The Make It Last Build Series Newsletter, Build #2, Dispatch #5
Greetings! Welcome to our fifth dispatch for the Robot Plant Build. We've enjoyed this sojourn into the land of robotic plant life, and hope you have as well. This week, we'll finish building our robot plant, and configure it to bloom properly. We'll also revisit the topic of capacitive touch sensing, to clear up a misunderstanding from last week.
See full details on the Make It Last Build Series, this robot plant contest, and a rundown of prizes, including a $200 gift card and copies of Make: Electronics on the contest landing page.
Cheers,
Matt and Gareth
back to top Get Your Entries In!
We're anxious to see what your robot plants look like! Remember that all entries need to be in by December 22nd, but you can submit your entry as soon as you're ready. As a reminder, here are the awesome prizes that the top plant builders will win:
The grand prize winner will receive:
- $200 in the form of a pre-paid gift card
- Make: Electronics
- Energizer® Night Strike Swivel Light
And the runner up will receive:
- Make: Electronics
- $25 Maker Shed gift certificate
- Energizer® Night Strike Swivel Light
back to top Finishing the Plant
We're almost done building the plant; there are only a few more steps that need to be taken to get everything working. Check out the final part of the tutorial to see how we finished ours!
back to top Tech note: Touch sensing, revisited
After last week's capacitance sensor discussion, I noticed an odd behavior with the touch sensor circuit for the robot plant: it worked fine while powered by something that was grounded to earth, but not very reliably when powered by batteries. Upon closer inspection, it appears that the circuit was not working in capacitive mode at all; instead, it worked by detecting the 60Hz hum that's present in the air, but is absorbed and radiated out through our bodies! To test this, touch the probe of a running oscilloscope -- it should pick up a sine wave that oscillates at about 60Hz (or 50Hz, depending on where you live). I captured the above screenshot from my o-scope, which shows a 2v peak-to-peak wave(!). I had a little fun turning on and off lights and watching how they affected the brightness. When I turned my two fluorescent lamps off, the signal became much less noisy.
Unfortunately, since we want the plant to work without being plugged in, it means that this sensing method won't work. Instead, we can switch to a simpler method -- resistive touch sensing. The idea here is to think of a finger as a resistor with a really high resistance. Then, we use the same low-power current source and exposed wire sensor as before, but we add a second ground wire next to it. When the user touches both of the wires, the sensor pin is shorted to ground, which causes the sensor voltage to drop noticeably. Although it's not as cool as the capacitive method, it is more reliable, and should be much less frustrating to get working. See this week's build for instructions on how to make the resistive touch sensor.
back to top Microchip Discount Code
Want to pick up a PIC programmer or development kit? As part of the contest, Microchip is offering a 20% discount on the following development tools:
PICkit3
PICkit3 Debug Express
ICD3 In-Circuit Debugger
XLP 16-bit Development Board
F1 Evaluation Platform
F1 Evaluation Kit
If you've been thinking about getting started with PIC programming, this could be a great opportunity to get a good deal on a programmer. To request a discount code, send an email to makeitlast@makezine.com.
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