Lunetta Pulse/Trigger/Gate Delay

nathanxl · Joined: Apr 24, 2012 Posts: 59 Location: Wa

Hi.
I have a need to variably (prefferably with a pot) delay a pulse (a gate from a 4024 shift register) from about a milisecond up to maybe 2seconds long.
Basically I have the 4024 outputs going through gate to trigger circuits and then Im using the triggers to drive small motors that pluck strings... Now I want to be able to dampen the strings with solenoids just after they are plucked...
Im sure there is a simple Lunetta solution to this that maybe uses one chip and a few other components.
Do any of you gurus have experience with this kind of circuit?
Im aware I will need to beef things up to drive the solenoids its just the delay part that has this noob baffled.

PHOBoS · Posted: Thu Jan 24, 2013 7:51 am Post subject:

first thing that came to mind was a 555 in monostable mode but that just changes the length of the pulse, maybe if you invert it
add some extra logic that might be usefull.

something with a shiftregister ? clk sets delay time, but you would need a flip flop or something at the data input if the pulse
is shorter then a CLK cycle. You could use some logic to control the time digitally too by setting the number of places it shifts.
(a mux that selects where you take the pulse out)

PHOBoS

I couldn't let it rest so I tried something. I wanted to make it with only 1 IC and managed to get it working with a CD4093 Cool

U1c and U1d form a S/R flip flop which 'captures' the pulse. Because it works on active low levels there's an extra transistor
inverter at the input. So when the "Pulse In"- get's a high signal U1d's output becomes high and stays high even when the
input is low again. The 4.7uF capacitor is (slowly) charged through the 500K pot until it reaches the treshold level
of U1a. When this happens the output of U1a will become low (inverted by U1b for the pulse out) which will then (slowly)
discharge the 1uF capacitor (500K) untill it drops below the treshold level of U1c. Which will reset the flip flop.
The 4.7uF cap is now quickly discharged through the diode, the output of U1a become high again and the 1uF is quickly
charged.

If the pulse input is still high after the cycle ends the pulse out will also stay high untill the input becomes low again.

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nathanxl · Joined: Apr 24, 2012 Posts: 59 Location: Wa

OMG PHOBoS, that is fantastic. I knew this was the right place to get an answer.
Great work.
Im away from the "Lab" for the next few days so cannot put this together myself ATM but I thankyou from the bottom of my heart.
I look forward to getting this up and running. Just on first glance, once I have decided on the desired Pulse length of this circuit would I be able to switch out the pot with a resistor permanently or will this value always need adjustment? Because this circuit is going on an (already complicated) instrument that I use in a live performance situation I like the idea of having just one pot to adjust the delay time of the pulse... I get confused easily when on stage.
WOW, cant wait to get back in the shed.

PHOBoS · Posted: Fri Jan 25, 2013 6:11 am Post subject:

you're very welcome Very Happy

a pulse delay was something I have been thinking about before, just hadn't taken the time yet to come up with a circuit.
so I'm glad you asked. And yes you can of course replace the pot with a resistor. You could even leave out the pot/diode/cap
attached to pin 8 alltogether and just connect it directly to pin 3. But then the pulse might be too short.
You could also leave out the LED's but I think the active LED is very usefull.

I'm looking forward to see it added to your amazing sculpture Cool