Category Archives: DIY

Prepping the IR proximity sensor


Preparing the Sharp 0A41SK for testing. Soldered a 10uF bypass cap, as per the data sheet and others. Don’t have any JST PH connectors, so the leads have also been soldered. Covered the exposed bits with polyamide tape prior to soldering the cap. Hot glue will be used for strain relieve in protecting the solder joints.

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Filed under DIY, edX, Robots, Soldering

Voltage window comparator front end

The Problem

The comparator circuit presented in the course detected flex in the sensor for only one direction, as this is a single rail circuit. This presents a challenge, as my robot platform uses a single long whisker straight out the front to detect collision. An obstacle could strike the end of the sensor and bend it in either direction. Thus it is crucial that the micro controller be able to read a flex in the sensor in either direction.

My original flex sensor bridge circuit with comparator front end.

The Question

Having measured the voltage differential across the Wheatstone bridge flex sensor circuit, I noticed that a negative voltage when the sensor was flexed in the direction opposite from what the datasheet advises. It stood to reason that this change in voltage might be detectable.

After posting my circuit and question on the discussion board, Thomas_91 was kind enough to supply the following voltage window circuit. It is a dual comparator circuit that detects an upper and lower reference, and gives a HIGH when within the window of the reference and LOW when outside the reference, or the inverse. In my case, I want to measure when the sensor is at rest, and provide a HIGH signal when outside the window. This solves my design problem of wanting to detect a flex in either direction.

Thomas_91’s circuit posted on the EE40LX discussion board. He used a sine generator to demonstrate the window effect.

Thomas_91 produced a graph to demonstrate the window effect using a sine voltage generator.


Having installed LTSpice but never really used it in solving a problem, I recreated Thomas_91’s circuit, and adapted it for my purposes. Verifying the window and threshold behaviours in simulation, I breadboarded the circuit and included trimpots to adjust the upper ceiling of the voltage reference. The flex sensor is measured with a trimpot for adjustment to its balance.

My window comparator circuit. R6 = flex sensor, R9 & R2 = trimpots.

My voltage window circuit with flex sensor (out of frame) and trimpots.

I created a small program in Arduino to read the output of the dual comparator on a digital pin. After fine tuning the trimpots, I was able to get a very satisfactory sensitivity range.

The Sensor Rig

A nut captive in some hot glue allows mounting on the front end of the robot.

After reading the many comments about how this particular flex sensor can break easily, I followed the instructor’s example and mounted my sensor on a tie wrap. Smooth sided tape was used to fashion a sheath that allows the sensor and tie wrap to bend together without bunching up.

The flex sensor bends together with the tie wrap without bunching up thanks to the green sheath. The sheath is affixed to the tie wrap with hot glue.


Filed under analog, DIY, edX, ICs, Robots

Hackable ornament: video demo

Happy holidays from hellocrispyking!

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2014-12-19 · 00:46

Help the CaTrakr find a home


Present incarnation of the CaTrakr receiver, tests OK

Look at this poor homeless CaTrakr receiver. Its guts are hanging out, all vulnerable. The smallest tumble could be its last. What sort of home would you put it in, and why?

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Filed under DIY, ICs, micro-controllers, radio, regulators, Soldering, Teensy

Hackable ornament: my circuit and code remix


Glowing demo with ShiftPWM and ATMega328P

After first coming across the Dangerous Prototypes Christmas ornament design in the summer, I’ve been thinking about how I might build on the design to make the light show a bit more exciting. One aspect that stuck out for me was how all 10 LEDs flash on and off at once. I decided my goal should be to have individual control over as many LEDs in the Christmas tree as possible.


Two shift registers connected to an Arduino

My first experiment solving the problem envisioned one or two 74HC595N Serial In Parallel Out shift registers. A nice glowing PWM effect is achieved through PWM via a CTC interrupt function. While I was able to wire this up with my Arduino’s Mega328, the Mega-only ShiftPWM library is too bloated for the tiny13’s 1K. Although the tiny13 Arduino core does have untested ShiftOut() functionality, the shift register idea is out, as the necessary program at my skill level puts it outside of time constraints. Although unsuitable for now, but shift registers have been added to my list of fun future projects.

Another option I considered was charlieplexing, or using the tri-state properties of MCU i/o pins to control 3 LEDs per pin, set up in an array. However, this would also mean current flowing through i/o pins. Scratch this idea,  but add it to the experiment list, as it might come in useful later.


How to groups the lights up with transistors

Ultimately, I decided to keep it simple and divide the 10 LEDs into groups of two, expanding from a single transistor switching 10 LEDs to 5 BJTs switching groups of 2 LEDs each. The same interrupt-based PWM technique as mentioned above is used on all 5 tiny13 pins. PB5/Reset is kept free for ISP access. Why not drive the LEDs directly from the tiny13 i/o pins? After all, the combined max current from the LEDs is 40mA, and the max current on a tiny13 pin is also 40mA. Good question.

When approaching hackability from a design perspective, maximizing the useful life of a device is a key focus. Future needs change, and if your design lends itself to being adapted along with that change, it will continue to serve a useful purpose for a long time. Building in tolerance and redundancy above the bare minimum requirements increases resiliency to the unforeseen.

Let’s ignore for a minute the fact that a simple code change to open up all 5 pins puts the tiny IC dangerously close to its 200mA limit of VCC to GND. While the two parallel LEDs on each pin might be enough for our current purposes, what if the LEDs are replaced with a heavier load? The i/o pins would probably be okay with the 2 LEDs at the duty cycle as programmed, but the pins would surely fry driving a motor. On the other hand, the NPNs can handle 600mA of continuous collector current, and might one day the board could be repurposed to power a little robot to amble along for our amusement. Plus, they’re insanely inexpensive. BJTs FTW!


Original schematic with my grouped LED remix

Another concern is the ISP port, which shares the serial lines with two groups of LEDs. The 40mA demand of the LEDs could potentially disrupt the flash programming sequence or damage the programmer. Isolating the load from the serial lines keeps things safe for reprogramming the tiny13.

match footprints with printout

Pro tip: printouts can be used to match footprints

Minor design changes included moving most of the cluttered components to the back of the board, arranged in a fashion to allow for a small area for a handwritten message at the top of the board. When designing PCBs, using a block of silkscreen can be handy for writing notes on boards. The switch and battery clip were swapped for components I had available. The switch fit an existing part in Eagle (Omron 1000), which made light work of swapping out the original design’s SMD switch in favour of the through-hole I had on hand.


Creating devices from scratch in Eagle is a 3-step process

The battery clip had no datasheet and no part that came close. Out came the caliper and a notebook, then the SMD pad size and spacing were created in Eagle’s package editor. The placement silkscreen with chamfer to indicate polarity was also created. A tip: when replacing components in Eagle’s schematic view, make sure the polarity of the new part matches your diagram. While it wouldn’t have broken the circuit, the silkscreen indicator for positive would have been reversed, creating future confusion.

The final result is a slightly larger (5 * 8 cm) remixed work, with most of the unsightly components moved to the backside. The battery clip and switch are slightly different. The code has been replaced with the interrupt software PWM implemented by funkfinger based on Atmel Application notes. The sleep function from Ian’s code rounds things off to replicate the battery saving functionality. Here is the code from the original Dangerous Prototypes ornament (hopefully those links work for you. If not, please let me know):

Here’s my frankencode:

The effect of my circuit and code remix is 5 groups of two LEDs glowing on and off, all at different rates. This produces a pleasing twinkling effect, and is sure to light up the season. As of my writing this, my DirtyPCBs order has been batched and sent to the board house. With any luck, I’ll have an update before Christmas.

Original design, bottom layer

Original design, bottom layer

Circuit remix bottom later

Circuit remix bottom later

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Filed under Battery power, C++, code, DIY, fabrication, Hacking, ICs, Lights, PCB design

Then there was three…


Remember the new PV modules I barely mentioned previously? Well, it turns out that two was much better than one. Now we shall see how three in parallel will compare. This time I’ve stuck to nonpermanent adhesive (ha ha) so it can easily be remodeled. Lesson learned from a previous module multiplexing experiment.


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Filed under Battery power, DIY, Solar Stage, Soldering

Testing rig


To allow for current measurement, a Solar Stage has been assembled with clips on both power interfaces (PV module and battery) for connecting an ammeter in series.
Of course, the sun disappears shortly after it’s ready to go. More to follow.

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Filed under Battery power, DIY, Solar Stage, Soldering