Electronic – Circuit to detect presence or absense of current

current measurement

I'm trying to come up with a bad-bulb detector for a circuit that will control the turn signals and brake lights in a car or truck. I'm defining "bad bulb" here as an open circuit, which is what typically happens when a light dies in a vehicle.

The allowable current range per output is pretty large: 100mA to 10A. This will allow the installer to use a single LED bulb on each output or several old-fashioned incandescents in parallel. In order for the MCU to detect a bad bulb, it only needs to know that current flowed or didn't flow, not necessarily how much.

Ideally, the circuit will have an extremely low part count and very inexpensive components (but still work in an automotive environment). This is mostly because the circuit will need to be duplicated several times on the PCB, so any excess will grow the size of the PCB and the budget. And the budget for this project is extremely small.

Here's one idea I've come up with so far using only passive components:

schematic

simulate this circuit – Schematic created using CircuitLab

When M1 is turned on, C1 charges up and power is available to the bulb. When M1 is turned off, one of two things may happen:

  1. If the bulb was good, C1 will supply additional current to the bulb for a very short amount of time. NODE1 will very quickly drop to zero volts, which will cause NODE2 to also drop off quickly. If the GPIO pin samples NODE2 a few milliseconds after M1 was turned off, it should see a digital low signal. A low signal indicates the bulb is good.

  2. If the bulb was bad, the only load to drain C1 will be the resistor divider R1 and R2. But those resistors are large, so the voltage across C1 will stay higher longer. If GPIO samples NODE2 a few milliseconds after M1 is turned off, it should see a digital high. A high signal indicates the bulb is bad.

Everything outside the dotted box is already committed in the schematic, so I'm just adding two resistors, two capacitors, and two diodes per output (D1 and D2 are just to protect the MCU from load dumps). These parts might add up to 25 cents per output, which is great. This is an adequate solution, albeit untested outside of simulation so far.

I'm hoping to get some ideas for clever ways to do this using even less components or for making it more robust. I would also appreciate critiques on the circuit I've presented above.

Best Answer

You might want to consider an 'old school' method - using a reed switch with several turns of wire wrapped around it. When current flows through the wire the magnetic field closes the switch.

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The switches do come as spdt types or you can reverse the 'logic' with a simple transistor inverter or logic gate.

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