# Design and Construction of an LED-Based Excitation Source for Lock-In Thermography
## 2 Hardware Design
### Circuit Topology  
In this section, a description of the functionality of the driver circuit is given. The circuit diagram of the LED excitation source is shown in Figure 4. Connections to the controller are indicated as gray labels. The circuit simulation projects in LTSpice, the firmware for the Arduino Nano, the STL files for the 3D-printed parts, and schematics for the hardware are included in the Supplementary Material of this article. The three LED strings are connected in parallel with one common output transistor. Each string has a separate shunt resistor, which also acts as a balancing resistor. To drive the LEDs, a switched topology and a linear topology are considered. The switched topology is based on a hysteretic step-down converter, designed to have a ripple of 300 mA (10% of the maximum current) at a switching frequency of approximately 150 kHz. The linear topology approach directly controls the LED current with the output transistor using a proportional-integral (PI) controller. Both topologies are simulated using the freely available electronic simulation software LTSpice(r) [25]. \begin{table}
\begin{tabular}{l c c} \hline \hline
**Parameter** & **Symbol** & **Value** \\ \hline LED forward voltage & \(U_{\text{F}}\) & 42 V \\ LED forward current & \(I_{\text{F}}\) & 3 A \\ Voltage level, analog input & & 3.3 V \\ Voltage level, digital input & & 5 V \\ Maximum frequency & \(f_{\text{max}}\) & 1 kHz \\ Amplitude response & \(G(f)\) & \(\pm 1\) dB \\ Phase response & \(\varphi(f)\) & 1\({}^{\circ}\) \\ Rise time, fall time & \(t_{\text{r}}\), \(t_{\text{f}}\) & 10 μs \\ Propagation time & \(t_{\text{prop}}\) & 10 μs \\ Jitter & \(A_{\text{J}}\) & \(\pm 5\) μs \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Specifications of the LEDs and the driver circuit. The switched topology shows a rise time of 12.3 us and a fall time of 5.5 us, whereas the linear topology shows a rise time of 2.0 us and a fall time of 0.7 us. Thus, the switched topology approach does not meet the criteria of Table 1. In addition, the slow rise time of the switched topology leads to a lower effective duty cycle when the output is pulse-width modulated. This deteriorates the modulation linearity. As the hysteresis controller of the switched topology only oscillates above a threshold of half the ripple current, the LEDs cannot be dimmed at low current values. Due to the better transient performance and linearity, the linear topology approach is chosen over the switched one. The circuit is powered by an external adjustable switched-mode power supply unit of the type Mean Well HEP-185-36A. The output voltage of the power supply unit can be varied in the range from 33 V to 40 V. The voltage chosen is 4 V above the forward voltage of the LEDs, leaving enough regulation reserve for the output transistor. When choosing the supply voltage such that it is only 4 V over the LED forward voltage, the power dissipation of the output transistor is 12 W, which is low compared to the electric power of the LEDs of 101 W. If the voltage reserve is set lower, the control loop tends to oscillate. A relative voltage of 4 V is chosen as a compromise between control stability and power dissipation. The 15 V supply consists of a 7815 TV linear regulator, which is connected to the LED supply voltage. Z-diodes are added to the input of the linear regulator to meet its input voltage limit of 35 V.  
Figure 4: Circuit diagram for the LED excitation source. Connections to the controller are indicated as gray labels. For clarity, the units of resistor and capacitor values are omitted and only the SI prefixes are shown; decimal points are replaced by SI prefixes, for example 2k7 instead of 2.7 k.  
Figure 3: Infrared LED excitation source with additional lenses to focus the beam of the individual LEDs (**a**). To further focus the beam, a parabolic reflector is attached to the LED lamp (**b**). The currents through the LED strings are measured with individual shunts.