Friday, March 31, 2017

Telegraph Sounder Player

Yesterday I put the finishing touches on my latest project, the telegraph sounder player.

Video link first if you want to see it in action before you read:

The device on the left is the telegraph sounder player, and the device on the right is the telegraph sounder. The telegraph sounder itself is a vintage telegraph module. The player allows for up to four sounders to be plugged in to it, and plays back random snippets from text files loaded up on an SD card. It uses American Morse Code (as opposed to international). I loaded the SD card with excerpts from my favorite novels as well as full novels from Project Gutenberg.

The project was made for the owner of the telegraph sounder, Don. Don used to work as a telegrapher for a railroad company (US land based telegraph operations used American Morse code). He had a first version of the player built for him during the most recent Build Madison event and came back to Sector67 with notes for some features he would like for the next version.

Features I included in this telegraph player include: SD card playback, 4 sounder support, knob for speed of playback, separate knob for inter-sentence interval length, status led and ON/OFF toggle, CNC milled hardwood box with brass corners, battery power (est. 10 hours of playback), battery charging circuit onboard.

Design process in brief:

I had four 5V relays I found in a bin here at Sector, so I chose to support up to 4 sounders and to use a 5V Arduino for the brains. The relays are each switched by a BJT transistor circuit, and when switched they connect the sounder port to the battery pack.

The ports are barrel jack connectors with a 10uF capacitor across the leads and a high power rating. low resistance resistor. The resistor is mostly just in case the leads going to the telegraph are placed so that they are touching each other.

I had been playing around with trying to minimize the currents induced by the sounder turning off (the clack, as opposed to the click). A large ferrite choke close to the telegraph with many windings of the connector cable worked, but was a little unwieldy. The capacitor doesn't entirely solve the problem, but helps make the clack sharper. Too large a value and the clack actually bounces presumably as the capacitor charges on the induced current, and then discharges back into the sounder and re-enables it somewhat. The 'problem' is in any case limited to the Arduino resetting only when connected to a computer during the disconnection of the sounder, so it was possible to ignore. 

The SD card is connected through a bi-directional level converter. The Arduino has limited memory, so the code handles things by grabbing small chunks of the files into four small buffers for the four ports. The Morse codes are stored as 2-byte unsigned int objects, with the exception of a few special 4-byte unsigned long objects for some of the punctuation. The code can be found here: Arduino Code.

Access to the Arduino is through the bottom of the box, which is attached with screws to corners that are glued inside the box.

The project uses two battery cells that each have overcurrent and undervoltage protection built in. The charging circuit is as described here.

You can find more photos of the project and an additional write up here:

And a video here:

Friday, March 17, 2017

Painting Robot Compilation

A while back somebody donated an old robot arm kit to Sector67. I built a small model of it with glue, tape, foamboard, and potentiometers, and hooked it up to an Arduino. The project is very simple, but it has been very popular at science fairs (and has survived many hours of use requiring minimal fixes only). Here's a compilation video.

Friday, March 10, 2017


Another set of magnetic tile polyhedra. This time, three different trapezohedra.

I used this resource to define the kite and dihedral angles, but the hexagonal and heptagonal did not work out (and I believe the octagonal also has issues). Fixing them is on my to do list. I plan to write a script that actually generates them from the antriprisms.

Tuesday, March 7, 2017

Talking Calipers 2

Preface: Voting on a contest I submitted this to is open right now. You can vote once a day through this link.

Before going all in on the research for my minimal-cost minimal-footprint all on one custom PCB version of the Talking Calipers, I went through my bin of parts and made one more with just what I had already available. It uses:

The video demonstrates that the reading is taken when the button is pressed, so the display can potentially change to reflect that the measurement has changed since the reading began being spoken aloud. The button is a limit switch super glued next to the thumb wheel.

It was a useful learning experience to build, and it was good to have it done quickly for demos and such, but in the end I built something very, very similar to the Adafruit Wave shield ( but slightly more expensive.

Feature list (all were tested on the breadboard, though some haven't yet migrated to the somewhat portable prototype):
  • Rechargeable battery on board, recharges via USB
  • Separate amp with lower gain for headphones for hearing safety
  • Headphone detection for automatic headphone/speaker output switching
  • SD card for easy file loading--can be made to work in any language by switching out files
  • Relay mutes headphones to reduce pop
  • Sleep mode that wake up on request for a reading, for longer battery life
Features not yet added:
  • Filter to get rid of a hum caused whenever the SD card is read
  • Low battery indicator (through sound or vibration ideally!) (though thanks to a circuit built in to the battery header, it will turn itself off to protect from under-voltage to the battery)
  • Inches mode protocol is different and currently reads off incorrectly
  • Audio files and code support for reading numbers more naturally (currently reads off each digit separately)
  • Change sleep mode to trigger only if the readings are constant and the button is not pressed for a certain time interval (right now just based on button)
I actually recorded my voice for this project since I couldn't find everything I wanted online. I used to create the files and then used to make sure the files were 44.1kHz sampling rate and 16 bit sample size and then volume adjusted to a max volume that did not clip. (Note: the wav shield played most any wav files I could find fine, but the Teensy audio library was picky about the settings, necessitating use of the sox tool).

I estimate the battery life at about 10 hours of use (assuming it never enters sleep mode). The main power switch does lose the zeroing, but is convenient to have (out of the box, these calipers draw enough current to drain the coin cell in half a year to a year, unless the battery is removed). Auto-entering sleep mode when the user is away helps extend the battery life even more.