The MQTT letterbox Part 2: Sending alerts from MQTT // Project

Solar Panel

In the previous video I used a small solar panel, but I found it really hard to find a cheap way of securing it to the letterbox.

So I picked up this one from Jaycar. It had everything reliably water-proofed.
You could probably pick one up cheaper, but make sure it’s a 6V version as the DFrobot charger can’t handle anything over that.

Under my studio lights it was generating an open-circuit voltage of around 9v, whilst the loaded voltage dropped down to around 2.6v. Nice. Note the open-circuit voltage of this panel can hit around 10v, so you have to be careful when connecting up.

So, I’ll need to add a weatherproof connector so I can change the cell out if I need to. This was a 2way Deutsch connector, which guarantees no water will get in.

It’s pretty easy to connect it up. Check my video on how to do this.

The other end of the cable I trimmed and tinned so I could screw into the DFrobot LiPo solar charger and allow me to remove either the panel or ESP8266 box for maintenance.

Before connecting up make sure you black out the panel, because the open-circuit voltage could potentially damage the LiPo charger. The voltage levels were all in spec and predictable. Chucking a battery on and checking the voltage. Yup everything seems to be working, and the solar cell will charge the LiPo when there’s enough photons hitting it.

In a later version of this letterbox, I’ll remove the Lux sensor and just use the LiPo charge indicator to see the current charge rate.