 # simple current sensing circuit for small gearmotor

I'm wondering if there's a simple way to use an op amp that radio shack sells (LM741 or LM324) to measure the current drawn from SparkFun's mini gearmotor. 90mA @ 6V 800mA stall current @6V

If I power it with 5 volts, could I measure the voltage difference across a 1 ohm resistor in series with the motor, with one of these op amps? Not quite sure just what other resistors I'd need to get an output that an analog pin could read.

option #2: I just found this: http://www.romanblack.com/current.htm This looks like it would do just what I need. The schematic doesn't give a Rsense value, it says to have .05 volts. So, a 1 ohm resistor would probably work if I'm using 5 volt to power the motor?

SouthernAtHeart: option #2: I just found this: http://www.romanblack.com/current.htm This looks like it would do just what I need. The schematic doesn't give a Rsense value, it says to have .05 volts. So, a 1 ohm resistor would probably work if I'm using 5 volt to power the motor?

The resistors value has to be calculated based on what motor current value you want the alarm to go off on. That article give the explanation of how to calculate and build the sensing resistor:

The value of R1 can be calculated by this formula; ohms = 0.05 / amps (so for example to get a setpoint of 2A needs 0.05/2 or 0.025 ohms) Low value resistors can be made by putting resistors in parallel so 0.025 ohms would be 0.1 ohm resistors x4 in parallel. Alternatively, a thin strand of wire or fine pcb track can be used as a low-ohms resistor but make sure it does not get hot in normal operation. For example, a 25mm (1") length of PCB track of width 0.010" has a resistance of 0.025 ohms. If you want to calculate PCB track resistors you can download a great freeware calculator program here.

Lefty

Do you want to MEASURE the current or just have an overcurrent indication? The circuit you referenced really only does the second one.

-- The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons

My Sparkfun gearmotor, running on 5V, draws 89 mA with no load. If I put a bit of load on it, (what I consider more than adequate for my installed application) it's about 150-200 mA. So I will make it a little above that. This is just what I've been looking for...