize the parts. How's this? Now you can see all the RC relationships and the signal path - the signal charges C4 through R3->D4A, discharges C4 through R72, is pulled up by R1 in the absence of a signal input, and is clamped to VDD by D4B.

Excuse the wrong diode symbol and the wrong name for the rail... (Edit: fixed)

If I were drawing this into a full, proper schematic, I'd go one step further and annotate it with the voltage thresholds and the expressions for the delay constants.

Also, for noise immunity (and depending on how well the power rail is decoupled), I'd reroute C4 to ground instead of VDD. Won't make a difference at all in operation. Otherwise you're AC-coupling any power supply noise into the signal. Of course, a Schmitt inverter won't be too susceptible to that.

[OlivierNum]

Thanks!

[Rufus]

Care to upload an awesome drawing of yours?

The scribble on my pad looks like this:

[oPossum]

It's missing a ground node.

[c4757p]

Possibly, but as far as I can tell without seeing the schematic, it makes sense as it is. Besides, then R1 and R3 don't really make much sense, unless the signal input was AC coupled. No point in bias resistors on a DC coupled signal.

[MrsR]

You took the circuit from the board and that is what you do when debugging a circuit, Draw the circuit you are working on then move onto the next section untill you find the fault that is what all good Techs. do if they don't have a schematic to work from.

ALL THE BEST
Rachael