Scaling a car's injector pulses to count and time them?

Question

What would be an efficient way to reshape a car's injector pulses to digital 0-3.3V pulses, so a microcontroller can count and time them? The waveform can be seen on this page under "Example waveform".

This question might be considered a duplicate of Efficient spike detecting Microprocessor or DSP architecture but none of the answers is really clear for me. I wonder if the transistor solution works with Vb >> Vc. The flip-flop solution misses the voltage scaling stage.

Requirements:

• the circuit must present a high impedance to the coil (arbitrarily set at > 1Meg)
• must correctly detect pulses between 5 and 150Hz
• should survive common faults (wire disconnected and reversed polarity)

Assumptions:

• voltage spike amplitude is not constant, and can go anywhere from 30V to 120V (again, somewhat arbitrary).
• voltage spike duration is not constant, and can go anywhere from 200us to 4ms (again).
• Processor is likely to be an Atmel SAM D20 or D21 (3.3V).
• Supplies available: automotive 12V, not regulated; 5V, regulated, 3.3V, regulated.
• the microcontroller has edge-triggered interrupts and input-capture mechanisms

A solution based on the signal levels instead of the spikes could also be acceptable, assuming that it works with a high voltage anywhere between 9 and 16V.

One simple solution would be a 1Meg resistor in series and a 3.3V zener (or even a 3V one, or maybe a shottky diode to Vcc), but the high-value resistor (and thus low current) means that we're in an area that is often overlooked in diode datasheets. In a previous case, I had conduction in an IC's protection diodes, causing incorrect behavior in other parts of the IC. I'd like to make sure that this doesn't happen. I'm also not sure whether the input-compare can deal correctly with such an high input impedance.

Thanks

Answer 1

That's not real hard, assuming the coil voltage is a rectangular (or even trapezoidal) waveform.

^{simulate this circuit – Schematic created using CircuitLab}

This will not respond to 30 volt pulses of less than about 100 usec, although it will respond to somewhat shorter 120 volt pulses. R2 keeps the circuit well-behaved if disconnected, and D1 protects against reverse polarity.

The only possible problem is the relatively slow rise and fall times (about 20 usec), but if this is an issue, add another transistor with an emitter resistor of 5k, and get rise and fall times of 2 or 3 usec. Alternatively, add a Schmitt trigger such as a 74C14 and get rise and fall times of 10 nsec or so.

EDIT - So how do you get 5 nF for C1? Use $$\frac{dv}{dt} = \frac{i}{C} $$ $$ \frac{\Delta v}{\Delta t}=\frac{i}{C}$$ $$C = \frac{i\Delta t}{\Delta v}$$ At low capacitor voltages, all the current through R1 goes to charging C1, and at about 0.65 volts the transistor base-emitter junction will begin to conduct and turn on the transistor. At 30 volts, 1M produces 30 uA, so $$C = \frac{i\Delta t}{\Delta v} = \frac{3 x 10^{-5}\times 10^{-4}}{.7}=4.6x10^{-9} = 5 nF \text{(close enough)}$$