PC Recycler

a 100 pF, In914 diode, and a multimeter. to test a crystal oscillator

Arduino capacitor for crystal circuit choice

Texas instruments testing crystal circuits http://www.ti.com/lit/an/slaa322b/slaa322b.pdf

http://www.diyelectronicsprojects.com/2012/09/ttl-pulse-reading-logic-probe-circuit-diagram.html pulse detecor circuit - logic probe

http://www.talkingelectronics.com/projects/LogicProbeMkIIB/LogicProbeMk-IIB.html logic probe circuit

http://www.swansontec.com/sprobe.html logic probe circuit

http://www.zen22142.zen.co.uk/Circuits/Testgear/lprobe.html logic probe

http://ecelab.com/circuit-logic-probe.jpg

http://www.pdamusician.com/dpscope/overview_se.html

http://www.instructables.com/id/DPScope-SE-the-simplest-real-oscilloscopelogic-/ very thorough description