Many electronic-control systems have digital outputs that use transistors. One method of improving the security in these outputs is to use an oscillating signal to represent a logic-high state instead of a fixed voltage level (Figure 1). This type of signal, a dynamic variable, can drive the circuit shown in Figure 2. This circuit connects to the output transistor of the electronic-control system. The dynamic- variable signal connects to the output transistor, whose collector connects to a pulse transformer.