Blinking an LED once when a slide pot is pulled to ~zero ohms

Question

I am developing a little circuit to control a servo linear actuator that is used to control fine injection onto a microscopy set-up. I am using a 555 circuit (see below) to generate the PPM signal required to control the actuator position with a couple of slide pots.

^{simulate this circuit – Schematic created using CircuitLab}

It contains three pots, a "coarse" slide pot to control the servo position, a "fine" slide pot (with audio taper) and a pot that sets the range of the "Fine" pot. The user will preset the coarse and range pots and then, at a particular time-point, gradually slide the fine pot over its full range until it reaches zero resistance. When it reaches zero resistance I would like an LED, positioned close to the microscope, to give a brief flash (~35ms). This way we can align the moment of injection with the video recording of our microscope slide. Does anyone have an idea how to add that to this circuit?

Thanks!

Answer 1

Untangling the schematic a little we get this.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. The OP's schematic redrawn.

Not it's a bit clearer that when the Range Fine pot is reduced towards zero that we should see 5 V appear at 'A'. To get your required output you need to do two things:

1. Add a voltage comparator at A and arrange it to turn on at 50 mV below the 5 V rail or whatever tolerance you decide. You will need a comparator with rail to rail inputs because you are working right up to the 5 V supply. (Strictly you don't need it to work to 0 V but you're unlikely to find one that goes to V+ and doesn't go to zero.)
2. Arrange for the comparator output to drive a pulse generator to blink the LED. Since you're comfortable with the 555 it would make sense to use another for this.