Sensor devices, Light-emitting diodes (LEDs) and Neons.

Sensor devices

Photoresistive cells (light-dependent resistors)

The simplest fonn of light sensor is the photoresistor, a common example being the ORP12. Absorbed light produces electron-hole pairs in the material of the photoresistor, causing the resistance to decrease.

A typical cell has a resistance of around 2 Mil in the dark and l 00 Q in room lighting. This represents a change of I 0,000 to l, allowing very simple circuits to be used. A circuit for a light­ operated relay is shown.

Photoresistive cells nonnally have a peak response around 600 nm, at the red end of the spectrum. The peak response frequency is detennined by the choice of semiconductor.

Photoresistive cells are slow devices, taking several milliseconds to respond to step changes in light. The response is also non-linear and temperature-dependent. They are thus best suited to on/off detector circuits.

Photovoltaic devices (solar cells)

A photovoltaic device is a specially designed PN junction which generates a voltage across its tenninals when illuminated with strong light. The power generated is small; a single cell produces between 20 rnA and 100 rnA at 0.4 V in sunlight.

The open circuit voltage/incident light relationship is logarithmic, making them useful for photographic light meters. The short-circuit current/incident light response is linear, and this mode is used in some limited range light meters.

Photovoltaic cells ·are expensive, and this restricts their use to specialist applications. Unless a totally self-contained battery-less circuit is required, photodiodes or photoresistors are more cost­ effective devices.

Photodiodes

The photodiode consists of a back-biased PN junction. Under dark conditions the only current flowing is the minority carrier leakage current. When the junction is illuminated, electron-hole pairs are generated and the current increases.

Changes in characteristics in a photodiode are not as marked as in a photoresistor. A typical photodiode goes from I 0 iJA dark current to around 100 iJA in strong light. The photodiode is a low level, high impedance device, and requires more complex circuits than the photoresistor. A typical circuit using a photodiode is shown, where and op-amp is used as a buffer amplifier.

The response of the photodiode is relatively linear, allowing it to be used in photometer applications. Its main advantage, however, is its high speed of operation. The response time of most photodiodes is under 200 ns, allowing them to be used in conjunction with high speed circuits. They are widely used in high speed tape readers and opto-isolators.

Phototransistors

The phototransistor can be considered as a combined transistor and photodiode, as shown. The photodiode replaces the normal base bias resistor, and the light-dependent leakage current supplies the base. This current is multiplied by the normal transistor action. Unfortunately, the dark current of the diode is also multiplied, so the dark current of a phototransistor is rather high.

A typical phototransistor has a dark current of several microamps. The current rises to several milliamps when the device is illumi­ nated. Although faster than photoresistors, the phototransistor is not as fast as a photodiode, so can only be used up to 100kHz. The response is somewhat nonlinear, limiting its use to on/off detecting circuits.

Other devices

Theoretically, any semiconductor device can be made into a sensor. Photo-FETS, photo-darlingtons and photo-thyristors all exist, but these are rather rare.

An interesting recent development is the production of a combined photodiode and integrated circuit amplifier. These are combined in a small case and only require two external components, as shown. These are designed for on/off applications, and are both fast and reasonably priced.

Comparison of devices

A comparison of the characteristics of the four common sensor types: photoresistor, photovoltaic, photodiode and phototransistor, follows:

Light-emitting diodes (LEDs)

An LED is a PN junction diode which emits light when conducting. The mechanism by which the light is produced is somewhat complex. Basically, electron-hole pairs are formed which emit light as they recombine. Fortunately it is not necessary to have a detailed knowledge of how LEOs work in order to use them.

Electrically, an LED looks like a normal semiconductor diode having low resistance in the forward direction and high resistance in the reverse direction. The only major difference is the high forward drop of around 2 V and the low PIV of around 5 V.

An LED is a current-operated device, so it must always be operated with a series resistor (or driven from a constant current source). For most applications the current required will be between 5 rnA and 30 rnA. Because the eye has a logarithmic response to light, the apparent light output does not vary greatly with current once the LED has attained a reasonable intensity.

In the circuit given in (a) the value of the series resistor, R, is given by:

where Vr is the LED forward drop and I the required current. These are obtained from the data sheet.

The low PlY can present design problems. If an LED is to be driven from an AC source the LED should be protected by a reverse diode, as shown in (b). Because the LED is bei(lg illuminated for only one half-cycle, a higher value of current is required.

LEDs are almost perfect indicator lamps. They are very efficient, and run cool. Unlike normal incandescent light bulbs, they are immune to shock and vibration, have a long life and no surge current at tum-on.

LEOs can be obtained in a wide range of colours, the commonest being red, yellow and green (ideal for model railway enthusiasts). Special LEOs, working in the infrared region of the spectrum, are also available for use with photocells.

Incandescent bulbs

Normal lamp bulbs have almost entirely been superseded by LEDs for panel indicators. The maximum intensity available from an LED, however, does not yet match that available with bulbs. Where a high intensity is required, the designer therefore has little choice.

Life of a bulb is inherently limited to a few thousand hours. This can be extended in several ways. The first (and obvious) way is to underrun the bulb. Operating a bulb at 10-20% below its nominal voltage can double its life.

Cold resistance of a bulb is considerably lower than its hot resistance. At tum-on there is a considerable current surge which may fracture the bulb filament (and cause noise problems in the rest of the circuit). This current surge can be reduced by always keeping a small 'lamp warming' current flowing through the bulb, so that in the 'off' state it glows dimly. The absence of a current surge extends the life of the lamp considerably. A typical indicator panel is shown, with lamp warming resistors and a lamp test switch.

Neons

The neon is a gas discharge device suitable for use with high voltages. Once common, it is now mainly used for 'mains on' indication. It is a current-operated device, and like an LED needs a series resistor. The value is obtained from the equation given for LEOs earlier. Typical values for Vr are around 100 V.