Digital
gear display

 

 

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February 12th 2013

 

 

 

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Digital gear display

 

What is the sense or nonsense of a digital gear display? Is this just for pussies or for BMW riders? Well, if you ask those guys you will be surprised about the answers you get.

Unfortunately you find this useful extra only at a few motorcycles. We have installed this little extra on our motorbikes and Heidi my wife won’t miss it any more. I the meantime I ride my standard round around my region based on brake points and the associated gear to it. Approach the curve, at breakpoint switch down from fourth to third gear, to second, accelerate full in the middle of the curve and then up again to the fifth. This in combination with a gear switching flash indicator does give a lot of fun.

7-segment digital gear displayThe left picture shows the first prototype, where the 7-segment display is still installed in a carton housing. This system was the later on integrated into our board computer project. But that’s another story. Nevertheless, during that project we have tried to avoid complicated computer programs or micro-controller circuits and tried to make it happen with zero software development. The use of low cost devices like the standard  TTL chips on the market was therefore our goal number one.

Robust digital components, you can buy just around the corner or at any web-shop. So we focussed mainly on the parts costs during that project. The less components you build in a project like this, the less components can fail during operation, killed my the motorbikes vibrations. We found this out by try and error and the first five designs we have made, have died the vibration death.

Based on many request we now publish the electronic circuit diagram and a description of the whole design for reproduction. Nevertheless, if you plan to reproduce this unit, you definitely should have in depth knowledge of electronic designs.

 

The shifting process

There are four switching situations in a gear system of a motorcycle.

  1. The shift lever is in neutral position which means, there is no up or down pressure applied to it and the gear is in neutral position. The green NEUTRAL lamp is lit.
  2. The shift lever is in neutral position and the gear is in any gear position.
  3. The shift lever is in the upper position, in order to switch up a gear level (UP).
  4. It is in the lower position, in order to switch a gear level (DOWN).

The sequence of the gears is also not similar to the standard numbering system: 1-0-2-3-4-5-6, the 1st gear is the first one in the row. It’s not the NEUTRAL. Nevertheless, the electronic device was designed to handle any amount of gear on a motorbike and it does not care if you have a 4-gear gear box or a 6-gear gearbox installed. It will always count correctly.

 

Sensing the switch lever status information

The information of the four switching status can be sensed with two different technologies. On one hand with micro-switches, on the other hand based on an electronic circuit called ”electronic window discriminator”.

    For the micro-switch solution you use switches with toggle function that are installed in such a way at the switch lever, that if the lever is in the extreme UP or DOWN position, the appropriate switch toggles.

    • Advantage: It’s an inexpensive solution
    • Disadvantage: It is a very sensitive mechanical construction that must be adjusted properly and secured to avoid humidity and dirt to kill it.

The more elegant method is with the use of an electronic window frequency discriminator. How does that work. Well, you need to purchase small electronic rudder servos that usually are used and installed in model airplanes to run the rudders. You can buy these rudder servos for less than 15 Euro in any model airplane store. This rudder servo replaces the two micro switches. In the rudder servo is regulated resistor pot installed, which is handle by the rudder servo arm and feeds back the rudder position by a resistance. In addition you need a bowden cable which you may purchase in the same model airplane store. One end of the bowden cable you fasten with a metal angel on the switch lever, the other on the rudder servo arm. The servo itself can now be installed at a secure place on the motorbike, that is protected against humidity and dirt. These rudder servos are usually water proof and the variable resistor is mechanically decoupled from the rudder arm. This setup offers maximum protection during daily operation on a motorbike.

The transmission inside the servo drive is not required and an be removed. The three wire connection cable of the servo unit is cut off the pcb and soldered directly onto the three pods of the variable resistor (potentiometer). The two outside contacts of the servo potentiometer are connected to 5V, whereby the middle connection of the potentiometer that connects to the variable part of it, is connected to the window frequency discriminator circuit (WFDC). We have used a ready-to-go kit, which we purchased from Conrad (Conrad electronics po-no. 19 96 30).

It’s now irrelevant whether you may have a short or long gear switching lever or you have to move that lever a long or a short distance to enter the next gear. With the help of the WFDC you can adjust and adapt to any setup easily. Two relays on the circuit board of the WFDC have take over now the function of the two micro switches. We do not publish the circuit diagram of the WFDC, since it is available as a standard kit with Conrad electronics or any other electronic store.

Now let’s explain the setup and adjustment of the WFDC circuit. Two potentiometers on the circuit board are used to setup an upper and lower trigger level for the so-called window. Since the shift lever is connected with the servo through the bowden wire, the position of the lever will end up in a voltage level. If you press the gear lever down into the most DOWN position BEFORE the lower next gear gets in, the position will measure approx. 1V. If you press the gear lever into the full UP position, before you get into the next upper gear, you will measure approx. 4.5V. Within that ”window” from 1V to 4.5V no relay must switch. Then this setup is ok.

If the gear shift lever now is pressed downwards fully to switch the next gear down, then the WFDC circuit input voltage will drop below 1V. We leave the defined window and the WFDC circuit notifies the voltage drop below the defined window and will switch the appropriate relay. The electronic circuit ”counts” one gear down.

If the gear shift lever is no pressed upwards fully to switch the next gear up, then the WFDC circuit input voltage goes over the threshold of 4.5V. We leave the window and the WFDC detects the voltage increase above the predefined window and will now switch the other relay. The system now ”counts” one gear up.

 

How does the system know that it is in NEUTRAL?

Below outlined circuit diagram outlines the complete setup and function for gear analysis and NEUTRAL detection.

circuit diagram

The incoming switching pulses, either sent from the two micro switches or from the WFDC relays are first buffered and debounced with the IC 7400 circuit. The counter 74193 counts the individual gear and will be set back to ”1” by the circuit 78C05. The ICs 7404, 7432 and 7411 make a ”0” from the ”1”. These circuits form the gear sequence  -1, - 0, - 2, - 3, - 4, - 5, (- 6) *if available.

The exits - A, - B, - C are attached now to a 7-Segment display. This is now up to you which type of 7-segment LED you choose. There is also no ”must” for a common anode and/or cathode system. We used an extra a bright 7-Segment LED type from Conrad electronics with common anode. The appropriate decoding circuit that decodes ABC to 7-segment is also installed with the digital LED display in the display housing.

 

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