Using a RHT03 (aliases: RHT-22, DHT22, AM2302) temperature/humidity sensor from Arduino

I picked up a nice compact little temperature and relative humidity sensor called the RHT03 for a project from Little Bird Electronics. It and very similar parts appear to go by the names RHT-22, DHT-22 and AM2302. You can find the part at SparkFun, Adafruit, etc too.

It took a lot more work to get it working than I expected, so I thought I'd write it up here for anyone else who is looking into it. There's sample code at the end of this post, but you should probably read the details because this is a quirky beast.

UPDATE: I since found a library on GitHub: nethoncho/Arduino-DHT22 that does a better job more simply and compactly. It works fine with my sensor. It needed some changes for Arduino 1.0 and some further tweaks to work how I wanted, so I've uploaded a fork here: https://github.com/ringerc/Arduino-DHT22.

Extending Arduino example CIRC-05 to use hardware SPI control

For kicks, I've extended the basic Arduino shift register LED control example CIRC-05 from www.oomlout.com to use the Arduino's hardware SPI routines instead of software signalling.

As someone who has done very little with low-level electronics and who didn't know what SPI even was until today, this was embarrassingly easy. Kudos to the excellent Arduino libraries and the great documentation for making this simple.

I'm posting the re-written example for CIRC-05 here. It has the original software-based control as well as support for SPI, so you can see how similar the methods are.

(BTW, if you were wondering what a "latch" is in the IC, see this example.)

SparkFun Inventors Kit CIRC-03 - Motor not working (spinning)? It's an error in the instructions

I've started playing with some basic tutorial/toy electronics stuff using the Arduino platform and the "SparkFun Arduino Inventors' Kit" (hardly "inventors'", but anyway...) after picking it up as part of an order from the awesome outfit Little Bird Electronics. While generally good, I've hit an interesting issue with the kit that's worth documenting for anyone else who has it.

The short version: If you're using the SparkFun kit that specifies a 10kΩ resistor and the test circuit doesn't work (the motor won't spin) you might need to use a lower valued resistor between the transistor and pin 9 of the Arduino board.

If this is the case, you'll find that when you flick the motor's drive around with your fingers so it spins, sometimes it'll spin down slowly and sometimes it'll stop suddenly, depending on whether the Arduino is currently trying to drive it or not.

Check for all the usual errors before assuming the issue described here is what's wrong with your circuit. You might've reversed the flyback diode, made a poor connection on a power rail, etc etc.

Windows command line survival: findfiles, find+xargs's dim cousin

Sometimes, we have to use the Microsoft Windows cmd.exe command line. Maybe it's at a client site or on a user machine where installing PowerShell, Cygwin or Mingw isn't appropriate. Maybe you're on a domain member so locked down by group policy you can't do anything if you want to. Either way, you have to use cmd.exe, and you're swearing.

I find out about the odd useful command that makes the Windows shell more usable, and I'll be trying to post them here. The first is forfiles, a basic alternative to find -exec or find|xargs.

Bulk conversion of OpenType (OTF) to TrueType (TTF) or Type 1(PFA/PFB) fonts using FontForge

FontForge is a great tool for converting fonts. I needed to convert a lot of OpenType fonts to TrueType or Type 1 format to use them with Apache FOP, as FOP doesn't yet support OpenType fonts with CFF glyph formats. Doing the conversion in one FontForge script didn't work well, because (a) it leaked memory until it got OOM-killed and (b) it only used one CPU. So I wrote a helper shell script to control the job - posted here in case anyone else needs it later.

PostgreSQL: Great even when you can see the warts

I’m a very happy PostgreSQL user, and was interested to read a recent post by an MS SQL Server DBA advocating PostgreSQL.

This post is a response, enumerating areas where Pg may not be so hot after all. You might say it's my tendency to always see the negative, or perhaps it's just my desire to inform others of the downsides of something so the loud advocacy doesn't leave them feeling let-down when the reality hits. I'd like to think it's the latter. Telling someone how it is from the start is much better than showing them the rosy-goggles view until they're committed. So, from an enthusiastic PostgreSQL user who spends a lot of time helping out on the mailing lists, here's a list of some of the warts and quirks I'm aware of, so you don't have to find out about them the hard way:

This post is based on PostgreSQL 9.2. Each version improves a great deal, so don't be surprised if some of these issues are gone soon.

No further work on Kobo

This article is part of an extended series on Kobo development and investigation

I've stopped work on the Kobo software and dev env.