Fan pulse alarm using two stage comparator for speed detection

Abstract

The alarm circuit uses speed detection so that at a fan slowdown speed below a critical low level, a buzzer alarm starts sounding at a low warning volume and then as the fan slows down further the volume increases; that is, the buzzer volume or the buzzing frequency is inversely proportional to the fan speed. The circuit advantageously can be used for a wide range of fans. A pulse detector of a first stage comparator of this circuit detects the pulses or speed of the fan. More particularly, a power supply isolation resistor with an AC coupling capacitor block the DC voltage levels at the fan so that only the AC or pulse component of the fan is picked up. The isolation resistor prevents the power source from attenuating the pulses. The second stage comparator of the circuit sets the fan speed at which the alarm is to start buzzing and powers the buzzer accordingly. Particularly, the pulses are amplified by the first stage comparator and they periodically discharge a charging capacitor. If the pulses are too infrequent, the capacitor will charge up to the level of an error comparator which will trip and sound a buzzer. Since the circuit uses only discrete components and one (multi-vendor) QUAD comparator for all signal processing including alarm activation, it is inexpensive. It is also very small allowing for greater mounting flexibility.