Photoelectric keyboard for typewriter - has LEDs supplied from astable multivibrator and clocked photosensors that set bistable via OR:gate

摘要

Key actuation interrupts the light beam from an LED (36) to a photo sensor (38). The diode energy supply is provided by an astable multivibrator that generates a pulse every 5 milliseconds. The pulse is shaped to a 50 microsecond pulse and gates a supply transistor. When a key is actuated an OR gate generates an output to set a bistable. The bistable output is gated by a clock signal to operate a counter to generate an output after 89 msec. The flip-flop is then reset and key states clocked into a shift register. The shift register output may be recorded and finally converted. The locked gating system ensures that correct information is recorded.