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Topic: Pins stop working on Motorshield R3 (Read 849 times) previous topic - next topic

MarkT

Quote

According to what I see online, an AG3 buttoncell battery is 1.5V and 35mA.  Since the device came with 2 x AG3's in series, that should be 3V at 35mA.  Based on this it looks like I should be under the 40mA limit,


That doesn't make sense - to find the voltage and current you'd need to know the response curve for the laser diode and the internal resistance of the battery and find the intersect - it'll certainly be less than 3V, and could easily be more than 40mA.

I think you found the capacity rating for an AG3 in mAh and confused that with current?  Capacity is the amount of charge a battery can hold, not the rate at which is can supply it.

For instance an LR44 (somewhat larger alkaline button cell) is rated at 160mAh but a new one can push 0.7A through a short-circuit indicating an internal resistance of about 2.2 ohms.  The AG3 probably has internal resistance of about 6 ohms and thus could push 0.25A or so max.
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kanejp63

Okay, yes, I see now that I certainly confused the capacitance with the current.

Based on my last post I went out and bought a pack of 100 ohm resistors.  I was going to move the laser over one pin, put a 100 ohm resistor on in the line, crack open a beer and hand out high-fives, but now I'm thinking that isn't a great idea.

How do I find the "response curve for the laser diode" and the internal resistance of the battery so I can "intersect" them?


MarkT

Now you're confusing capacity with capacitance!

Your 100 ohm resistors will protect the Arduino pins but will probably stop the laser diode from lasing - laser diodes need a minimum current to work (unlike an LED which just glows dimmer at lower currents).  Without data on the exact laser diode there's no easy way to know how
to guarantee it will work.
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kanejp63

Okay, haven't experimented yet to see if the resistor in the line will work with the laser diode, but assuming that it doesn't... what if I just used the original two button cell batteries in circuit with the laser, and then just used a mosfet or something to trigger the laser on and off?

kanejp63

In case anyone else checks out this thread looking for information for their own projects:

The laser works perfectly well with a 100ohm resistor in the line.

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