The 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

Leave a Reply

Your email address will not be published. Required fields are marked *