2 Stage Shift light

       Here's something I've had on my car for a while now. I decided to post it as others might have a need for it. This is actually the third and final revision and the first one was made from way back early 2000 or so.

       It's a simple shift light that lights a bi-color LED in two different colors depending on the set RPM range. On my unit, the indicator lights up blue at about 3000RPM and turns red at 5000RPM. (Engine redlines at 6000) The red LED also lights up when the engine is stopped or there is no tach signal going to the board.

       The circuit based on the LM2917 where it converts frequency to voltage. In my car, dividing the engine RPM by 30 results to the tach output in Hertz. So to light up at 3000RPM, I just set it to trigger at 100Hz and so on. An LM358 is used as a comparator with hysteresis to light up the two LEDs with the trimmers setting the trip points. The op amp in the LM2917 is set up to short the 6.8V zener reference of the comparators so that the red LED lights up when there is no tachometer signal (engine stopped) which can be helpful in fault finding or tracing for the tach signal in the gauge cluster with the engine idling.

       Here are the circuit schematic and PCB layout for my rev-3 design which has been in daily use since 2011 and still working great.

Circuit schematic.

PCB layout.

Animated GIF of my unit in action.

Page created and copyright R.Quan © 24 Mar 2014.