“This is an under-speed sensor module that provides an audible warning and signals output when the speed of a machine is below the set point. The module can be used in applications such as Engines, Machines, Sports machines, Fitness Equipment, Speed of the machine or engine can be detected using a magnetic pickup sensor. The project can be used in many applications where an under-speed monitor is important. The project is based on the LM2907 chip which works as a frequency to voltage converter, interfacing of magnetic pickup as shown in the figure below. A typical magnetic pickup for automotive applications will provide a thousand pulses per mile so that at 60 mph the incoming frequency will be ~16.6 Hz. If the reference level on the comparator is set by two equal resistors R2 and R6 then the desired value of C5 and R4 can be determined from the simple relationship:
The buzzer is activated when Frequency ≥ (1/(2XR4C5))
From the RC selection chart, we can choose the suitable values for R5 and C5. In the current circuit, the frequency is set to 500Hz approx. A frequency trigger point can be changed as per user requirement using the above equation. The module supports any sensor with voltage levels 20mV to 28V.”