GB2054151A - Proximity Switch - Google Patents

Abstract

A switching network for a hot-air hand drier or the like comprises. An aerial, preferably in the form of an open mesh across the outlet of the drier, which is capacitatively coupled in the network in the path of the repetitive pulse signal from a generator ICA to a trigger circuit ICB, such that alteration of stray capacitance by the proximity of a human operator causes the signal to change and the generation of an operating signal for the motor driven blower and heating element of the drier. <IMAGE>

Description

SPECIFICATION Proximity Switch for Hot-air Hand Driers and the Like This invention relates to proximity switches primarily though not exclusively intended for the control of hot-air hand driers.

According to the invention there is provided a switching network disposed to provide an operation signal in response to proximity of a human operator to an aerial, and comprising a signal generator which provides a repetitive pulse signal to a trigger circuit via a capacitator voltage divider of which one part comprises the aforesaid aerial which is available for approach by a human operator; in which network the divider is arranged such that alteration of stray capacitance of said aerial by the approach is sufficient to impede the passage of the said signal to the trigger circuit, which thereupon provides said operating signal.

Reference will hereinafter be made to the accompanying drawing, which schematically illustrates the principal operating of one embodiment of the invention.

The present embodiment is intended for use with a hot-air drier. It is intended that the drier be operated in response to the approach of a hand to an aerial, which may, for example, be fitted across the outlet for air from the drier in the form of an open mesh; alternatively the aerial may be disposed close to the outlet for air in the form of a mesh or metal foii. The present embodiment may be arranged such that when a hand approaches the aerial, to within for example 75 mm, the switching network causes the drier to operate.

Withdrawal of the hand from the proximity of the aerial and the stream of hot air will cause the switching network to stop the operation of the drier.

An incoming main supply is connected to the line and neutral conductors L and N; output contact, fed from the line supply through the contacts of relay supplies power to the motor for a fan blower and a heating element. The relay is controlled by the switching network to be described. A double wound transformer T provides isolation for a low voltage power supply which is derived from the secondary of the transformer via a diode bridge rectifier. The rectified supply is smoothed by a capacitor C1, the smooth voltage being partly dropped across a 1 kilo, 1 watt resistor R1, which reduces the available voltage to 7.5 volts, regulated by a zener diode D3. A diode D2 isolates a relay coil R, from the capacitor C1.

An integrated circuit type CD4093 is a quadruple 2 input Schmitt trigger NAND device.

A NAND gate A is connected as a free running square wave oscillator producing repetitive output at, for example, 1 megahertz. The output of the oscillator is fed into a 62 picofarad capacitor C3 and a 10 picofarad capacitor C4, which is carefully balanced against the stray capacitance associated with the aerial. Normally the available signal is fed into a 1 nanofarad capacitor C4 which switches the NAND gate B on and off, thereby forward biassing diode D4 when the output of the latter gate goes high; the conduction of the diode D4 charges the associated 1 nanofarad capacitor C5. By means of the gate C the transistor TR, which the collector is connected to the relay coil, is cut off.

When, for example, a human hand approaches the aerial, the small additional loading of the aerial causes the shunting of the signal from the capacitor C6; there being insufficient signal at the input of NAND gate B, the 100 kilohm "pull up" resistor R3 causes the output of the NAND gate B to go "low". The diode D4 is reversed biassed, the capacitor C5 is discharged through the 100 kilohm resistor R4 to ground, causing the output of the NAND gate B to go high and switching on the transistor TR and the relay coil.

Claims

1. A switching network disposed to provide an operating signal in response to proximity of the human operator, comprising a signal generator which provides a repetitive pulse signal to a trigger circuit via a capacitative voltage divider of which one part comprises an aerial which is available for approach by a human operator, the divider being arranged so that alteration of stray capacitance associated with the aerial is sufficient to impede the passage of the said signal to the trigger circuit, which thereupon provides the operating signal.

2. A hot-air hand drier including a motor driven fan for providing a blast of air, a heating element disposed for heating said air, an electrically operated relay or switch for control of the motor and heating element, and a switching network as set forth in claim 1 arranged to provide an operating signal for the said switch.

3. A network as set forth in claim 1 or a drier as set forth in claim 2, wherein the aerial is in the form of a mesh or other conductive member disposed across or in close proximity to an outlet for air from the fan.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Proximity Switch for Hot-air Hand Driers and the Like This invention relates to proximity switches primarily though not exclusively intended for the control of hot-air hand driers. According to the invention there is provided a switching network disposed to provide an operation signal in response to proximity of a human operator to an aerial, and comprising a signal generator which provides a repetitive pulse signal to a trigger circuit via a capacitator voltage divider of which one part comprises the aforesaid aerial which is available for approach by a human operator; in which network the divider is arranged such that alteration of stray capacitance of said aerial by the approach is sufficient to impede the passage of the said signal to the trigger circuit, which thereupon provides said operating signal. Reference will hereinafter be made to the accompanying drawing, which schematically illustrates the principal operating of one embodiment of the invention. The present embodiment is intended for use with a hot-air drier. It is intended that the drier be operated in response to the approach of a hand to an aerial, which may, for example, be fitted across the outlet for air from the drier in the form of an open mesh; alternatively the aerial may be disposed close to the outlet for air in the form of a mesh or metal foii. The present embodiment may be arranged such that when a hand approaches the aerial, to within for example 75 mm, the switching network causes the drier to operate. Withdrawal of the hand from the proximity of the aerial and the stream of hot air will cause the switching network to stop the operation of the drier. An incoming main supply is connected to the line and neutral conductors L and N; output contact, fed from the line supply through the contacts of relay supplies power to the motor for a fan blower and a heating element. The relay is controlled by the switching network to be described. A double wound transformer T provides isolation for a low voltage power supply which is derived from the secondary of the transformer via a diode bridge rectifier. The rectified supply is smoothed by a capacitor C1, the smooth voltage being partly dropped across a 1 kilo, 1 watt resistor R1, which reduces the available voltage to 7.5 volts, regulated by a zener diode D3. A diode D2 isolates a relay coil R, from the capacitor C1. An integrated circuit type CD4093 is a quadruple 2 input Schmitt trigger NAND device. A NAND gate A is connected as a free running square wave oscillator producing repetitive output at, for example, 1 megahertz. The output of the oscillator is fed into a 62 picofarad capacitor C3 and a 10 picofarad capacitor C4, which is carefully balanced against the stray capacitance associated with the aerial. Normally the available signal is fed into a 1 nanofarad capacitor C4 which switches the NAND gate B on and off, thereby forward biassing diode D4 when the output of the latter gate goes high; the conduction of the diode D4 charges the associated 1 nanofarad capacitor C5. By means of the gate C the transistor TR, which the collector is connected to the relay coil, is cut off. When, for example, a human hand approaches the aerial, the small additional loading of the aerial causes the shunting of the signal from the capacitor C6; there being insufficient signal at the input of NAND gate B, the 100 kilohm "pull up" resistor R3 causes the output of the NAND gate B to go "low". The diode D4 is reversed biassed, the capacitor C5 is discharged through the 100 kilohm resistor R4 to ground, causing the output of the NAND gate B to go high and switching on the transistor TR and the relay coil. Claims

1. A switching network disposed to provide an operating signal in response to proximity of the human operator, comprising a signal generator which provides a repetitive pulse signal to a trigger circuit via a capacitative voltage divider of which one part comprises an aerial which is available for approach by a human operator, the divider being arranged so that alteration of stray capacitance associated with the aerial is sufficient to impede the passage of the said signal to the trigger circuit, which thereupon provides the operating signal.

2. A hot-air hand drier including a motor driven fan for providing a blast of air, a heating element disposed for heating said air, an electrically operated relay or switch for control of the motor and heating element, and a switching network as set forth in claim 1 arranged to provide an operating signal for the said switch.

3. A network as set forth in claim 1 or a drier as set forth in claim 2, wherein the aerial is in the form of a mesh or other conductive member disposed across or in close proximity to an outlet for air from the fan.