super simple pulse stretcher

What's wrong with a 5-10ms flash? I'm flashing some at 400us and they are easily visible in my shop with florescent lighting.

NPN emitter follower driving R-C to ground, then a comparator. Use an opamp as the comparator.

Supplies? ...Jim Thompson

I'm probably going to need to be using something like an LM324A. Will they work OK in that role?

Inject the pulse via a diode to the (+) input of a op-amp. The (+) input will have a small cap on it going to common for the delay off with a R also going to common as part of the RC calculations. Put a voltage divider on the (-) input of the op-amp of lets say 2.0 volts, that will be your threshold for the comparator.

You may also want to have a low current feed back from the output to the (+) input to aid in a little snap, hysteresis, for the off state.

If for some reason your reference pulse appears to be very low current in nature, it could have issues driving the cap you select. If this is the case, buffer the pulse with another op-amp in unity mode or use a NPN trannie in emitter follower mode to drive it, in which case, you won't need the diode.

Jamie

Looks like it's going to be +5V virtual ground split into +2.5 and -2.5 at this point so they'll be low-voltage opamps.

That will work just fine. Just keep in mind that a 324 gets closer to the bottom rail than it does the top rail. Infact, it can become detached but I don't think that should cause you any issues.

Jamie

I'll explore that a bit in simulation, thanks.

So your input pulse is -2.5V to +2.5V _relative_ to your virtual ground? ...Jim Thompson

Nosir, it will be 5 volts relative to the overall common, i.e. the -2.5 volt point wrt virtual ground.

Isn't that what I just said ?:-) ...Jim Thompson

Sure. If you have two sections available, use one open drain to discharge the capacitor, and the other section as the threshold sensor. Quiescent high, RC pulsed low. Easy.

We sometimes have a "trigger" LED that is basically a stretched input. In some cases, we want to catch very short pulses - a couple ns - but also indicate a static high input. Laser drivers want that sort of indication. We use a fast one-shot (SN74LVC1G123) and OR its input into the output.

Correct.

I'm not sure I follow what you mean by using one "open drain." If you have a moment could you show me in LTSpice? Also I only got 4 hours sleep last night and jesus I'm lazy today.

Oh, sorry, I was thinking LM339, the comparator. I'm starting to forget those antique part numbers.

If you use opamps, you'll need a diode or a transistor to discharge the cap. An active discharge part (transistor or mosfet) can give fast discharge and high stretch ratios.

and here's an oldie

GND From /-\ V+ supply ___ V+ supply ( ) Threshold ref -|___|- \+/ ___ ___ | |--+|___|+-+++|___|-GND V -> + | R4 - | R3 + | short pulse + | | |/ | |\| ||-+ o--------o-| NPN +-+|-\ longer Pulse || | >-+---o-------+--------+||-+ + +++|+/ | + | | |/| | | +-----+-----+ ___ | | | +--+|___|+--+ | + .-. R2 | --- | | | C1 --- | |R1 | + '-' | | | | | | | === === + GND GND GND (created by AACircuit v1.28.6 beta 04/19/05

C1 and R2 is the charge constant hold time before it goes below Threshold.

R2 is a + feed back to aid in a little snap action for the off cycle, make sure to select a value that does not hold + input higher than the reference input while output is on, otherwise it'll latch.

Jamie

Cool, thanks!

I think I may go with option #2 - should the op amp be configured as a comparator in that example?

The one with the 2N7002?

Opamps usually work OK as comparators, as long as you don't need super fast response or rise/fall times. They usually don't oscillate so generally don't need hysteresis.

So there's not really a "configure as a comparator" issue.

Oh, there's one gotcha. Some brands of LM324 do weird things when used as a comparator. When one section rails, it messes up the bias of the other three sections. That's because opamp sections use shared current sources. The original National part did that, maybe still does.