IC1 (74HCT125):
Buffers 2 and 4 are configured as open-drain pulldowns for SDA and RESET (they either pull the line low or they float). Buffer 1 is used as a tristate buffer to drive SCL. Buffer 3 is used to buffer SDA onto the S7 input of the parallel port.To disable the interface, and allow normal remote control operation, output 0x7 to the PC parallel port data register. To enable the interface, set bit 2 of the data register to zero. This causes buffer 4 to pull the RESET line low and reset the microcontroller inside the remote which disables its drivers. It also enables buffer 1 to drive bit 1 of the parallel port data register onto the SCL line. Setting bit 0 of the data register to zero causes buffer 2 to pull the SDA line low. The remote eeprom can be read/written by setting SDA and SCL according to the protocol specified in the eeprom datasheet.
R1-R3 (1k):
These resistors protect IC1 by limiting the clamping current that flows when the interface power supply is off and the PC parallel port signals D0-D2 are driving a logic 1.
R4 (1k):
This resistor provides a minimum current flow from the 78L05 regulator. Without any load, the regulator output would be too high.
R5 (100k):
This resistor pulls the SDA line high when there is no remote connected, which keeps the input to IC1-buffer3 from floating (all of the remotes have a 10k pullup resistor on SDA, so R5 must be large enough to not lower the effective pullup resistance). The software interface check depends on this.
D1-D4:
These diodes can be any type of diode that can handle 100mA, but they must all be the same type (match as closely as possible). D1-D2 prevent the batteries in the remote from being drained through the interface power supply which is at a lower voltage. D3 lowers the VDD voltage by the same amount added by D4. D4 raises the output of the regulator VDDH (+0.7V for a silicon diode) to compensate for the forward voltage drop of D1-D3.
D5-D6:
These diodes must be silicon diodes so they will lower VDDH by about 1.3V. This lowers VDD from +5V to about +3.7V. Note: this technique of lowering voltage is only accurate for small ranges of current.
SW1:
When the switch is closed, VDD is +5V (use for remotes with 4 batteries).
When the switch is open, VDD is +3.7V (use for remotes with 2 batteries).
78L05:
+5V voltage regulator (can substitute 7805)
0.1uF and 22uF capacitors:
Both capacitors should be connected as close as possible to the 78L05 regulator. The 0.1uF capacitor should be rated for at least 20V.
Power Supply:
Use a small (cheap) 9VDC adapter. The output should be filtered (DC, not AC) but does not have to be regulated. Cheap 9VDC adapters actually output up to 14V with no load, which will work fine. Any adapter that has an unloaded output voltage of less than 16V will work fine (be sure to measure it with a voltmeter).WARNING: Once you have built this box, please take great care to connect the remote correctly. It is very easy to either get the connector upside down, or off by a pin, which would fry the remote!
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