Schematic critique request

[turbosupramk3]

This is my first time drawing a schematic outside of MsPaint, to set some expectations. There are also probably a few glaring oversights because I'm a newb. If anyone has for import, an Arduino Mega config file into Microcap, that would be appreciated for referencing.

I drew orange boxes around the circuitry that will be part of the Arduino PCB, the rest is existing circuitry that the Arduino Mega will be interfacing with. I couldn't find the standard to label the Arduino's pins, so I used ucAx for analog and ucDx for digital. I also do not have any schematic information for the existing proprietary circuitry.

My end goal is to have an overengineered (for robust protection) circuit to control some things in an automotive application. If there are any questions, please ask.

[PerryBebbington]

You are trying to use an NPN transistor, Q7, as a high side switch. As a BJT has a base emitter voltage of about 0.6V, 1.2V for a Darlington pair, 5V on the base will result in about 3.8V on the emitter, not the 12V you want.

U33 has SDA and SCL without pull up resistors. I2C needs pull up resistors, maybe 2k2.

I can't comment about the rest of the circuit because I don't understand what it does.

[turbosupramk3]

Thank you. I have edited the first post with the information you requested, as well as made the updates you suggested. If more information is needed, let me know.

[JohnRob]

Without commenting on the functional circuit design.....

An automotive electrical system is an electrically ugly environment. Protecting ALL wires (signal and power) going in and out is a must.

A few rules to consider:

1. Physical placement in your enclosure is critical. ALL wires must enter and exit in close proximity to each other. VERY close to the entrance/exit point there should be a 0.1µf 50v capacitor. Here lead (or clad) length is your enemy.

2. Input power can have 100 ms surges to 50 volts (80volts for older vehicles). Input power with the engine running can be between 13.5 to 16V. The 16v is for a weak battery.

3. Ground is not ground. In the vehicle the steel chassis is considered ground. However car mfg require designs accept a +/- 2.5 volt variation between different locations on the chassis.

[PerryBebbington]

10k is probably too high for I2C pull up resistors, I suggest 2k2, and 4k7 is common.

[turbosupramk3]

Circuit descriptions corresponding to the number in the updated picture. The mcp4725 is on a breakout board and has 10k pull up resistors according to its schematic from Adafruit.

#1 - A square wave signal that has a duty cycle of 50%, a varying frequency of 1hz per 5rpm and an amplitude of 0v / 12v . I want to read this frequency without affecting it, so I'm hoping the resistor values are high enough to not draw amperage that could impact the existing ECU's, but not high enough as to where transient external signals could provide an unwanted impact

#2 - This is a momentary contact switch that grounds a ~12v source wire which has a pullup resistor, that when grounded changes settings within the vehicle. I believe the ECU monitors this source wire for state changes and after a full state change of near 0, back to ~12v, a configuration setting will change. I would like to also monitor this with the Arduino, as well as to simulate its functionality and automate it

#3 - This a knob with multiple selections that will send varying voltage values through what is in essence a voltage divider circuit, which when untouched is a ~12c source wire which has a pull up resistor. Each differing voltage value indicates a different setting, which the ECU monitors for a state change to the lower voltage value and back to the default of ~12v. I would like to monitor this and automate it.

#4 - This is a momentary contact switch that grounds a ~12v source wire which has a pullup resistor, that when grounded changes settings within the vehicle. I believe the ECU monitors this source wire for state changes and after a full state change of near 0, back to ~12v, a configuration setting will change. I would like to also monitor this with the Arduino, as well as to simulate its functionality and automate it

#5 - This is a momentary contact switch that grounds a ~12v source wire which has a pullup resistor, that when grounded changes settings within the vehicle. I believe the ECU monitors this source wire for state changes and after a full state change of near 0, back to ~12v, a configuration setting will change. I would like to also monitor this with the Arduino, as well as to simulate its functionality and automate it

#6 - This is a pressure sensor with a range of ~1 to 4.5v . I would like to intercept this signal at the sensor and reproduce it to the ECU, for the purpose of controlling the floor and ceiling threshold values output under various scenarios.

#7 - Is meant to energize a relay coil to power a solenoid

[noobmastha]

For a start, rotate all for 90 deg. clockwise, it would be easier to follow, now it looks upside down.

[turbosupramk3]

How does this look? And does anyone see any design issues with my comments? @PerryBebbington @noobmastha