Wide Voltage Automotive Status To Microcontroller Input

Question

I need to monitor the status of a wire in an automotive application that can have 10-40VDC on it at any time. What is a good approach to limit the voltage to a microcontroller input across such a wide range? I plan to use a 10nF cap and TVS for ESD and other transients but I'm not sure the best approach to limit the expected voltage. If I can reliably detect 10V, 40V may burn up the input resistors (or resistor and zener?) or I can limit for 40V and not sense 10V.

The signal is a simple on off status line that I'm tapping into. It won't be carrying any data or change state very often. I will be sampling it occasionally in the micro to determine if it's high (1V to 40V) or low (GND to under 1V).

Thanks.

Edited to add that my microcontroller will be running at 3.3V and a logic low needs to be under 1V. Any solution I use must include a low value cap between the input and GND and either diode clamps to Vdd and GND or a Transil.

Answer 1

Using a resistor and Zener diode will work fine. The simulation below shows what you get with a 4.7V zener.

You can use a very large resistor, because the microcontroller I/O pin draws virtually no current. With a 100kOhm resistor and approximately 5V output voltage, the power dissipation is (40-5)^2 / (100,000) = 12mW, well within what the resistor can handle.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

I'd be inclined to use something simple and dirty like this

^{simulate this circuit – Schematic created using CircuitLab}

Vary the value of R3, or omit it, depending on what you want the lower threshold to be.

Note that base emitter junctions are pretty tough, and tend to fail short circuit when they do. If it dies, it will save the protected circuit, and can be easily replaced.

D1 is to protect from negative voltage. Car electrics are supposed to be able to tolerate the reverse connection of a jump start battery without damage, as well as 160v load dumps and 24v truck battery jump starts, pretty harsh!

Answer 3

Selvek's answer should work. I would add that another simple alternative to the zener is to clamp the voltage to VDD using a diode.

^{simulate this circuit – Schematic created using CircuitLab}

In fact, most ICs already include such a diode internally so if you get the resistor right you don't even need to add a diode.

One thing you need to be careful of when you use this method is that the minimum power draw of your circuit must always be greater than the current through D1 at 40v. In this case, the current is 0.7ma. R2 in this case creates a minimum load of 1ma, which will protect the supply from being raised over 5v.