EEC136 LED Driver and Trans impedance update

The LED driver circuit was simulated and populated this week. An early problem encountered by Jon and I was that with no LED's as a load the circuit demonstrated erratic behavior. Without the probe plug, the supply produced by the regulator dropped below +5v. The DAC's also failed to provide full supply. The LED driver will produce current depending on the bias selected from the DAC3/4 from micro controller. If the current has no where to go but through the bottom transistor. This problem was remedied with construction and introduction of the finger probe.

The LED driver was tested by using the I2C DAC LABVIEW program to modify the voltage on the base of the bias transistor. The brightness was modified by changing the voltage from the peak (+5v) to (0v) transistor off. We need to investigate just how hard we can push these LED's as the brightness can improve the signal coming out of PD. To do this, we will need to modify the emitter resistance and see just how far we can push.

The finger probe was constructed in the following manner. First, a pre-prepared probe from Masimo was acquired and modified. The wire was cut and the wires were stripped. The color code is as follows:

ORANGE - LED IR

RED - RED LED

WHITE - PHOTO DIODE 1

GREEN - PHOTO DIODE 2

METAL - GND

I took a copper prototype board and cut traces. Then I soldered the wires into specific traces. I think soldered a 16-pin male connected on the POX board where the LED output and PD input's come in. Then, I soldered a 16-pin female connected to this copper prototype board. The final result is displayed below:

I'm nearing completion on the work of the transimpedance stage as well. I've populated the entire subcircuit but am running a couple of last minute simulations to choose feedback and input resistances. A problem was discovered by Shanit in which VCC and VSS are wired backwards. To remedy this, I will have to cut a trace and use jumper wires to each of the 3-vcc/vss pads. I will post another update with progress on this front in the next few days.

Megan has been given the task to complete an integrated software solution. The software solution will enable control of all 4 (four) DAC's, the PGA, and the ADC. This will allow modification of the intensity of the LED's, DC subtraction, and programming the gain amplifier. At this point the team will regroup and focus for the rest of the quarter on software development.