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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

Fuel Economiser uses
strain gauge on accelerator

Click for larger image

This project helps a driver to avoid excess fuel consumption by monitoring pressure on the accelerator. It uses a pressure sensor made from a piece of conductive foam (as used to store ICs) and mounted between two sheets of PC board. The sensor is mounted on the accelerator pedal and its resistance drops when pressure is applied to it.

The pressure sensor is teamed up with a PICAXE-08M. The PICAXE determines whether excess pressure is applied to the pedal and provides a warning if it does. It will allow gentle acceleration to your peak speed. Two stages of high acceleration (as determined by the software) can be detected and a different tone is produced for each.

The PICAXE-08M senses pedal pressure via the ADC pin 6. This looks for a change of the resistance of the pressure sensor. A 5kΩ pot, VR1, provides a sensitivity adjustment. The PICAXE runs through tests at 100ms cycles and if the value of W0 (the reading derived from the transducer) exceeds your threshold, it triggers a piezo speaker to sound an alert.

The circuit uses a 78L05 regulator and diode D1 is included for polarity protection. The piezo speaker should be a fairly quiet unit, not rated at 105dB/1m which would scare the daylights out of the driver.

When making the sensor, remember that the assembly must be slightly compressible and must resume its shape when pressure is removed, so you cannot simply use nuts and bolts.

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