Keyboard with touch keys for high temperature applications

摘要

The keyboard (1) consists of a number of touch keys (21) which control terminals (22) in a circuit (2). Each touch key is supplied by a HF generator (4) connected to the input (24) of a comparator (23). The voltage generated is compared with a reference voltage delivered by a circuit (5) connected to the second input of the comparator. The HF generator includes an oscillator (41) whose output is amplified by a transistor (42) in circuit with a tuning potentiometer (44) and receiving a positive voltage generated by a transformer (46). A diode (27) is connected between the HF generator and the key so that a change of state of the key does not affect the signal from the generator.