Tester for Inductive Sensors Circuit Diagram

This  tester  uses  a  LED  to  indicate whether an inductive sensor is generating a signal. It can be used to test the inductive sensors used in ABS and EBS systems in cars, with engine cam- shafts and flywheels, and so on. The circuit is built around an LM358 dual opamp IC. The weak signal coming from the sensor (when the wheel is turning slowly, for example) is an AC voltage. The first opamp, which is wired here as an inverting amplifier, amplifies the negative half cycles of this signal by a factor of 820. The second opamp is wired as a comparator and causes the red LED to blink regularly.

 

In order to judge the quality of the signal  from the sensor, you must turn the wheel  very slowly. If the red LED blinks, this means  that the sensor is generating a signal and the  distance between the sensor and the pole  wheel (gear wheel) is set correctly. If the distance (air gap) is too large, the sensor will not  generate a signal when the wheel is turned slowly, with the result that the LED will remain  dark, but it will generate a signal if the wheel  is turned faster and the LED will thus start  blinking. Irregularities in the blinking rate can  be caused by dirt on the sensor or damage to  the pole wheel (gear wheel).

 

Tester for Inductive Sensors Circuit diagram:

Tester for Inductive Sensors Circuit Diagram

If you connect an oscilloscope to the LED with  the engine running, you will see a square-wave signal with a pattern matching  the teeth of the gear wheel, with a frequency equal to the frequency of the  AC signal generated by the sensor. You can also use this tester to check  the polarity of the connecting leads.  To do this, first dismount the sensor  and then move it away from a metal-lic object. The LED will go on or off  while the sensor is moving. If you now  reverse the lead connections, the LED  should  do  exactly  the  opposite  as  before when the sensor is moved the  same way.

 

The circuit has been tested extensively  in several workshops on various vehicles, and it works faultlessly. The author has also connected the tester to  sensors on running engines, such as the cam-shaft and flywheel sensors of a Volvo truck  (D13 A engine). With the camshaft sensor, the  LED blinks when the engine is being cranked  for starting, but once the engine starts running you can’t see the LED blinking any more  due to the high blinking rate.