Fig. 1 Test setup Arduino controlled constant current source.
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Arduino Controlled Constant Current Source

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Adding a single switching transistor enables microcontroller control of the output. Current limit can also be set.

For more on the original constant current source see Constant Current Source Theory Testing.

Fig. 2 Original variable constant current source circuit using LM317.
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The original CCS circuit is shown in Fig. 2. The LM317 circuit sets the emitter-base current. This controls the emitter-collector current through Q1.

Fig. 3 Switching transistor in ground side LM317 constant current source.
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By adding a TIP120 transistor to the ground circuit allows an Arduino to switch the current ON-OFF. This also enables the use of pulse-width-modulation to control the output.

Iset now determines maximum current while Arduino or other microcontroller controls overall power output by varying the duty cycle - or full on.

Fig. 4 Wiring diagram Arduino controlled constant current source.
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Fig. 4 is the circuit used in the demo video.

Fig. 5 Electrical test schematic Arduino controlled variable constant current source.
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Fig. 5 are the load connections in the test circuit.

Code

#define pwmPin 11

int val;
void setup() {
  // put your setup code here, to run once:
  digitalWrite(pwmPin, 0);
}

void loop() {
  // put your main code here, to run repeatedly:
  val = analogRead(0) / 4;
  analogWrite(pwmPin, val);
  delay(100);
}