I will need to monitor a pressure transducer that out puts scaled voltage of .25 to 5.5v dc. I need a trigger circuit that will send out a signal if the transducer voltage reaches approximately 5.26 volts which is at a psi red line on the pressure gauges. The circuit should have a trim pot or rehostate that I can adjust and set to make an LED turn on when it is at the psi is at the red line.

Idealy I will apply a pressure to the the transducer and set it at the reading that warrents a warning light....at that time I will adust the pot to get the trigger. This will calibrate the light to the red line on the gauge.

What circuit would do this?

 

Below is the LTspice sim of a circuit that uses an inexpensive, programmable voltage reference as a comparator to turn on the LED at the desired voltage from the transducer:
The TL431 reference turns on (conducts) and lights the LED (red trace) when the voltage at its Ref input (green trace) reaches 2.5V (here for a 5.266V transducer voltage (yellow trace)).
The trigger point can be adjusted by pot U2.

 

Below is the LTspice sim of a circuit that uses an inexpensive programmable voltage reference as a comparator to turn on the LED at the desired voltage from the transducer:
The TL431 reference turns on and lights the LED (green trace) when the voltage at its Ref input reaches 2.5V (here for a 5.266 transducer voltage (yellow trace)).
The trigger point can be adjusted by pot U2.

View attachment 318346

Thank you for your insights and I will be bread boarding this. What is the resistor accross the LEd for?

 

What is the resistor accross the LEd for?

The TL431 draws a small amount of bias current (<1 mA) before it turns on, so the resistor bypasses that current to prevent the LED from glowing dimly below the trip point.

 

Does the LT431 REALLY switch on like that?? That seems quite amazing.

 

Does the LT431 REALLY switch on like that?? That seems quite amazing.

Yes, it's a very versatile IC.

When used as a shunt regulator (similar to a programmable Zener) with a resistor in the cathode and feedback to the Ref input, it starts to conduct when the Ref voltage reaches 2.5V and the adjusts the current to maintain the 2.5V.

When used as an open-loop comparator it again starts to conduct when the Ref voltage reaches 2.5V but, since there's now no feedback, it just keep conducting until it reaches a minimum cathode voltage of about 2.5V.

Below is an example comparator circuit from the TI data sheet, which also shows the TL431 equivalent circuit.
From that you can see that it is basically acting as a comparator with a 2.5V reference at the minus input.
Here R1 and R2 are just to reduce the minimum 2.5V output to a lower voltage, such as to drive a logic gate.