Assemble the circuitry on the basis of the attached connection diagram. It is recommended to print the direction on a piece of paper in advance so that the mirrors could be positioned in a degree of 90o relative to each other as accurately as possible, and so that the direction of the IR-LED and the photodiode could be adjusted with the appropriate accuracy. Using a collector lens put in front of the IR-LED, focus light on the photodiode. This could be achieved by using a smartphone put to the future position of the photodiode, – opening up the photographing programme of the phone – because the phone detects IR waves. (The focus will be set not to the first mirror, but to the telephone set placed to the anticipated position of the photodiode, because mirrors only increase the distance of the image after the lens, they do not transmit the focused light. But this can be clearly seen on the phone.) In order to demonstrate the successful experiment it is sufficient to connect the photodiode to a multimeter and follow the variations of the voltage readings, the value of which will depend on whether or not the photodiode can detect any IR waves. Pending on their colour, the opening voltage level of the LEDs used in the experiments falls between 1.7 and 2.5 V. Readily available voltage sources have typically 1.5 volts, or an integer multiple. In other words a single 1.5 volts penlight battery would be insufficient, two is too much. The load bearing capacity of these tiny LEDs is very low, therefore it is definitely necessary to use a preventive lead (switch resistance) to protect the LED which is usually connected to the circuit before the anode.

Implements required for the experiment

– A LED emitting infrared light (the type used in the experiment: IR-Emitter Harvatek HE3-245AC, enclosure type 5 mm, wavelength 940 nm)
– Photodiode (the type used in the experiment: Photodiode PIN quick reaction, 5mm, radiation angle ±20 ° 400-1100 nm Osram Components SFH 203)
– Concave metal LED socket in a diameter of 5 mm
– Two flat mirrors
– 100 Ω metal film resistor as preventive leads to protect the IR-LED
– 3 volts power supply source
– Multimeter to measure the changes in the voltage level of the photodiode
– Collector lens
– A sheet of paper on which directions are pre-printed – Wires