Amatuer electronics answer:

A little "positive" feedback from the output of the op-amp to the
non-inverting input will add hysteresis to the switch over point
and make it less sensitive to noise "jitter".  There is a way to
calculate the hysteresis based on the amount of positive feedback,
but I'll leave that up to you to research.  You could just
determine how much hysteresis you want by experimenting with
different feedback resistors.  Start by making it large compared
to the input resistors.  This will give you a small amount of hysteresis.
Decreasing the feedback ressitor will increase feedback and thus
increase hysteresis.

On Fri, 4 Sep 1998 12:18:39 -0700, Brian Scearce wrote:

>Amateur electronics question:
>
>I'm working on a battery-powered infrared transmitter, and I'm
>concerned by the fact that a working IR LED looks exactly the same as a
>non-working IR LED in the visible spectrum, so I'm adding a "battery
>low" indicator.
>
>I'm sure that there are a large number of clever ways to do this,  but
>I happen to have some LF411 op-amps on hand, and I was planning on
>doing something like (sorry, no ASCII art):  divide-by-3 voltage
>divider on the 9V supply going to the + input of the op-amp, a
>divide-by-2 voltage divider on the 5V regulated supply going to the -
>input of op-amp.  Output of the op-amp is "battery above 7.5V".
>
>But I'm haunted by Horowitz and Hill saying "an op-amp is never (well,
>almost never) used without feedback".  Am I properly exploiting one of
>the "almost never" cases, or am I misusing the op-amp?  I'm aware that
>the op-amp will be unstable near the crossover point, and that I could
>cure this with feedback; maybe I'll add this later.  I'm only wondering
>if this circuit will work without feedback.
>
>Construction questions:
>
>The PCB will have ordinary 0.1" headers for connecting the LEDs.  Where
>do you get 1-hole, 2-hole or 3-hole female header plugs to fit these
>things?  I know they exist, I've seen them on hobby servos, and for
>attaching a PC's panel buttons to its motherboard, but the only ones I
>can find in catalogs and surplus places are 20-plus hole plugs for
>ribbon cables, not for two or three conductors.  Do I have to keep
>cutting up DIP sockets?
>

Look for "snap off" connectors.  You can snap off the size you want.

>
>I'm also working on an H-bridge motor driver.  I've got a working
>prototype, but the MOSFETs are pretty delicate.  I'd like to be able to
>easily replace them in the final version.  What do I use for TO-220
>sockets?

I wouldn't socket anything driving high current unless you use very
heavy copper or brass lugs and nuts.  Design the H-bridge right and
you won't have to replace the MOSFETS.  I am currently working on an
h-bridge using TO-92 bipolar transistors for a low current motor. A
primary design criteria of mine is "smoke-proofing" the circuit.  It
works very well and doesn't even get warm.  Bipolars are much cheaper.
the only reason I am still working on it is because I want to add
circuitry for using the "back-emf" of the motor for speed measurement.

>
>
>Thanks,
>Brian
>