The LED stimulator is built on a double-sided printed circuit board (PCB). A detailed circuit diagram is presented in Fig. 3(a). The power is supplied by a lightweight lithium polymer battery (output: 3.7 V, 10 mAh; size: ; mass: 0.52 g; Guangzhou Fullriver Battery Ltd., Guangzhou, China), which is converted to dc 5 V by a boost converter (TPS61222, Texas Instruments Inc., Dallas, Texas). The front side accommodates an IR receiver module for a carrier frequency of 38 kHz (RPM7138-R, Rohm Co., Ltd., Kyoto, Japan), a microcontroller (PIC12F683-I/SN, Microchip Technology), a programming port, and a socket for a battery [Fig. 3(b)]. IR receivers for carrier frequencies of 30 kHz (TSOP4830, Vishay Intertechnology Inc., Malvern, Pennsylvania) and 56 kHz (TSOP4856, Vishay Intertechnology) are also used for multiband IR reception. The backside of the PCB is used to mount the boost converter and receptacles (R861-83-050-10-001, Tokiwa, Tokyo, Japan) for three output LEDs [Fig. 3(c), LED1 to LED3]. The receptacles are also used to attach the LED stimulator to implanted LEDs [e.g., Fig. 4(d)]. Small blue LEDs (, ; LP-1608H183BC, LED-Paradise, Saitama, Japan) were used to activate channelrhodopsin-2 (ChR2). We used two different LED configurations. One had three small LEDs [Fig. 3(d), LED1 to LED3] attached to plugs [R860-10-050-10-001, Tokiwa; Fig. 3(d), arrowhead]. The other had two small LEDs [Fig. 4(c), right LED and left LED] attached to plugs [Fig. 4(c), arrowhead] and connected to LED1 and LED2 of the LED stimulator through an adaptor [Fig. 4(d)]. The supply of current to each LED is limited to 20 mA by a resistor [100 Ω, Fig. 3(a)]. The output power of the LED was measured by an optical power and energy meter (PM100D, Thorlabs Japan Inc., Tokyo, Japan). The height and mass of the LED stimulator including the battery (10 mAh) were 10 mm and 2.4 g, respectively.