Create something that is really usable in practice - i.e. it must have sufficient sample rate to at least cover the audio range (DC up to well over 20 kHz), and a wide input range (from Millivolts up to several Volts). The present design actually is usable up to about 150 kHz (sample rate is 1 MHz = 1 million samples per second). The maximum voltage range is -15 to +20 Volts, but can also go down into the low mV range.
Use only readily available, inexpensive parts which don't need special skills to assemble - e.g. no tiny surface mount components. The whole scope can be put together with a cheap soldering iron, a small wire cutter and flat-nosed pliers.
Keep down cost as much as possible and design it in a way that all the components are easy to procure and assemble, so any moderately skilled hobbyist can build his/her own (see circuit description later).
Make it easy to use so even a user without much experience with oscilloscope gets a quick start, and make it compact so it doesn't use much desk space.
Last but not least, make it an open design so anyone can improve upon it. Note that the design may be used without any restrictions for personal, non-commercial use only. Any other use is strictly prohibited without explicit, written permission by the author.
You can get additional information as well as download the original design files (schematic, layout) and scope software from my oscilloscope homepage. From this site you can also obtain the bare printed circuit board and (as long as I have some left) the fully assembled scope and accessories.
In case you wonder what LCS-1M stands for, it's Low Cost Scope, 1 Megasample/sec.
Disclaimer (yes, this is a litigious world :-):
The author of these pages does not assume any responsibility whatsoever regarding the design, construction or use of the described circuit. The author cannot be held responsible for any damage to persons or property connected with the described design. This includes (but is not limited to) damage to your computer, fitness for a specific task, and specified performance. If you decide to build the oscilloscope and use it, you do so at your own risk. Observe safety guidelines when soldering, as well as when using the oscilloscope. Never apply any voltage exceeding 20V to the oscilloscope inputs.
Step 1: Filling a Niche
Unfortunately, even with significant reductions in cost in recent years, a usable oscilloscope remains out of reach for many people who could benefit from it - especially young students just starting out, and many hobbyists on a limited budget. A good low-end standalone scope easily comes in at over US$1000, and even PC-based scopes (which connect to a computer for display and control) usually cost US$300 and above.
On the other hand, most "hobby" solutions (sound-card based or purely microcontroller based) lack sufficient performance and usually are not much more than toys without much practical use. Most of the time they are more like "proof of concepts" that lack any decent frontend (to amplify small signals or attenuate signals) and have bandwidths much too low even for audio.
Of course it is often possible to pick up a decent used analog scope on Ebay for a good price, but most such scopes (cathode ray type) are rather bulky and bothersome, especially for someone without the luxury of a spacious electronics lab, and they often are difficult to use without a good dose of prior experience.