Detecting current

Hi all,

What is the easiest way to detect an increase / decrease in current?

I've got an RC car and wanted to setup an intelligent lighting kit for it with my arduino. I'd also like to be able to detect small changes also if possible. e.g(10% throttle, 20%... Full etc)

I'm pretty sure i can code it up no problem, just need to find the best way to detect throttle increase / decrease and reversing.

thanks all :slight_smile:

It might be easier to decode the signal going to the speed controller.

Since you have both forward and reverse, you'd need to detect both positive and negative current flow (a nuisance with a unipolar power supply). Plus you have to insert the sensor into a relatively low-voltage, high-current (compared to most Arduino projects), electrically-noisy circuit without wasting too much power or getting fooled by noise spikes.

Ran

You could try a ACS712 from Allegro. If you run it off 5V, its output is biased at 2.5V and it swings above and below that to indicate positive and negative current. Allegro has versions that go up to 200A if you need it.

The easiest and most common way to sense current is to put a small value resistor inline with the current you want to measure and detect the change in voltage across it.

It is probably a bad idea to directly connect an A/D input to the motor drive circuit. It might be better to buffer, low pass filter and limit the voltage of the signal into the Arduino to less than +5V since it is likely it will be full of crap from the DC motor and could damage the input.

If the voltage is too high for the Arduino (ie > +5V) then use a divider or an opamp to buffer the signal and reduce it down to a level the A/D can handle.

If the voltage difference is too small to measure, then perform a differential measurement using an opamp to measure the voltage across both sides of the current sense resistor and amplifying the difference.

The resistor should be kept small (as in low resistance) so as to not waste power, but large enough to allow you detect a meaningful voltage drop across the resistor, as well as being rated high enough to handle the power dissipated at the maximum current drawn by the motor.

The precise value you use will depend on the amount of current drawn...

One of the nice things about the Allegro Hall effect devices is that you get 2.1 kV isolation from the circuit you are measuring. They also have an adjustable low-pass filter. Just one capacitor. One of their applications is motor speed control current sensing, so they are ideal for your application and they only cost $4.50.

Thanks everyone, I've now got a good starting point to work from.

I think I'll try the Hall Effect Sensor and see how that travels, though i have seen some really great tutorials on Hijacking the RF signal from the receiver.

I've also thought about making a completely seperate system using a 2axis Gyro.

keep the posts coming if anyone's got more idea's on this.