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Would it be safe to use a 1.5 volt motor with the adafruit motor shield? It is rated for 4.5 to 36 volt motors so I am unsure what will happen. Also, what would be a good way to determine the voltage of a motor if you have no information on it at all?

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Many motor drivers can directly drive a 1.5 volt motor from a 5 V (or more) power supply, because they have built-in current limiters ("chopper circuit").

Alas, the Lady Ada motor shield FAQ specifically says: "Can this shield control small 3V motors? Not really, its meant for larger, 6V+ motors."

It appears to me that you can add a few power resistors to (inefficiently) drive a 1.5 V motor from the adafruit motor shield using the "L/R driver" system.

The "Current Control of Stepper Motors" document describes the older, less efficient "L/R driver" system, and compares it to the more modern and efficient (but complicated) "chopper circuit".

This motor shield uses the L293D motor driver chip, which simply drives the output high or low, without any limit except its thermal protection limit. To convert it to a L/R driver system:

  • Ir: Find the rated current Ir (if it's not written on the motor and you can't find the datasheet, measure the motionless coil resistance Rcoil with a multimeter and calculate Ir = 1.5 V / Rcoil). If this is more than the 0.6 A that the L293D can provide, do the remaining calculations with Ir = 0.6 A instead. (Example: measure 3 Ohms; Ir = 1.5 V / 3 Ohms so Ir = 0.5 A).
  • Vdd: What is the voltage Vdd of your power supply? (Example: VDD = 5V, but lots of people prefer 12 V.)
  • P: Your power resistors need to dissipate up to P = Ir*(Vdd - 1.5 V)/2. (Example: P = 0.5 A * (5 V - 1.5 V)/2 = 0.88 W)
  • R: Your power resistors need a resistance of R = (Vdd - 1.5 V)/(2*Ir). (Example: R = (5 V - 1.5 V)/(2* 0.5 A) gives R = 3.5 Ohms).
  • Get or make 4 identical power resistors that have approximately the above resistance and at least the above power rating. (Example: a 5 W, 4.7 Ohm resistor will work nicely ... or a series of 4 quarter-watt resistors in series, each one 1 Ohm, produces a 1 W, 4 Ohm resistor, which will also work nicely.)
  • Connect one end of each resistor to one of the 4 halfbridge terminals on one side of the motor shield, the other end to one of the 4 pins of your bipolar stepper motor.

I assumed your motor was a 4 wire (bipolar) stepper motor. Does this L/R thing work for DC motors also?

The L293D datasheet says it requires something in the range of 4.5 V to 36 V to function properly, so I wouldn't bother trying to power the adafruit motor shield from anything less than 4.5 V.

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It won't drive a motor with a 1.5V supply, you need an H-bridge with very low saturation voltage transistors. The LB1630 driver chip might work, it's OK with 3V motors.

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  • \$\begingroup\$ Ok. What would happend if I tried to run it on the shield? Even if I used an external power supply such as a double a battery? \$\endgroup\$ Feb 11, 2011 at 22:14
  • \$\begingroup\$ The Adafruit shield won't drive a 3V motor (I've tried it), so it isn't likely to work. \$\endgroup\$ Feb 11, 2011 at 22:27
  • \$\begingroup\$ What happened when you tried? Did anything fry or did it just not work? \$\endgroup\$ Feb 11, 2011 at 22:30
  • \$\begingroup\$ Nothing much. It tried to turn but there just wasn't enough voltage. \$\endgroup\$ Feb 11, 2011 at 22:59
  • \$\begingroup\$ Enough? I was thinking it would overvolt it. Hmm. \$\endgroup\$ Feb 11, 2011 at 23:04

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