| 摘要 |
A battery-powered alarm and reset circuit for a conventional electromechanical countdown timing mechanism which drives a set of cam-actuated main contacts. The circuit renders both a visual and an audible alarm at zero countdown time. The rotation of a timer knob closes the main contacts which readies the digital NAND gate logic of the circuit to a state preliminary to oscillation. At zero countdown time, one of the NAND gates goes into oscillation to generate a sequence of relatively low-frequency pulses in response to the opening of the main contact at zero countdown time. The low-frequency pulses activate an LED light to render a visual blinking alarm and an electrical horn to render an audible beeping alarm. Both alarms continue indefinitely until momentary manual closure of a reset switch stops the NAND gate from oscillating.
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