The Datum

Observations From The Hobby Engineering Lab

Counterfeit Money Detection US currency has numerous anti-counterfeiting features. The one showcased on the left is a UV sensitive watermark that is invisible under normal lighting. I used one of my UV LEDs to make a simple counterfeit bill detector. What You Are Looking At The first image shows part of a US $20 bill back-lit by the UV LED. The second image is the same bill back-lit by an incandescent light bulb. What To Notice The color of the first image is a bit unnatural due to the purple-tinged visible light output from the LED. That green stripe behind the words "USA TWENTY" is visible in the UV-lit image and invisible with incandescent light. If you are seriously checking for counterfeits, make sure those little words match the apparent currency amount. Some counterfeit bills use washed smaller denominations as a paper source since its not easy to get the right feel of currency paper. While you can wash out the ink, you can't wash out the watermark so you end up with a bill that looks like a $100 but has a watermark that says something else. Relay Contacts Most small relays have sealed, opaque cases that hide the mechanism. What fun is that? I decided to take the top off the DPDT relay used in my Shuttle Motor Controller project to see if I could expose the mechanism without destroying the functionality. It worked! I plan to make a video of the circuit controlling a scissor lift model. I'll make sure to include a shot of the relay mechanism in action. What You Are Looking At This is a close-up of the switch contacts with the coil power off. What To Notice There are three pairs of contacts. In this view the pairs are arranged in vertical columns. The middle "common" contacts are touching the left "non-energized position" contacts. When the coil is energized the armature will push the common contacts to the right until they touch the "energized position" contacts. Don't Judge A Motor By Its Cover Motors look pretty simple on the outside, but the farther into them you dig, the more complexity you find. Thanks to informal industry standards, there is a reasonable level of external similarity among a wide range of motors. For each common size, there are at least hundreds of variations in magnets, windings, brushes, bearings and other components. Matching motors by external appearance doesn't have a very high probability of providing matching performance. What You Are Looking At This image shows the brushes from 3 motors that look almost identical on the outside and which all fit into the same set of gearboxes. What To Notice The brushes vary significantly. The first is from a low performance 1.5V motor and has dainty little brushes. It might not be clear in this picture, but a good part of the apparent thickness is lubricant. The middle motor is from a "high performance" 5V motor. The last looks pretty industrial. It is from a 12V motor used in automotive accessories and is intended to last the lifetime of the car. Shuttlle Motor Controller Its fairly common to want a motorized mechanism to shuttle back and forth between two set points. Almost everyone who thinks about this problem quickly realizes that its pretty easy to install switches which get bumped when the mechanism reaches the limit of travel. The rest of the system is a little less obvious. You need a mechanism to reverse the direction of current flow and one bit of memory to remember which switch was hit last. This sort of problem is often handled by a microcontroller and h-bridge, but that is considered over-kill by some. What You Are Looking At The breadboard circuit uses a DPDT relay to reverse the current flow and two NAND gates configured as an RS Flip-Flop to provide the bit of memory. What To Notice This is a fairly simple circuit to build and understand. If anything goes wrong you can debug it with a voltmeter. This project will soon be fully documented on the Hobby Engineering web site.

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