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Shift Indicator & Rev Limiters For Cars

If you drive your car for optimum performance, you will want this Shift Light Indicator to indicate just when to change gears. As a bonus, it incorporates a Rev Limiter which throttles back the fuel injectors.

By John Clarke

If you are interested in driving your car for best acceleration or fuel economy, you will know that an engine’s torque peaks at a lower RPM than the peak power. You will also know that when driving for maximum fuel economy, it is wise to keep engine revs reasonably low and to get into the highest gear as soon as possible.

Click for larger image
The gear shift and rev limit points are indicated by four LEDs. The LDR at far left is part of the dimming circuit.

But whether driving for best acceleration or economy, you don’t want to be watching the tacho to judge each gear change. That would distract your attention from the road. Having a Shift Light Indicator is the way to go. You will see LEDs light up without having to divert your eyes from the road.

Our Shift Light Indicator has three LEDs to indicate shift points and a fourth LED for the Rev Limiter. How you set the individual LED RPM values is up to you. For example, you could set the three LEDs to give a ‘ready’, ‘set’ and ‘go’ indication for each gear change.

Rev limiting can be hard or soft. Hard limiting simply switches off power to the fuel injectors and the engine immediately "dies"; power does not came back until the RPM falls below a threshold value. Soft limiting reduces the fuel injector duty cycle in stages so that the power is not killed abruptly. Either way, the engine is protected from damage due to over-revving.

Note that many cars these days already have inbuilt rev limiting, so you may choose not to implement this feature.

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