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I would like to add an Arduino to control a toy with a 1.5V battery and motor in it. The toy is currently controlled by a SPST switch in series with a single 1.5V AA battery and a cheap motor.

I could put in a transistor like How to switch an external circuit with Arduino? or To connect a simple toy circuit and trigger with an arduino? but I would think the voltage drop of the transistor would be significant in the 1.5V circuit.

What would you recommend for switching a 1.5V circuit?

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You would be better off using a MOSFET which, when on, has quite a low resistance. For example the RFP30N06LE datasheet shows a RDS(on) of 0.047 Ω at 30A and 5V.

Using Ohm's Law we therefore expect a voltage drop of:

V = IR
V = 0.5 x 0.047
V = 0.0235

Thus, only 23 mV drop if you are drawing half an amp. I measured using a toy that drew 200 mA and found a 5 mV drop at 1.5V (compared to the theoretical 9 mV).

I have a page about driving motors that illustrates how you might wire up such a device.

MOSFET driver

The 12V on that schematic could just as easily be 1.5V.

The diode on the schematic is a snubber diode which protects the transistor from flyback voltages from the motor.

Comparison to transistor

BJT transistors will have a higher voltage drop between collector and emitter. For example the 2N3904 has a 200 mV drop (0.2V) (called VCE(sat)) when conducting 50 mA. This is a somewhat larger drop than the 5 mV drop that I got with the MOSFET, and a somewhat larger percentage of the 1.5V that you are starting with.

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  • Cool, so if I snub the motor, I could jumper the Drain and Source terminals to my switch terminals.
    – Dave X
    Commented Dec 22, 2017 at 2:24
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    Yes, the MOSFET will act as a switch. As per the schematic you want the Drain to be on the positive side. The Source would be the lead connected to the negative side of the battery.
    – Nick Gammon ♦
    Commented Dec 22, 2017 at 3:16
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    Note that it is an N-channel MOSFET. Check the specs for the MOSFET you choose to make sure it can handle the current of whatever device it is that you are controlling. You could work that out by using a multimeter to bridge the switch connection (in amps mode) and get a current reading.
    – Nick Gammon ♦
    Commented Dec 22, 2017 at 3:21
  • I'd always been a little shy of MOS devices because of static sensitivity from way back when my dad wouldn't let me solder a 4K dynamic memory board, but the device you linked looks like it has plenty of internal protection.
    – Dave X
    Commented Dec 22, 2017 at 5:37
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    Those were the days! These days however I am careful but not obsessive about handling MOS devices. I mean, you don't dance around on a nylon carpet and then pick up a MOSFET, but if you calmly handle them after discharging any stray voltages by touching something which is grounded, you should be OK.
    – Nick Gammon ♦
    Commented Dec 22, 2017 at 6:02

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