The RHT03 digital temperature and humidity sensor

RHT03 Digital Temperature and Humidity Sensor
RHT03 Digital Temperature and Humidity Sensor
Accurate but expensive - the RHT03.

While the use of analogue sensors for detecting temperature are common in the world of open source electronics, digital devices are less so. The RHT03 could help change that, offering a low-cost high-accuracy sensor which connects easily to most prototyping platforms.

The first thing to notice about the RHT03 – also known as the DHT-22 – is its breadboard-friendly layout. Mimicking a T-style package, the legs are properly spaced for connection to any common breadboard type, while also allowing for the component to be soldered to a through-hole PCB for a more permanent project.

Sadly, the RHT03 is clearly made on a budget: the legs are extremely thin, and it can be fiddly to get the breadboard to accept the component without bending one or all. If you’re using the RHT03 in a project where it’s going to be frequently moved around, consider adding some reinforcement.

The RHT03 is an odd beast: although digital, it’s not a One-Wire device and doesn’t work with any common libraries. Thankfully, resourceful hackers have fixed that problem: a GitHub project page provides a simple library for the component plus sample code which spits out the current temperature and humidity.

There are limitations, however: query the RHT03 too quickly and it will return an error, something which is never a problem with an analogue sensor. That restriction – due to the digital nature of the device – comes with an corresponding upside: unlike a thermistor, there’s no complex calculation to carry out in order to arrive at a human-readable figure.

Connecting the RHT03 to an Arduino and running the sample script results in two figures: temperature in Celsius and humidity as a percentage. Using a calibrated multimeter with K-type temperature probe proved that the temperature was accurate, and the humidity didn’t seem far off. Accuracy is official stated as ±0.5°C and ±2% RH.

Compared to using two separate components, the RHT03 has a dual advantage: the sensors are located together for better accuracy, and it requires only a single input pin on your controller along with VIN and ground connections. It’s also battery-friendly, drawing around 1.5mA when reading and 50µA when in standby mode.

If you’re using a non-Arduino prototyping platform, the RHT03 will likely still work – thanks largely to a wide supply voltage range of 3.3-6V – but you may find yourself doing a bit of hacking in order to implement the DHT22 communications library.

There is a catch in all this, however: at £8.51, the RHT03 is an extremely expensive option compared to analogue sensors. If you need accuracy, it’s a good option, but be prepared to pay for the privilege.

Pro: Accurate temperature and humidity readings from a single pin.
Con: Very expensive compared to analogue equivalents.
Supplier: Proto-Pic, £8.51
Score: 6/9

The Arduino ProtoShield v5

ProtoShield v5 Clone
The breadboard fits perfectly on the ProtoShield.

Arduino ‘shields’ – add-on boards that connect to the Arduino’s headers to add additional capabilities – are handy things, but sometimes you need something a little more custom. Although it’s possible to make your own, the Arduino’s famously non-standard pin spacing makes it difficult, but there’s a solution: the ProtoShield.

As its name suggests, the ProtoShield is a shield which makes prototyping on an Arduino significantly easier. Often supplied in kit form, the ProtoShield’s design is open source. As a result, it’s possible to get the device pre-made from a variety of sources, which sadly means you’re often taking a gamble on quality unless you buy directly from a reputable supplier.

The shield on test, unfortunately, is from no such source: manufactured by an unknown Chinese OEM and sold through Hong Kong gadget site DealExtreme, the design is based directly on Adafruit’s implementation of the ProtoShield with the logo removed before the PCB has been printed. That’s in direct contravention of the Creative Commons licence under which the open source design is provided, and we’d recommend you look elsewhere if you’re planning on buying one.

The shield itself arrives as two separate components: the ProtoShield, plus a mini-breadboard with an adhesive pad on the underside. This breadboard is specifically designed to fit on top of the ProtoShield, allowing you – if you so choose – to combine the two into a portable prototyping platform.

The other option is to use the mini-breadboard on another project, and concentrate on the ProtoShield itself. The PCB is covered in through-hole soldering points, and a glance at the underside reveals a combination of connected and disconnected circuit paths. There’s room for a wireless module, an SOIC solder pad, and in addition to the usual Arduino headers there’s an additional five for ground and 5V on many designs.

The concept is simple: prototype the layout of your custom shield’s components with the breadboard, and when you’re ready solder the components in place directly onto the shield to create a permanent custom creation. It’s a neat idea, but there’s a problem: the ProtoShield isn’t cheap. Even as an unlicensed knock-off shipped from China, the ProtoShield will set you back around £7, which compares poorly with some stripboard and a set of angled headers.

As a prototyping platform using the stick-on breadboard, however, the ProtoShield is great. If your project calls for LEDs, you’ll be pleased to see two already form part of the shield’s design, along with a handy switch. The fact that the reset switch is brought to the top is also a welcome sight, as many shields forget how inaccessible the Arduino’s version can be when the shield is in place.

Pro: It’s a great portable prototyping platform when combined with the breadboard.
Con: While easy, it’s an expensive way to make your own shields.
Supplier: DealExtreme (uncredited clone of Lady Ada’s ProtoShield)
Score: 7/9