External sensor mod

[Tony]

First of all the PCB mod - if you have one of the original batch sold in 2013 you'll need to add a wire link to connect the sensor to the PIC.

Link the 2 pads as shown, between R18 and R13.



For the external sensor you'll need another DHT11, a length of 3-core cable and a 3.5mm stereo jack plug,

Use the 3-core cable to connect the sensor to the plug as follows -

Plug tip - 5V - DHT11 pin 1
Plug centre - data - DHT11 pin 2
Plug body - 0V - DHT11 pin 4

Holding the DHT11 sensor with the grill facing you and the pins pointing down, pin 1 is the leftmost of the 4. Pin 3 is unused.

The thermometer will display the external temperature in place of internal temperature whenever the external sensor is plugged in. If you want to display both alternately link the pads marked RX/LK1 on the PCB, just above R14. External temperature will be indicated by the DP on the centre numerical nixie.

[sean4star]

I'm looking at adding an external sensor to see if it fixes the problem I talk about HERE. One quick question:

Is it possible to use a DHT22 instead of a DHT11 for the external sensor? The DHT11 specs show a temp accuracy of +/-2C, which seems pretty lousy to me, vs the DHT22 accuracy of +/-0.5C.

[Tony]

That's why it is only used for humidity. The software reads and uses both bytes of data so a DHT22 should work, I know I've answered that question sometime before but it may have been by email.

[adminbear]

Well, after having the common issue of my unit not showing a valid temp, I did the external sensor deal and it seems to be right on!

Gonna order another DHT-11 and instead of hanging it off a cable, just going to put it directly onto the plug so it sits immediately behind the unit so that it goes back to being one piece.

[Tony]

You could also try changing L1 for a 33uH, it will reduce the display brightness slightly but also cuts the power dissipation.

[Higginz]

This weekend I picked up all the items to make the external sensor from my local Radio Shack and eBay. Everything was under $20 so wasn't too bad. I put together the harness with a DHT22 as mentioned in some previous posts. It did work, but not quite right. Humidity displayed 01%, where actual was over 20%. Temperature seemed to be right at 71.6 F. If I hold the sensor in my hand and breathe warm air onto it, the displayed humidity goes up to 03% and temperature goes down, to the 40 degree mark! No way I can blow air that cold! I did make the modifications to the underside of the board for the external sensor. I have ordered a DHT11 to replace the DHT22. In the meantime, Tony, is there any checks I can do to the board to make sure my issue lies within the sensor?

[Tony]

The sensor output is digital, if you can read the temperature from it then it's connected correctly and the board is working OK. $20?

[Higginz]

Thanks for the info Tony. Ill have to wait and see how the DHT11 works out when it gets here. I had picked up the DHT22 from eBay for $4.50 and $1 shipping. Expensive for a single component I know, but I didn't want to buy 10 or wait two weeks for one to be shipped from China. Radio Shack had an 8 foot headphone extension cable for $9 and that was the shortest stereo cable they had in stock, and $5 for a pack of assorted shrink tubing. So, not too bad, and I have plenty of shrink tubing now.

UPDATE!

Came home today to find the DHT22 external sensor to be reading 32.5 degrees Decided to try shorting the external sensor cable from 8 feet to 2.5. Made no difference. Seems the DHT22 is reading opposite of that it should. Took a new fancy DHT11 that came in the mail today and soldered it up. BINGO! Temperature now reads within 0.8 degrees of actual and humidity reads within 1% of actual. Maybe the DHT22 didn't like me...

[sean4star]

Hey all, just getting back to some of my projects after a long hiatus to take care of family and work.

I finally put together my external sensor, using the dht22 that I ordered a while back. And what do you know? I'm getting a good temp reading, but the humidity is just plain wrong. After letting the external sit for an hour I'm getting a constant 71.8℉ and 1% humidity. When I unplug it, going back to the internal sensor, I'm getting 77.0℉ and 44% humidity.

Of course, that's when I logged-in here and see higginz's comments. Got to say I'm pretty frustrated. I suppose there's something different about the dht22 that the thermometer just doesn't like.

Tony, any suggestions? Or should I follow higginz's lead and just get the dht11?

[Tony]

I'm not sure what the problem is with the DHT22 but they seem somewhat unreliable. If you compare the datasheets the data timing and format is identical and being checksummed if it wasn't read correctly would be refused, causing it to drop back to the internal sensor.
From my own tests though the DHT11 seems to be accurate as an external sensor and much cheaper. I would also recommend changing L1 to a 33uH part.