The Pencil Box Logic Designer is a complete digital circuit design instrument . It can fulfill modest requirements for the design and study of gates, counters, multiplexers and can even interface directly to a microprocessor. The unit contains 8 LED logic indicators, 8 logic switches, a clock and two debounced pushbuttons. The logic indicators are a transparent latch and thus serve the dual function of 8 independent logic probes or two four bit output ports which can be connected to a computer. Similarly, the eight logic switches can serve the dual function of logic switches or two four bit input ports for interfacing to a computer.