Roborugby Optical Parts

These are electronic components, using either visible or infra-red light. You can use them, for example, to detect and follow lines on the RoboRugby table, or to detect the presence of a ball in the robot.

Reflective Optical Sensor

This optical sensor is easy to use, as it has the source and detector in one package. However, it is only suitable for detecting light reflected from objects in a 5 to 10mm range, as the source and detector are angled sharply towards each other. Each kit has two of these sensors already wired up as shown below, with a plug to fit the Handyboard.

For those who want to build more sensors, the basic sensor component is available in two versions - the only difference is the connection arrangements:


Optek OPB704 - data sheet	Optek OPB704W - data sheet
This version has short pin connections. You can solder wires directly to these or bend them and solder them into a circuit board. Great care is needed when soldering, as the pins are short - too much heat will destroy the device in seconds!	This version has long insulated wire connections. You can solder these wires into a circuit board, or connect them directly to other components or wires. The wires are colour-coded to indicate which is which. A mistake here can destroy the device!

Light source

These devices use an infra-red light-emitting diode (LED) as the light source. See details of LEDs below. Like other LEDs, this one needs a series resistor to set the current flowing in it at the required value.

The maximum current which the LED can survive is 40mA. It is normal to operate it at 10 to 20mA. Higher current will give more light output and increase the range of the sensor. The range is is limited by the sensor geometry anyway, so there is little point in using more than 20mA.

At a current of around 20mA, the voltage across the LED is typically 1.6V. With the 5V supply from the Handyboard, this leaves 3.4V across the series resistor. A series resistance of 180 ohms will give a current of about 19mA. The circuit that we have used is shown below, along with the pin connections of the sensor.

Light detector

The light detector is a photo-transistor, which can control the current flowing in an external circuit. When it is dark, it only allows a very small current to flow - about 100nA. As the light intensity increases, the photo-transistor allows more current to flow, up to a maximum of 200µA.

As the Handyboard analogue inputs respond to voltage, you have to make this current flow through a resistance. The Handyboard inputs already have a resistance of 47kohm connected to the 5V supply, which is ideal for this purpose. The circuit that we have used is shown above.

This circuit will give a high voltage (close to 5V) when the photo-transistor is dark. As the light intensity increases, more current flows, and the voltage falls, reaching 0.3V at a current of 100µA. This means that on the Handyboard, smaller numbers correspond to brighter light.

Light-emitting Diodes

A diode allows current to flow through it in one direction only. A light-emitting diode emits light when a current flows through it. The larger the current flowing through an LED, the more light will be emitted.

There is a limit to the current which an LED can survive. It is important to connect a resistor in series with an LED to set the current at the required value. Too much current will destroy the LED!

A current of 10mA is a good starting point - it will produce plenty of light from the LEDs provided.

Details

LED type:	Red	Infra-red
Package colour (for identification):	Red	Blue
Part details: (click for data sheet)	Kingbright L531D	Temic TSUS 5202
Peak wavelength of light output:	625 nm	950 nm
Angle of half intensity:	30°	15°
Maximum current:	30 mA	100 mA
Typical current:	10 mA	20 mA
Voltage at typical current:	2 V	1.3 V

Using an LED

As the Handyboard provides a constant voltage supply, of 5V, you must connect a resistor in series with an LED to set the current at the required value. Connecting an LED directly to the 5V supply will destroy it! The diagram below (centre) shows the circuit which you need.

The diagram above (left) shows the symbol for an LED, and the ways of identifying the pins on the package: the cathode (negative terminal) is the shorter pin, and is also marked by a flat part on the side of the LED package. Connecting an LED backwards will destroy it!

The equations above (right) show how to calculate the resistor value needed. You need to know what current you want to flow in your LED, and the voltage across your LED when this current is flowing. You can assume that this voltage will be the value at the typical current given in the table above. An LED is not like a resistor - it does not obey Ohm's law - the voltage remains fairly constant for a wide range of current values.

Light Detectors

These are light-sensing elements with a built-in amplifier circuit.

The detectors operate from a 5V supply, and give an output voltage proportional to the light intensity detected, up to a maximum of about 4V. Any further increase in light intensity will cause no change in voltage.

Details

Detector type:	Visible	Infra-red
Package colour (for identification):	Clear	Dark
Part details: (click for data sheet)	Texas Instruments TSL250	Taos TSL260R
Response to light:	450 to 950 nm	850 to 1000 nm
Angle of half response:	55°	45°
Minimum output voltage (dark):	3 mV	4 mV
Maximum output voltage (bright):	3.5 V	3.8V

Using a light detector

These light detectors can be connected directly to a Handyboard analog input port. The Handyboard provides the 5V supply needed. The detector output voltage is within the 0 to 5V range of the Handyboard inputs.

The diagram above shows how to identify the pins on the light detector package, and how to connect these pins to the Handyboard port. The 47kOhm resistor shown is part of the Handyboard input circuit - it does not have to be added. Take care with the connections - connecting the wrong pin on a light detector to the 5V supply will destroy it!