Phototransistor and LED

As discussed in the previous chapter, a phototransistor is a transistor that is activated or turned on by light. When combined with a LED and a rotating slotted wheel in a vehicle speed sensor, a phototransistor can supply vehicle speed data to a computer.

In this type of sensor the LED is aimed at the phototransistor. When the slotted wheel is rotated by the speedometer cable, it breaks the beam of light. The beam of light is interrupted 20 times per revolution. The ECU supplies a reference voltage to the collector of the phototransistor and the emitter is connected to ground. Each time the light hits the phototransistor, it turns it on just like a toggle switch. Each time the phototransistor is turned on, the wire from the ECU is connected to ground and the voltage is pulled down to 0 volts. The ECU can count these pulses and calculate vehicle speed.

PHOTOTRANSISTOR SWTICH

PHOTOTRANSISTOR SWTICH

Phototransistor Sensor
Phototransistor

This type of sensor is also used as a G Sensor or deceleration sensor on the Celica All Trac and Trucks equipped with ABS. This sensor has two LEDs aimed at two phototransistors that are separated by a slotted plate on a fulcrum. When the vehicle is decelerated, the plate pivots on the fulcrum and the slots in the plate line up with one or the other or both of the LEDs and phototransistors-depending on the rate of deceleration. These signals are sent to the computer so it can determine the deceleration rate for ABS to operate properly.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

Don't pay hundreds of dollars to find out what is wrong with your car. This book is dedicated to helping the do it yourself home and independent technician understand and use OBD-II technology to diagnose and repair their own vehicles.

Get My Free Ebook


Post a comment