First draft of ground censor circuit. It can monitor the battery voltage, ambient light. surface and subsurface soil temperature. The sensors are powered off, only enabled by the MOSFET to take readings; this will reduce standby power use and also interference when transmitting wifi. The beacon LED can be used to locate. I happen to have LM324 on hand so using that to buffer the analogue readings, and a 4051 to multiplex because the ESP8266 only has one ADC. The LDR monitors light, so we can transmit readings less often overnight to reduce battery (and logging the daylight will also be interesting) - and maybe helpful for diagnostics and tuning, because the solar panel may be partly obscured by a crop.