I bought an ESP8266 NodeMCU module for £7 just to have a play and see what everyone is talking about.
dx.com is one supplier (£6.57). It has a 80MHz or 160MHz CPU. And is very small (few cm x few cm).
The SDK including GCC is free, and it has Wifi on board plus GPIO and one ADC.
Add a breadboard for all of £1.50 ish (then saw it in half to fit the NodeMCU’s wide leg spacing). Job done.
Now grab a photoresisitor, some nicely trimmed connecting wires and a 10K resistor (see dx.com). In the old days we used to have to strip our own wires…blah blah..
Next is to load the firmware and create a lua script. I am not going to go into it here, other that to give a list of useful links:
Firmware loader and NodeMCU Lua flash binary.
I left it running for 2 days sampling light levels by a window every 3 seconds. Worked like a charm (see the frame below, though only tested on Chrome, it exceeds the browser stack size on Safari – work in progress).
Next step is to ad a Wacom GSM modem I have lying around and make it a truly go-anywhere logger for next to no money per unit.