In designing the sensor unit, I deviated from the original spec and design parameters determined
earlier. I created a LED driver circuit using a Bipolar Transistor current source. The schematic can be seen below. The Resistor on the sensor was selected based on previous test results. The resistors on the LED drivers were selected to keep the BJT in saturation and to run ~50mA of current through the LED. A general purpose NPN transistor, 2N3904, was selected from components immediately available. 2N3904 has a maximum Ic rated at 100mA, hfe is around 60 to 100 depending the parameters of operation. In my calculations hfe = 60 was assumed. These transistors, I believe, are available at Radio Shack, which means replacement parts are readily available. All unlisted data are available on datasheets online. Vcc is 5V. Below are pictures of the sensor module.
In designing the sensor unit, I deviated from the original spec and design parameters determined earlier. I created a LED driver circuit using a Bipolar Transistor current source. The schematic can be seen below. The Resistor on the sensor was selected based on previous test results. The resistors on the LED drivers were selected to keep the BJT in saturation and to run ~50mA of current through the LED. A general purpose NPN transistor, 2N3904, was selected from components immediately available. 2N3904 has a maximum Ic rated at 100mA, hfe is around 60 to 100 depending the parameters of operation. In my calculations hfe = 60 was assumed. These transistors, I believe, are available at Radio Shack, which means replacement parts are readily available. All unlisted data are available on datasheets online. Vcc is 5V. Below are pictures of the sensor module.
