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Gear Selector Indicator  lite

We are pleased to offer this stripped down version of a part of electrical system I’m developing for my car. In essence it is a stand alone gear display that will indicate which gear I'm in on 5x7 LED matrix. Full version has support of options like selecting if its manual or automatic transmission, dual color option and communication bus to other systems. Where as this one does not have those options. All characters / symbols can be made custom by altering the character lookup table within the program. As such it still can be used with either manual or automatic transmissions

Main design feature of it was to make it as least component dependant and simple. There are 2 main components to the circuit
    1. a processor, Microchip anything PIC16f627/8 or PIC16f648 or you can adopt it to any PIC processor you like
    2. a display any Common Anode column dot matrix in any color you like.

A few passive components to limit current for LEDs and small signal diodes i.e.. 1n4148 or 1n914.
Since the processor can only handle 5v a 5v power source is required. Any type of regulator will do with sufficient filtering for clean operation of  the processor and open to your imagination.

The Hardware

Since the processor has only 18 pins 2 of which are used for power and another 12 are used to drive the LED display that leaves only 4 pins that could be possibly used to sense which gear you are in.
Since we wanted to make the circuit as small as possible and not use expensive or large pin count processors we ended up multiplexing display and gear sensing as the processor has enough processing power. To prevent possible problems with driving display and gear  sensing we are using small signal diodes as isolation and additional pin from processor as driving signal.

Basic layout seen in above schematic  illustrates of how things are laid out. There are 2 possible ways to setup gear sensing  and both are in the schematic. One is to use switches which would be activated by the gear stick, problem with that is that the switches have to be precisely mounted to activate the switch. Other option is to use hall effect sensors which are triggered by a small magnet mounted on the gear stick. This is probably better as the hall sensors just need to be in close proximity to the magnet to be tripped.. Disadvantage of it is that it requires some sort of buffer as the hall sensors do not have output control/enable pin hence we use the extra 74hc541 is a line buffer that we can put into hi impedance vi control signal from the processor.
In switch option you will not need the 75hc541 buffer nor the A3144 hall sensors. In the hall effect sensor version you will not need the switches.

Software operation

The code has 2 main loops, scan loop and display loop and run in perpetual loop. The scan loop looks for any pressed keys or negative signal on row drivers for the display if none is detected it displays neutral or N from lookup table. If there is a negative (ground) signal on any of the pins it looks up particular scan code and displays it on the LED. There are 9 possible characters that can be displayed as we are monitoring port(B) + N if nothing is sensed.
All characters are 5 bits by 7 rows, the display routine looks up 7 bytes from eeprom one for each row of the display. Characters in the sample code are set for manual transmission 1 through 7, R and N.

To create a characters is very easy and to understand and is illustrated in  following example.

In this example we are generating letter N and R. We are interested in only 5 least significant bits of the byte and need 7 bytes for each character. Each row is a byte stored in processors eeprom as 7 byte string. Hence we have 11  19  15  15  13 11 11 in the string for letter N

and 1e  11 11 1e 14  12  11  in this example for letter R. Though constrained to only 5x7 space the possibilities are limitless of what it can be displayed

So grab your self some parts and code found  here  and build your self a custom gear indicator.

Bill of materials

1    Microchip PIC processor PIC16f627 or PIC16f628  or PIC16f648
1    Any Common Anode row 5x7 LED matrix in color of your choice
5    120 or 220  1/8w resistors depending on brightness and color of  LED matrix you choose
8    1n914 or 1n4148  or any other small signal diodes
8    normally open switches or less depending on how many hears your transmission has 
or
1    74hc541  octal line buffer
8    Allegro A3144  or similar hall effect sensor and a small magnet
      7805 or similar 5v regulator and couple filtering capacitors to construct 5v power supply