Petrol Gas Switch For A Pajero


My current vehicle, a
Pajero, was modified for dual fuel – ie, petrol and gas. However, it’s
necessary to run the vehicle on petrol at regular intervals to stop the
injectors from clogging up. This simple circuit allows the vehicle to be
started using petrol and then automatically switches it to gas when the
speed exceeds 45km/h and the brake pedal is pressed. Alternatively, the
vehicle may be run on petrol simply by switching the existing
petrol/gas switch to petrol. You can also start the vehicle on gas by
pressing the brake pedal while starting the vehicle. The circuit is
based on an LM324 dual op amp, with both op amps wired as comparators.
It works like this: IC1a buffers the signal from the vehicle’s speed
sensor and drives an output filter network (D1, a 560kO resistor and a
10µF capacitor) to produce a DC voltage that’s proportional to the
vehicle’s speed.

Circuit diagram:

This voltage is then applied to pin 5 of IC1b and compared with the
voltage set by trimpot VR1. When pin 7 of IC1b goes high, transistor Q1
turns on. This also turns on transistor Q2 when the brake pedal is
pressed (pressing the brake pedal applies +12V from the brake light
circuit to Q2’s emitter). And when Q2 turns on, relay 1 turns on and its
contacts switch to the gas position. Trimpot VR1 must be adjusted so
that IC1b’s pin 7 output switches high when the desired trigger speed is
reached (ie, 45km/h). In effect, the speed signal is AND’ed
with the brake light signal to turn on the relay. The vehicle has been
running this circuit for several years now and is still running well,
with no further injector cleans required.

Author: J. Malnar – Copyright: Silicon Chip Electronics


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