Speed Switch: Running and Overspeed Controller with Magnetic Pickup

Overview

The Speed Switch is a microcontroller-based device designed in 2014 and mounted inside a controller panel. It measures the frequency of the signal coming from a magnetic pickup and, based on it, drives two relay outputs: one for a Running signal and one for an Overspeed signal. The input frequency can also be read on an LCD.

How it works

The magnetic pickup is mounted next to a rotating part and produces a pulse each time a tooth passes in front of it, so the pulse frequency is proportional to the rotating speed. The device reads this frequency and:

• shows it on the LCD;

• compares it with two limits set by the user: the Running limit and the Overspeed limit;

• activates the relay of each limit for the panel. The controller panel uses these two signals to make its decisions.

The limits are set with a few buttons on the device, with no extra tool or programming needed. The same device therefore fits different speeds and frequencies.

Specifications

• Supply input: 12 to 24 V

• Frequency input from a magnetic pickup

• Two relay outputs: one for the Running signal and one for Overspeed

• LCD connection for showing the input frequency

• Microcontroller based

• Several buttons to set the Overspeed and Running limits

Hardware design

I designed the board in Altium Designer, as a double-sided PCB. The heart of the device is a PIC16F877A microcontroller. The board carries two relays with NC, COM and NO terminals, the power section, the buttons and a few status LEDs. In the PCB layout, the display and the buttons sit in a separate part of the board. Because the device is mounted inside a panel, the relays and terminals are placed so that wiring stays simple.

The pictures below show the built board, the 2D PCB layout and its 3D view.

Result

A small, self-contained unit that measures the pickup frequency, shows it, and produces the Running and Overspeed signals on a single board, mounted inside a controller panel.