The digit driver board provides the necessary power to drive the larger 3" digits. This board can plug directly onto the timer board, or can be soldered to the dual row header for an inexpensive permanent connection. I recommend using a female header socket, as shown below, so that the board can be disconnected if necessary.
Bare board shown here with a bare timer board for size reference:
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Finished Timer with Driver Board attached:
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Finished Timer with Driver Board attached, side view:
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If you want a permanent connection, you can solder the driver board directly onto
the timer board display header connector. It is shown here with a female header
soldered to the underside of the driver board, and plugged onto the timer board header.
Display driver component layout:
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Schematic:
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IC1 can be any one of the following equivalent source driver chips, depending on availability:
UDN2981A, UDN2982A, MIC2981BN, MIC2982BN, MIC2981/82BN, or TD62783AP (Toshiba)
This is the back of one of the 3" digits, showing which pins connect to what:
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The segment lines A to G connect to each and every digit. That is, pin A on the driver goes to A (pin 7) on digit 1, A (pin 7) on digit 2, etc. This is usually easiest to do in a daisy chain manner.
The digit select lines 1 to 6 go to the common cathode pin (pin 1) on each respective lane digit you have.
Digit Select Pin 1 on the driver connect to CC (pin 1) on the digit for lane 1.
Digit Select Pin 2 on the driver connect to CC (pin 1) on the digit for lane 2.
etc., for however many lanes you have.
If you have less than 6 lanes, any unused digit select pins on the driver board should be left unconnected, and must not be allowed to short out against anything.