Wednesday, October 7, 2020

Fuel Pressure Monitor PCB

 Here is a simple circuit I am using to monitor fuel pressure for troubleshooting in my project car.  

 The circuit itself is really nothing special, just an op-amp used as a comparator with a potentiometer, R3, on the non-inverting (+) input and the signal from my fuel pressure transducer fed in to the inverting (-) input.  The 7805 regulator provides power for this circuit and my pressure transducer.  The pressure transducer outputs a voltage that is proportional to the pressure.  The output transistor is a logic level MOSFET that acts as a low side switch.  In my application I have this wired to turn on the check engine light when the fuel pressure falls below a certain point which is set by R3.  C7 is optional.

 Some special consideration was given to the power supply circuit, namely the addition of a TVS and a couple diodes to protect the regulator in the harsh automotive electrical environment. Use 50 volt caps on the unregulated side.

 CEL connects to the check engine light, and goes low when the fuel pressure is low 

 S+, S-, and SIG connect to your sensor 

 12V is the power supply to the board from the chassis 

 Spare pins are broken out for a 5V tap and the sensor output (SIG) for a datalogger. 

 There are probably hundreds of other possible uses for this circuit.  **One word of caution, don't try to use it as a thermostat or to drive high current loads.  No hysteresis is added to the circuit and when the sensor output is approximately at the set point the output transistor will operate in the linear region rather than saturation or cutoff, and will dissipate quite a bit of heat. For my application this provides a dim check engine light, which I don't mind as it kind of turns a single light in to a 3-state indicator.  If you want to, it would be quite easy to create a Schmitt trigger and make the circuit operate in decisive on/ off manner.

 I am sharing the schematic, BOM, and Eagle files here in case they are useful for someone, feel free to share/ modify/ make/ sell PCBs for these as you see fit.  Just don't claim my work as your own and don't blame me if something doesn't work right.  For reference it cost me $16 to have 5 of these PCBs made and shipped to me in 2020.


 Eagle files, schematic, and gerbers (.zip): Google Drive

No comments:

Post a Comment