The schematic for the sender is fairly simply, because the part count is low to reduce cost.

Noteworthy aspects include:

* The digital input is a general trigger to send data. That particular pin can also wake the ATtiny from a deep sleep (very low power mode). An optional pushbutton (SW1) is useful to manually send the data when debugging.

* Notice how the pins are reused for connecting to the transmitter, USB serial cable, and ISP6 Atmel programmer.

* The sender asserts high/low during transmission, but switches the TX pin to an input when idle. R3 gently pulls down the TX line to keep the transmitter board turned off. However, the USB serial cable connector (not shown) uses a 10 kilohm resistor to bring the signal back to serial idle standards (high). A simple trick to be compatible with both cases.

* The reverse battery protection diode (D1) has an ultra-low dropout. It is less than 0.1 V at LoFi's voltage and current draw: http://www.nxp.com/documents/data_sheet/PMEG1030EH_EJ.pdf