Last time around, we proved that the Raspberry Pi could indeed be integrated into a Heng Long Tiger I RC tank, and so control its functions. That’s a significant achievement, but there’s plenty more to do.
Firstly and most importantly, we can’t yet drive the tank around because the Raspberry Pi is still dependent on a power source, keyboard input and video output – each of which is a cable connecting the tank to a computer. On day nine of the build diary, we cut one of those dependencies, resulting in a Raspberry Pi that is powered from the tank’s internal battery. Compared to previous days, this was a relatively simple matter.
The tank’s power supply comes from a 7.2V NiCd battery. The Pi’s power input is a very strict 5V ±5%, so we sourced a component that could convert one to the other. The Pi’s maximum current draw of around 1A was also a consideration. A cheap voltage regulator such as the ubiquitous LM7805 could do the job, but it would need protective capacitors in parallel with it, and would also give us a heat dissipation problem – the regulator would get rather hot stepping 7.2V down to 5V at 1A, and the plastic body of the tank gives us no convenient heat sinks.
Instead of a regulator, a switching DC/DC converter was chosen, specifically a Recom R-785.0-1.0. This provides exactly the voltage and current output we need, with a higher efficiency so less heat dissipation. The tank’s 7.2V battery is well within its input range, even accounting for possible voltage drops when the battery charge is low.
For more detail: Tank Day 9: Power to the Pi