I'm trying to create a constant current source that can be turned on/off using Arduino's digital output.
I'm supplying the BC548 with a 5V vcc supply from the Arduino. The base is supplied with 5V using arduino's digital output. The output of the BC548 is sent to the input of the LM317LZ to produce a constant current source based on the design on http://www.bristolwatch.com/ccs/LM317.htm
The issue is that the voltage at the Adj pin of the LM317LZ changes with R1. With R1 set to 100 Ohms, the voltage at the Adj pin is 1.25 V as expected, but when R1 is set to 50 Ohms, by adding another 100 Ohms in parallel, the voltage changes to 1.73 V.
Here's the circuit diagram: