A light sensor as part of a voltage divider.

This is a light dependent resistor, or LDR

A LDR is a special type of resistor. The light-sensitive part of the LDR is a wavy track of cadmium sulphide. As more light falls onto the track, the resistance decreases...

What happens if one of the resistors in the voltage divider is replaced by an LDR? Again, like the switches above, it can be used to provide a voltage signal to our PICAXE which can trigger it into action.

The circuit can be built in either of two ways:

A sensor subsystem which functions like this could be thought of as a 'dark sensor' and could be used to control lighting circuits which are switched on automatically in the evening. On our Wishboard circuit 3V means connect to the +ve rail and 0V means connect to the -ve rail.

 

This circuit delivers a HIGH voltage when the light is present. A resistor value of 10kW is often used. On our Wishboard circuit 3V means connect to the +ve rail and 0V means connect to the -ve rail.

Sometimes there is too much light creating a "false" signal. An alternative is to use a Infra-red Phototransistor. We used this system in our Science Fair Laser Wars! combat game. The transistor is sensitive to infra-red so can be more useful in areas that are brightly lit. TV remote controls use IR LED's to trigger a phototransistor in the TV. "Normal" daylight does not cause a "false" alarm since our sensor reacts to "invisible" IR light.

Mighty Mouse uses LDR's to either run away from light or move toward light. You can even get him/her to follow a line drawn on the floor or desk!

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Temperature sensors

A temperature-sensitive resistor is called a thermistor. There are several different types.

The resistance of most common types of thermistor decreases as the temperature rises. They are called negative temperature coefficient, or NTC, thermistors. The Black Box! data logger uses this type of thermistor. The -t° next to the circuit symbol tells us that as the temperature rises the resistance falls.

It is possible to manufacture positive temperature coefficient, or ptc, thermistors. These are made of different materials and show an increase in resistance with temperature.

Lots of different types of thermistor are manufactured, each has its own characteristic pattern of resistance change with temperature.

Suppliers catalogues usually give the resistance at 25°C, which was 100 kOhm in the case of the Black Box! data logger.

Usually, catalogues also specify a 'Beta' or 'B-value'. When these two numbers are specified, it is possible to calculate an approximate value for the resistance of the thermistor at any particular temperature from the equation:

Where:

Using a spreadsheet it is possible to to generate characteristic curves for any thermistor by calculating resistance values for a range of temperatures. You can apply this equation to make sure that the voltage dividers you build will always be as sensitive as possible within an useful range of temperatures.

We used this system in our Black Box! data logger.

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Sound sensors

Another name for a sound sensor is a microphone. The diagram shows a cermet microphone:

To make them work properly, cermet microphones need a voltage, usually around 1.5 V across them. A SUITABLE CIRCUIT FOR OUTPUT TO A PICAXE-08 WILL BE UPLOADED SOON.

We used a sound sensor in The Professors electronic ear - a classroom noise level alarm; a must for all teachers!

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Resistance Chart

Resistors are too small to print resistance values on. Instead, a standard colour code is used.

The resistor above has four bands; two represent the first and second digit of the resistor value, the next band is the multiplier and the last band represents the tolerance. Some resistors have five bands.

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Capacitor Table

Capacitors may be marked to show their capacitance value, voltage rating and polarity, however, the value and voltage rating identification can be difficult because of the variety of systems in use.

Units: The unit of capacitance is the Farad, but this unit is extremely large. In practice smaller units are used such as the microfarad (abbreviated µF), nanofarad (nF) and picofarad (pF). Some capacitance values are commonly expressed by only one unit while others can be under two or more units, e.g. 0.0047 is often written as 4.7nF or 4700pF.

Values: Larger capacitors are marked in microfarads and indicate this by the abbreviations 'µF', 'µ' or even the obsolete 'MFD'. Smaller capacitors are marked in nanofarads or picofarads and may abbreviate the unit to 'n' or 'p'. If the value contains a decimal point the 'µ','n' or 'p' is sometimes put in place of the decimal point. Therefore a 4.7pF capacitor can be marked as 4p7. If no unit is given, a judgement, based on the capacitor's physical size, must be made to determine which unit is intended. For example, a small ceramic capacitor marked '4.7' is probably 4.7 pF, whereas a large plastic capacitor marked '4.7' is more likely to be 4.7µF. If the value is in nF then this is always written.

Another marking system uses 3 numeric digits to indicate the value in picofarads. The first two digits represent the first two digits of the value and the third digit is the multiplier or number of zeroes. For example, a capacitor marked 104, such as those used for noise suppressionon Mighty Mouse, would be read as 1, 0, 0000. This would be formatted as 100,000 pF and would commonly be known as 100nF or 0.1µF. Likewise a capacitor marked 472 would be 4700pF, also known as 4.7 nF.

Some common values and their possible markings:

microfarads	nanofarads	picofarads	EIA code
0.0001µF*	0.1n*	100pF	101
0.00022µF*	0.22n (n22)	220pF	221
0.001µF	1n(1n0)	1,000pF	102
0.0033µF	3.3n(3n3)	3,300pF	332
0.01µF	10n	10,000pF*	103
0.047µF	47n	47,000pF*	473
0.1µF (µ1)	100n	100,000pF*	104
0.82µF (µ82)	820n	820,000pF*	824
1.0µF (1µ0)	1000n*	1,000,000pF	105

(*) Not normally expressed in this form.

Voltage Rating: Voltage rating is usually marked and is often identified by the symbol 'V'. Most electrolytic capacitors clearly indicate their voltage rating. If the capacitance and voltage rating are both marked, a unit is also marked for at least one of the quantities so that the two cannot be confused. Polarity sensitive capacitors, such as electrolytics, are usually marked with a '+' or '-' symbol adjacent to one lead to indicate polarity.

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TRY THESE:-

1. Mighty Mouse - uses LDR's to either run away from light or move toward light. You can even get him/her to follow a line drawn on the floor or desk! (with code)
2. Bike Indicator - A Primary School student puts a PICAXE on her bike (with code)
3. Laser Wars! - Play a combat game in broad daylight instead of in the dark! See the Terminator!
4. Black Box data-logger - Add a sensor to this mobile PICAXE to record data anywhere, then download to your PC (with code)
5. The Professors electronic ear - a classroom noise level alarm; a must for all teachers!