RTD eh?

So okay, there is (again) this page: http://openenergymonitor.org/emon/buildingblocks/rtd-temperature-sensing. I think I can build something like that and fit it inside the PongSat… for a single temperature readout. But.

That’s powered off 5V. My uLog is powered off a 3.6V battery. You can’t get a lithium thionyl chloride battery with a higher voltage and enough amp hours for the job that’ll fit inside a ping pong ball. You can fit two 3.6V batteries inside a ping pong ball, but it won’t leave enough space for the uLog and the RTS divider and the amplifier stage; not the way I’ll be making them, anyway. (Again, if I etched my own PC board and surface mounted everything, that might be different.)

You can run an LM324 op amp on 3.6V, just, but the output will be limited to 2.1V. That means a third of the uLog’s 0 to 3.3V input range would be wasted. For similar reasons the openenergymonitor project used the 1.1V reference voltage for the ADC instead of the default 5V, and in theory the ATtiny24 in the uLog could, I think, be made to do likewise, if you reprogrammed it.

I don’t want to reprogram mine. No particularly good reason, just that it’s a skill and equipment I don’t have right now and at the moment I don’t feel like expending the effort and money to acquire them.

So either I have to eat the loss of precision, which would be no big deal really, this isn’t meant to be, excuse the expression, rocket science; or… I need to design something around a rail-to-rail quad op amp. Single supply. DIP package.

There aren’t many single-supply rail-to-rail DIP package quad op amps specced to low temperatures.

The TLV2764IN is one, and I think it’ll work. Not that I have much op amp experience. It’ll run off 3.6V; in fact, that’s its maximum supply voltage. Specced down to –40°C. (There are chips specced to –55°C but only in surface mount packages.) Sells for three bucks each, which is a lot more than an LM324, but I think I can afford a couple. OK, let’s breadboard this thing and see if we can make it work.

 

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