Feb 28, 2013


There are many places around the house where a small light would be useful but running a mains cable to the location is impractical. Corners of dark cupboards, over the telephone to light a note pad, by the front door to help find the keyhole at night, are just some of the applications which come to mind. None of these require very much light and high brightness light emitting diodes (l.e.d.s) can not only provide the illumination needed, but can also be readily fitted with a time delay circuit so that they switch off automatically, so saving battery power. The circuit described here offers such a solution. It is shown in Fig.1 To ensure that the circuit switches off after use and prevent having to change the battery too often, a timing circuit is required. For this a monostable configuration is used. A monostable has one stable state, in this case the off state.
                              When triggered into its on state, it will remain in that state for a preset period before switching off again. Some circuits of this type use two transistors (npn or pnp types) configured so that in the stable state one transistor is on while the other remains off. Following a trigger pulse, both transistors change state. A disadvantage of this circuit is that during the off state, one of the transistors is always turned on, and so consuming power. An alternative configuration is used here in which all transistors remain off when the circuit is in its stable state, so consuming virtually no current.

Touch Circuit
In the circuit diagram shown in Fig.1, transistors TR2 and TR3 form the monostable circuit, with capacitor C1 and resistor R2 determining the time for which the transistors remain on once the circuit has been triggered. This occurs when finger contact is made with touch pad TP1. The 50Hz mains “hum” normally present in all households will be induced into the circuit through the finger, causing transistor TR1 to turn on. This provides base current to TR3, turning it on, together with the l.e.d. (D2), whose negative-going pulse is generated acrosscurrent is buffered by resistor R3.When the collector of TR3 goes low, a capacitor C1, causing TR2 to turn on and provide more current to the base of TR3. When the contact with TP1 is broken, TR1 ceases to conduct, but TR3’s base continues to be held on via TR2. However, C1 starts to charge via resistor R2. Eventually, its charge rises to within less than 0·6V of the positive power supply, turning off TR2 and thus TR3 and the l.e.d. as well. Diode D1 inhibits any positive-going pulse generated across C1 when TR2 switches off. With the component values shown, the l.e.d. will remain on for about three minutes.Touch Down
It is worth noting that touch pad TP2 may be needed if the 50Hz mains “hum” introduced by finger contact with TP1 is not strong enough, or non-existent, as in a garden shed for example. Making finger contact between TP1 and TP2 causes a
small current to flow from the positive ine, though the finger and into the base of TR1. It is advisable to insert resistor R4 between TP2 and the positive line to prevent damage to TR1 should the two pads be shorted accidentally by an object with a low resistance. If the unit is found to be too sensitive, a high value resistor of about 10M can be connected from the base of TR1 to the battery negative. This will prevent the circuit from switching on inadvertently, especially in areas where the mains field is high.

The circuit is built on a small piece of stripboard having 12 holes by 8 strips, as shown in Fig.2. Only two track cuts need to be made and no wire links are required. Apart from the resistors, all other components must be inserted the correct way round. Power to the circuit should be supplied by a 9V battery. As the stand-by current is extremely low (basically the leakage current of the transistors), the expected life should be almost the shelf life of the battery, depending of course on how often it is switched on. Consequently, an on/off switch is not required. The finished unit should be mounted in an insulated plastic box of a size suitable for the battery and circuit board. The touch contact(s) can be made from any piece of metal such as a bolt or nail, but a drawing pin pushed through a suitable hole in the box and connected to the board via a short length of wire provides a neater, more attractive finish.

LED Considerations
When on, the total current is 6mA with the l.e.d. accounting for about 5·8mA. White l.e.d.s exhibit a forward voltage drop of around 4V, so two could be used in series to provide more light. Resistor R3 would then need to be reduced to 470ohmto maintain the l.e.d. current at around 5mA. The brightness of the l.e.d.(s) can be increased by reducing the value of R3 to increase the current flow. Do not allow the current to be greater than that permitted by the l.e.d., which should be stated in its data sheet and supplier’s catalogue. There appears to be little apparent increase in brightness beyond about 10mA. Data sheets normally quote an l.e.d. viewing angle and this describes the “off axis” brightness of the device. Unlike the filament in a light bulb, an l.e.d. chip emits light only from its surface, rather than all around, so the light comes mainly from the front of the device. This is modified to some extent by the plastic package and l.e.d.s are available with a more or less focused light beam. Depending on the use, a wider angled light pattern may be preferred.

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