Build this small oscilloscope. Frequency range is up to 40KHz(25uS full screen) In 4 selectable ranges. Input voltage is between 50mVpp and 50Vpp in 2 selectable ranges. Gain is adjustable between 1 and 100. Accepts AC or DC input . Auto-trigger sweep function to “freeze” the wave display. Free sweep is obtained by setting the potentiometer at either extreme setting. Y-axis adjustment to center the wave. Runs from a 9V battery.”

Link