This Instructable describes a preamplifier circuit for making smartphone scopes more versatile, more useful, and highly resistant to accidentally transferring lethal voltages into your audio jack. The input impedance is increased from around 2 KΩ to 1 MΩ, the voltage range is 10 mV to 50 V or more, and the safe overload range is equal or higher. The scale is easily calibrated with not much more than a volt-ohm meter (VOM.) No software is included; why reinvent the wheel when good apps are already out there for many platforms at a few bucks—or even free? The same basic circuit can be used, with minor changes, in many other systems including laptops, iPads, and Android tablets.
STANDARD WARNING: the complete circuit and your phone should be safe from accidental overvoltages of reasonable magnitude. But I take no responsibility for any damage that may occur to you, the circuit, or the phone. Nor can I guarantee that your particular brand/model phone will give good results. The frequency range will be limited by the parameters of your device; most should be usable from about 75 to 15,000 Hz (no DC). UNDER NO CIRCUMSTANCES SHOULD THE PREAMP OR YOUR PHONE EVER BE CONNECTED TO THE AC WALL JACK OR POWER LINE.