h-l logic tester

This is a TTL logic probe which, instead of the usual LED to indicate the logic states, uses a seven-segment Minitron or LED display to indicate `H` for a high or `1` state and ` L` for a low or `O` state. The circuit also detects when the probe input is open-circuit and the readout is suppressed, thus indicating that contact with the desired test point has not been made. This avoids the false readings that may occur with some types of probe when the input is not connected.