Choosing a Sensor

I’m back to discuss my choice of sensor for the Wireless RH/Temp Sensor Module. I wanted to capture a few simple requirements with the device.

  • Should be PCB mount, preferably SMT.
  • Should be capable of automated assembly, to avoid hand-placement in production.
  • Have an accuracy of ±3% RH or better.
  • Have an accuracy of ±0.5°C or better.
  • Have a resolution of 10-bits or better.
  • Have a response time of 10s. or better.
  • Needs to operate at the VCC rail of the device.

With regards to the system VCC I chose 2.5V to allow it to operate in the bottom-end of a Li-Poly battery charge life (around 2.8V), and to reduce current consumption. Keeping in mind that there would need to be LDO regulators to get down to 2.5V, and being conscious of their dropout voltage which might be ~0.1V. The radio works down to 2.0V, and most micros work down to 1.8V in some limited oscillator + power ranges. If for some reason a chosen device won’t work that low, I would have the flexibility to replace the 2.5V regulators with 3.0V or 3.3V ones, even if I didn’t get the same battery life.

One of the things I discovered when I went searching for a sensor device was that there is a great disparity in price with relation to accuracy, but it doesn’t scale linearly by any means.

I first searched to see what people might be using with Arduinos, and found this device at Sparkfun. Unfortunately, the minimum voltage is 3.3V, and it’s a little large.

I also found this board at Adafruit, using a Measurement Specialties device. It meets all of the requirements, so I looked the device up on Digikey. Although it has great specs, it’s also $12 per part in low volumes. So, I poked around on Digikey some more and found some Honeywell parts that met the specs as well. The problem here was the range of costs for similar parts. Most parts were ±4%, and ranged from $18-30 in SIP packages. The ±3% parts were even higher, some at nearly $80 a piece. I decided to take a look at Honeywell’s line guide on their site, and configured the part that I was looking for. The part I found was an HIH7130, which was difficult to buy on distributor sites and was costly ~$22 ea.

So I broadened my search on Digikey for different manufacturers, and eventually found a Silicon Labs part, SI7013-A10-GM1R. The only spec it didn’t meet was the response time (18s.) However, I realized humidity doesn’t change that quickly and I would only be sampling once a minute or less frequently. In addition, the per part cost was $5.13 ea. in low volumes at DK. At Mouser they are slightly less @$5.10 ea. In volume the costs are hard to predict because you can work with distributors and vendors to get the best pricing. But, it appeared that it would be around $4.50 / 1000 pcs.

The lesson here is that in product design, you may allow for one requirement (response time) to slip for another (cost.) In some client projects, there is little wiggle room and often something like response time may actually drive the rest of the requirements. For instance, if a sensor system is operating in a highly dynamic environment that requires continuous monitoring. RH isn’t a good example here, but if the sensor is monitoring the temperature of a mission-critical server node it definitely applies.

Datasheet of SI7013.

Silicon Labs SI7013 RH/Temp Sensor in T/R Strip.

Silicon Labs SI7013 RH/Temp Sensor in T/R Strip.