A first transparent strip having a series of alternate lines and spaces is mounted in a housing parallel and adjacent to a second transparent strip having a series of lines and spaces. A key inserted through a slot formed in the housing pushes the second transparent strip across the first transparent material. Photo emitters mounted in the housing emit light onto the first and second transparent strips. By moving the series of alternate lines and spaces on the strips with respect to each other, the intensity of transmitted light changes. Photo detectors mounted in the housing detect the light transmitted through the first and second transparent strips. Thus, a clock signal is generated as a function of actual distance moved by the second transparent strip. A magnetic head is mounted in the housing near the slot. The clock signal is applied to a circuitry that drives the magnetic head to enable the magnetic head to record a code on the key that is synchronous with the lines on the transparent strip. Since the key moves the second transparent material as the key moves past the magnetic head, the code is recorded on the key as a function of distance moved by the key past the magnetic head.