How many do I buy of Parts List items without a quantity specified?
A reader asked “I uploaded the BOM [Bill of Materials, i.e., the Parts List] to DigiKey. For the parts where no quantity is specified, does that mean any amount between QTY 1 and QTY10 works? I see a considerable price drop when QTY10 are ordered, but I am not sure I need too many left-over parts considering that I am not planning to specialize in analog circuits (of course, that thought may change as I complete the labs).
My response: Many of the resistors and capacitors in the parts lists do not have a value specified in the “Qty Needed” column because we aren’t really sure how many you need and we expect you to buy a kit of resistors and capacitors, not purchase individual values. This is because in some cases it either does not matter much which value you use (for example, pull up resistors in the digital labs) and in others you choose the value based on what works in the circuit. For example, in the filter circuits and the LM555 oscillator, you can use [almost] any resistor and capacitor value that gives you the correct time constant. So a 1k resistor and 100nF capacitor should work as well as a 10k resistor and 10nF capacitor (both result in an R * C time constant of 100uS).
With only the exception where we have specified a quantity of a particular part, we assume you will purchase a kit of most popular values, which usually includes at least 10 of each value – more than enough for the experiments in the book. (The 100uF polymer capacitor used in the switching power supply is a special low ESR [Equivalent Series Resistance] part so it has a quantity but the rest of the capacitors are pretty much generic so do not have quantity specified — but you could use a generic part in the switcher lab and still get an understanding of how switching voltage regulators work, you would just lose some efficiency.) Most of these types of parts make no sense to buy in single quantities – they are too expensive – but a quantity purchase makes little sense for an individual user. A kit is much more economical than buying individual parts for individual use. OTOH, someone stocking a university lab may prefer to buy the values listed at the quantity pricing which is why we have specified particular part numbers – but in reality, there are dozens of other parts numbers for each of the parts we list that would work as well.