In this project, I'll make a small circuit capable of controlling a servo, it is a servo tester made out of a 555 timer IC.

Just a signal generator making a 125 kHz square wave, coupled to a two transistor buffer pumping a tuned circuit.

The electronic version is also simple to make, and is a great introduction to the concept of circuits – the cornerstone of many maker projects and core knowledge for the budding robot-builder.

Figure 1 Simple circuit to indicate a low battery. For this application, the LED proved to be a decent reference diode. It has a temperature coefficient that tracks fairly well with that of Q1, and ...