Tag Archives: humidity

AM2302 (DHT22 ) Temp Humidity sensor and JAL on a 16F628 at 4MHz clock

Having decided I can’t be bothered with digital sensors with oddball serial interfaces like the DHT22 it was time to suck it up when I needed a number of sensors. Cost adds up with lots of sensors – though that Honeywell product more than paid for itself a few times over in much better hatch rates (fertile eggs are about £2 a pop by post, that’s how much of a loss you eat for every failure to hatch!) not every sensor application affects the bottom line like that. Sometimes low cost trumps accuracy, reliability and serviceability. Enter the AM2302, apparently a.k.a. DHT22, produced by the fine Aosong corporation. Their website looks like line noise on my browser, but apparently they are based in Guangzhou, which is China’s third largest city, a conurbation of nearly thirteen million souls.

AM2302, humidity and temperature sensor, a.k.a. DHT22 apparently

AM2302, humidity and temperature sensor, a.k.a. DHT22 apparently

The sensors are cheap, nasty and have poor accuracy, but the price is right, it’s the cheapest way to get a humidity and temperature sensor. Five for £17.70 or a unit price of £3.54 from a Chinese supplier on ebay, Buyincoins ISTR. They have a non-standard one-wire interface. That requires you to be able to tell a 30μS high duration from a 68μS high duration. No problem, eh, even with a PIC running on the internal 4MHz oscillator so each clock cycle is 1μS?

There was already a JAL library for this, called temperature_humidity_dht11.jal so I am in development heaven.

Except it doesn’t work – it acts up after about 20s in the video. It sort of works some times, tantalising short runs of OK in amongst loads of timeout errors. I fiddle with the power supply a little as the AM2302 is claimed to be finicky on the need for 5V. No luck. Tracing the library code I find it barfs around

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Humidity and Temperature sensor on Cosm IoT

The trouble with geese is that they are waterfowl, ie used to hatching near water, unlike chickens, so you tend to have to control the humidity of the incubator as well as the temperature. Humidity is terribly counterintuitive – unlike temperature humans have no real sense for it. So I was looking at how to make a humidity and temperature sensor.

Goslings

Goslings. Like most animals, geese don’t look so mean when young

The problem starts with our cheap Chinese import incubator. There’s no real calibration on the temperature dial, and you know it’s a bad sign when the manual tells you ‘when your small poultry is ready’. I was really sceptical about how well this would stabilise the temperature. Quite unfairly as it happens. Continue reading