MegaShift 7-Segment Gear Display

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MegaShift™ 7-Segment Gear Display

MegaShift™ v2.004+ code allows the user to set the state of the four gear indicator LEDs. Instead of using these outputs to drive discrete LEDs, you can use them to drive a 7-segment LED (or LCD) to display the current gear numerically.

To do this, you will need to construct a circuit using a 7-segment LED, a driver IC, and several resistors (on a proto board or something similar). Examples of such components are:

7-Segment LED: Digi-Key 160-1576-5-ND, $0.88 each.
4511 driver chip: Digi-Key 568-3124-5-ND, $0.61 each.

This is how these components should be connected:

The R1 to R7 resistors must be chosen to limit the current to 10 milliamps (the maximum for the 4511 chip). ~2.1 Volts are dropped in the LED itself.

With a 5 Volt supply, the resistors should be at least (5.0-2.1)/0.010 = 290 Ω
With a 12 Volts (nominal) supply, the resistors should be at least (14.5-2.1)/0.010 = 1240 Ω

1/8 Watt resistors are fine, but ¼W won't hurt either.

Note that the LED circuits on the GPIO board provide a ground for the LEDs. As a result, the circuits are grounded when "on", and 'floating' when "off". To provide a signal to the 4511 chip, these inputs must pull-up circuits. This is done by connecting each input to the supply voltage through a 1.0 KΩ resistor (R8 to R11 in the diagram). 1/8 Watt resistors are fine, but ¼W are good too.

Note that when connected like this with a pull up:

"on" in the tuning software is "low" at the 4511 input pin (i.e. the pull-up voltage is pulled low by the GPIO's conducting transistor), and

"off" is "high" at the input pin.

So for example, first would be set as:

LED1 = "off" - LED1 circuit is not conducting, so it is pulled high through the supply voltage via R8; and thus 4511 pin D0 (pin #7) is high,
LED2 = "on" - LED2 circuit is conducting, so it is pulling the voltage low; and thus 4511 pin D1 (pin #2) is low,
LED3 = "on" - LED3 circuit is conducting, so it is pulling the voltage low; and thus 4511 pin D2 (pin #3) is low,
LED4 = "on" - LED4 circuit is conducting, so it is pulling the voltage low; and thus 4511 pin D3 (pin #6) is low.

Then you configure the remaining LED outputs to give the appropriate number in each gear:

Value Displayed	D1	D2	D3	D4
0	"on"	"on"	"on"	"on"
1	"off"	"on"	"on"	"on"
2	"on"	"off"	"on"	"on"
3	"off"	"off"	"on"	"on"
4	"on"	"on"	"off"	"on"
5	"off"	"on"	"off"	"on"
6	"on"	"off"	"off"	"on"
7	"off"	"off"	"off"	"on"
8	"on"	"on"	"on"	"off"
9	"off"	"on"	"on"	"off"
blank	"off"	"on"	"off"	"on"

Click on the component links to the respective datasheets above for a complete explanation.

For neutral or park, 0 is an obvious choice. For reverse gear, you can use any number not already used for a gear, or you could blank the display. The display can be user-set to flash in reverse (you set two patterns - one can be blank - and the LEDs will alternate between them once per second).

Note the the decimal point ("DP" - pin #4 of the 7-segment display) is not connected. You could use one of the spare ports to signal some function using that LED, or you could tie it into the TCC output (with a resistor) to signal the TCC status.

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© 2010 Bruce Bowling and Al Grippo. All rights reserved. MegaSquirt^® and MicroSquirt^® are registered trademarks. This document is solely for the support of MegaSquirt^® boards from Bowling and Grippo.