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Table 1 A basic introduction of circuit components

From: An economical and highly adaptable optogenetics system for individual and population-level manipulation of Caenorhabditis elegans

While it is beyond the scope of this paper to provide an introduction into electronics, we want to provide some basic background information about the electronic components and their function in the circuit. However, more detailed information is readily available on the internet or in any electronics handbook.
 • LED (light-emitting diode): This component is a semiconductor that converts the energy of exited electrons into photons to produce light. One important characteristic of LEDs is their polarity, i.e., they have a positive and negative side. In order for the LED to function, it is paramount to connect its anode (+) to the positive side of the power supply and the cathode (−) to the negative side, so the current will flow in the right direction. Other key characteristics of LEDs are their tendency to produce heat and their sensitivity to heat. While you may have heard LEDs produce no heat and can be cool to the touch, substantial heat gets produced inside the device and this can change the electrical properties of the semiconductor. In the short term, this can decrease the output of the LED, but heat will also significantly reduce the LED’s lifetime or damage the LED permanently. In fact, the change in characteristics upon heating can give rise to “Thermal Runaway,” where the rise in temperature causes the resistance to lower and the LED to draw more current, leading to a further increase in heat production. This vicious circle will eventually cause the LED burnout. To prevent this from happening, it is important to power the LED with constant current.
 • LED driver: This component transforms the power from the power supply and delivers a constant current to the LED to prevent “Thermal Runaway.” The DC model used in our set-up has 6 leads. A red/black pair and a white/blue pair are attached to the positive and negative leads of the power supply and LED, respectively. In addition, it has an internal reference lead putting out 5 V DC (yellow) and a control input lead (gray). These last two leads allow external control over the current output and, by extension, the brightness of the LED.
 • Potentiometer: This is a passive electronic component that has a variable resistance. By placing a potentiometer between the internal reference and the control input, it is possible to regulate how much of the 5 V from the reference gets delivered to the control input. There is an inverse relationship between the voltage supplied to the input control and the current output of the driver, i.e., 5 V to the input will reduce output current to zero and 0 V will yield the maximum output.
 • Microcontroller: This is essentially a small computer. It can be configured to receive input signals, process them and return output signals. In our automated set-up, an Arduino Uno microcontroller is used to replace the potentiometer. This allows a more precise regulation of the LED intensity and timing of light pulses. In this set-up, the 5 V internal reference is used to power the Arduino and one of Arduino’s pulse width modulation (PWM) pins is used to supply voltage to the input control. Arduino pins can generally only switch between an off (0 V) and on (5 V) state. However, PWM pins can cycle rapidly between these on and off states to generate an output voltage between 0 and 5 V that is perceived as continuous, simulating the effect of the potentiometer.
 • Capacitor: This is a passive electronic component that stores electrical energy. Applying a voltage over a capacitor charges an electric field within the capacitor. In our automated set-up, a bypass capacitor is introduced, connecting the internal reference to the ground. The DC signal provided by the LED driver may not be “pure” DC and can display some high-frequency noise fluctuations, which can cause erratic behavior of the microcontroller. The capacitor will counteract these fluctuations by charging or discharging accordingly.