For example, I just finished building a heated mug for keeping my shaving brush and shaving cream warm. My initial design was based on four ceramic power resistors attached with thermal epoxy to the mug. A temperature sensor and Banana Pi (overkill, I know) running a PID (proportional, integrative, differential) algorithm and power MOSFET rounded out the simple resistor-based heater circuit. I priced out a simple thermal probe — with postage — at about $10 from a popular domestic supplier.
Thinking I might find a better deal on eBay, I searched for a similar sensor online. Not only did I find a thermal sensor, but it was attached to a complete Arduino-based PID control board with a three-digit LED temperature display and power MOSFET switch to drive the resistors. All this was for $11 — including shipping — from China. I found a dozen power ceramic resistors (also from China) for $3 including shipping. Needless to say, I ditched my original design which — by comparison — was simply cost prohibitive.
A month after making the original order for parts, I have a heated mug that works better than expected. I set the temperature to 110°F using the digital display for a guide, store my brush in the mug between latherings, and life is good. The PID controller drives the power MOSFET which cycles 12 VDC through the ceramic resistors. The thermal probe is attached to the side of the mug, providing temperature feedback. However, I can't really recommend the circuit to others because I have no way of knowing how long the circuit board will be available.
It turns out that if you’re using a no-name Chinese source for your parts, you’re fine — as long as you're building one-offs. However, when repeatability is an issue — such as when you or others need to create additional circuits — there's often no way to know if a vendor will have items in stock in the future, and often no clear way to find an alternate source for the same items.
For these reasons, you won't find many projects in Nuts & Volts that call for circuit boards sourced offshore from eBay. That's not to say that everything has to be purchased at your favorite domestic supplier — resistors are resistors, after all.
So, what’s the point of buying a completed circuit online? First, there’s a lot to be said for an experimenter who can integrate circuit modules. Second, who said experimentation in electronics had to involve low-level circuit designs? Sure, I wanted to work with the recently released Banana Pi and practice tuning my own PID algorithm, but my limited time and resources left no real options.
Before you embark on that new project, you might want to take a look at what’s available from the overseas suppliers on eBay. Some things are worth the wait. NV