Honda Mileage: July 1 2009 Update 3

Here, I have uploaded the schematic for the custom 2-wire BCD seven segment card. The reader will notice that this card uses up to four Microchip PIC 16F627/648a CPUs for interface and seven segment decoding function. Originally, I used these processors because they were all I had in stock.In other words, I had no seven segment latch/decoders. However, as it turned out using $2 CPUs actually lowered the cost and complexity of this card. It also allows me to customize the decoding function enough to invert the display output--just in case I want to try to read the display off from the windshield while keeping my eyes on the road. The primary drawback to this is actually seeing the displays in bright sun light.

The reader might also notice that this drawing shows the board with its own 5 volt regulator. A prudent design would isolate the power requirements for this card from the VID--given the relatively high current requirements of these seven segment displays. In my prototype, I used u the 5 volts from the VID card.


Unlike the VID card, this (3*4 inch) card is densely populated. It is, however, quite simple to understand. From the schematic the reader can see that I used a total of four PIC 16F627 class CPU devices to drive three common cathode seven segment LED devices and discrete LEDs. In this application, each processor's interrupts are connected to allow them to all process all transitions on the BCD_DATA and BCD_CLOCK input lines.

In essence, they are all looking for something specifically addressed to them. To configure their address, Each processor has three input lines tied to either logic high or low. For example IC1 has all of its address inputs tied to ground. It, therefore, processes all messages sent over the BCD_DATA and BCD_CLOCK lines, but looks for messages addressed to device zero. In a like fashion the other devices listen for their unique address and respond accordingly.

IC1- IC3 contain the exact same program. (IC4 is the voltage regulator.)IC6 contains a program that responds to address 3 but instead of driving a seven segment display it drives the decimal points on the other seven segment LED devices, or one of several status LEDs.

As in the case of the VID, the code for these chips will also be available to anyone that requests it via email.



Preliminary parts list for BCD Display Card

C1 0.1uf C5/2.5 ceramic capacitor
C2 .1uf C5/2.5 ceramic capacitor
C3 .1uf C5/2.5 ceramic capacitor
C5 .1uf C5/2.5 ceramic capacitor
C6 0.1uf C5/2.5 ceramic capacitor
C8 10 uf polarized capacitor 25 volts
D1 PCB LED different colors
D2 "
D3 "
D4 "
D5 "
D6 "
IC1 PIC16F627P PIC16F627P DIL18 microchip 1
IC2 PIC16F627P PIC16F627P DIL18 microchip 1
IC3 PIC16F627P PIC16F627P DIL18 microchip 1
IC4 7805T 7805T TO220H linear 1
IC6 PIC16F627P PIC16F627P DIL18 microchip 1
R1 2K 1/8 watt carbon
R2 33K 1/8 watt carbon
R3 33K 1/8 watt carbon
R4 33K 1/8 watt carbon
R5 33K 1/8 watt carbon
R6 33K 1/8 watt carbon
R7 33k 1/8 watt carbon
RN1 8R-N DIL16
RN2 8R-N DIL16
RN4 8R-N DIL16
RN6 8R-N DIL16
S1 10-XX B3F-10XX switch-omron 1
S2 10-XX B3F-10XX switch-omron 1
S3 10-XX B3F-10XX switch-omron 1
U$2 1INCHLED 1INCHLED LQ5 stanleyled 1
U$3 1INCHLED 1INCHLED LQ5 stanleyled 1
U$4 1INCHLED 1INCHLED LQ5 stanleyled 1
X2 22-23-2081 22-23-2081 22-23-2081 con-molex 1



Below is one possible PCB layout for this circuit. Here I am using 1 inch high LED display units from Stanley Electric.