user6628: 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? 

user10794: 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.

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.

6628: <p>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.</p>

<p>I could put in a transistor like <a href="https://arduino.stackexchange.com/questions/508/how-to-switch-an-external-circuit-with-arduino">How to switch an external circuit with Arduino?</a> or <a href="https://arduino.stackexchange.com/questions/1563/to-connect-a-simple-toy-circuit-and-trigger-with-an-arduino?noredirect=1&amp;lq=1">To connect a simple toy circuit and trigger with an arduino?</a>  but I would think the voltage drop of the transistor would be significant in the 1.5V circuit.</p>

<p>What would you recommend for switching a 1.5V circuit? </p>
 10794: <p>You would be better off using a MOSFET which, when on, has quite a low resistance. For example the <a href="https://www.sparkfun.com/datasheets/Components/General/RFP30N06LE.pdf" rel="nofollow noreferrer">RFP30N06LE datasheet</a> shows a R<sub>DS(on)</sub> of 0.047 Ω at 30A and 5V.</p>

<p>Using Ohm's Law we therefore expect a voltage drop of:</p>

<pre><code>V = IR
V = 0.5 x 0.047
V = 0.0235
</code></pre>

<p>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).</p>

<p>I have a page about <a href="http://www.gammon.com.au/motors" rel="nofollow noreferrer">driving motors</a> that illustrates how you might wire up such a device.</p>

<p><a href="https://i.stack.imgur.com/TwuX2.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/TwuX2.png" alt="MOSFET driver"></a></p>

<p>The 12V on that schematic could just as easily be 1.5V.</p>

<p>The diode on the schematic is a <a href="https://en.wikipedia.org/wiki/Flyback_diode" rel="nofollow noreferrer">snubber diode</a> which protects the transistor from flyback voltages from the motor.</p>

<hr>

<h2>Comparison to transistor</h2>

<p>BJT transistors will have a higher voltage drop between collector and emitter. For example the <a href="https://www.sparkfun.com/datasheets/Components/2N3904.pdf" rel="nofollow noreferrer">2N3904</a> has a 200 mV drop (0.2V) (called V<sub>CE(sat)</sub>) 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.</p>
