Pulse Detection with a coil

 

This experiment uses a coil to detect pulses. A magnet is moved past a coil and this creates a voltage in the turns of the coil. This is ideal for picking up the rotation of a shaft. It is non-mechanical and will have an infinite life. Reed switches have a very short life when used rapidly to detect shaft rotation and have a fairly low speed of operation. The output voltage of a coil is fairly low and needs two stages of amplification for the signal to be large enough to be detected by the input of a microcontroller. The clever arrangement on the front end of the analogue amplifier of the PIC LAB-1 board allows a microphone or coil to be fitted. The coil does not require the resistor, (it is required by the electret microphone) however it does not affect the operation. This demonstration program increments the 7-segment display.

This allows a count-of-ten however experiments on the web include a 2 and 3-digit readout from the 7-segment display and an RPM counter. The advantage of a magnetic pickup is the lack of switch-noise. The pulses from the pick-up are very clean but must be debounced for low-speed detection. The 2-stage amplifier increases the sinewave signal and over-amplifies it to produce a rail-to-rail signal commonly called a square-wave or digital signal.

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