GB2050609A - Manually operable switching network - Google Patents

Abstract

A manually operable switching network, for a hot-air hand drier or the like, comprises a touch button capacitatively coupled in a network in the path of a repetitive pulse signal from a generator A to a trigger circuit C, D, such that alteration of stray capacitance by the proximity of a human digit causes shunting of the signal and the generation of a timing cycle for the motor driven blower and heating element of the drier. <IMAGE>

Description

SPECIFICATION Manually operable switching network for hotair hand driers and the like This invention relates to manually operable switching networks primarily though not exclusively intended for the control of hot-air hand driers.

According to the invention there is provided a manually operable switching network disposed to provide an operating signal of predetermined duration after actuation, comprising a signal generator which provides a repetitive pulse signal to a trigger circuit via a capacitative voltage divider of which one part comprises a screened connection of which one end is available for approach by a digit of a human operator; the divider being arranged such that alteration of stray capacitance adjacent said end by the said approach is sufficient to impede the passage of the said signal to the trigger circuit, which thereupon provides the said operating signal.

The said end of the screened connection may be terminated by a touch button which, as will be apparent, does not rely on electrical contact between the skin of the human digit and the metal touch plate within the button; accordingly, the invention facilitates the mounting of the touch button with a certain immunity against damage or adverse operating conditions, such as damp atmospheres.

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

The present embodiment is intended for use with a hot-air hand drier. It may be mounted within the drier, which preferably has a front panel including a touch button protected against physical damage. It is intended that when the hand drier should be operated, the touch button be firmly touched with, for example, a forefinger so that the control unit (to be described) starts the hand drier. The drier may run for a preset period, for example 30 seconds, to enable a user to dry his hands. After the predetermined operating time, the control unit may automatically switch off the hand drier which will then be ready for another operation.

The control unit has line and neutral terminals L and N connected to an incoming main supply, preferably at 240 volts, 50 hertz. The line terminal is connected through the contacts of a relay to an output contact C which, when the relay is closed, supplies power to a motor for a blower fan and a heating element. The line and neutral terminals supply the primary of a double wound transformer T which provides a 24 volt A.C. supply to a rectifying bridge Di. The output voltage of the rectifying bridge is smoothed by a 100 microfarad electrolytic capacitor C1. A 1 K8 1 watt resistor R1 in series with a Zener diode D3 are disposed across the capacitor Cl to provide a regulated 7.5 volts for the switching network.A diode D2, in series with the capacitor C1, isolates the coil of the relay from the capacitor C1 and improves the regulation of the low voltage supply for the electronic circuit.

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 a repetitive pulse output at, for example, 1 mHz. The output from the oscillator is fed into an 82 picofarad capacitor C3. This is connected to the junction of a capacitative divider consisting of a 1 nanofarad capacitor C4 and a screened cable connected directly to the metal plate of the touch button.

Normally the output signal of the oscillator passes through the capacitative network and switches a NAND gate B on and off, so as to forward bias a diode D4 when the output of gate B goes "high" and charging the associated 1 nanofarad capacitor C5. A 1 meg ohm resistor R5 holds the input to a gate C high; this gate is cross coupled with a gate D forming a bistable circuit which is initially reset via a capacitor C7 when the circuit is initially switched on. Immediately thereafter the capacitor C7 charges through the forwardly biassed diode D7 and a 1 K ohm series resistor R8. The cross coupled gates remain in a stable condition when the capacitor C7 is charged because the input from the gate C is held "low". The circuit is thereby rendered ready for operation.

The capacitor C3 is matched to the stray capacitance of the touch button which is arranged such that the unbalance caused by the alteration of the stray capacitance associated with the touch button when it is touched by a finger will shunt the signal from the capacitor C4. If no signal arrives at the input of the NAND gate B, the 1 OOK pull up resistor R3 at the input of gate B will cause the output of this gate to go "low" and reverse bias the diode D3. The nanofarad capacitor C3 will discharge through the 1 OAK resistor R4 to earth, producing transient pulse across the capacitor C3 and thereby a negative going pulse at the input of gate C.This trigger pulse forces the output of gate C high and consequent rise in output through the 1 OK ohm resistor R6 switches on a transistor BOl 82L of which the collector emitter circuit is in series with the coil of the relay. The high at the output of gate C is also fed to the input of gate D; the consequential "low" output causes the discharge of the capacitor C7 through a 380K ohm resistor R7 and an adjustable resistor P1. A timing cycle commences and continues until the capacitor C7 charges to the lower threshold level of gate D to force the output thereof high. The output of gate 3 will then go low, to switch off the transistor and release the relay. The capacitor C6 isolates gate B from Gate C, ensuring that the touch button must be released and retouched before the timing cycle can be restarted.

1. A manually operable switching network disposed to provide an operating signal of predetermined duration after actuation, comprising a signal generator which provides a

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

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Manually operable switching network for hotair hand driers and the like This invention relates to manually operable switching networks primarily though not exclusively intended for the control of hot-air hand driers. According to the invention there is provided a manually operable switching network disposed to provide an operating signal of predetermined duration after actuation, comprising a signal generator which provides a repetitive pulse signal to a trigger circuit via a capacitative voltage divider of which one part comprises a screened connection of which one end is available for approach by a digit of a human operator; the divider being arranged such that alteration of stray capacitance adjacent said end by the said approach is sufficient to impede the passage of the said signal to the trigger circuit, which thereupon provides the said operating signal. The said end of the screened connection may be terminated by a touch button which, as will be apparent, does not rely on electrical contact between the skin of the human digit and the metal touch plate within the button; accordingly, the invention facilitates the mounting of the touch button with a certain immunity against damage or adverse operating conditions, such as damp atmospheres. Reference will hereinafter be made to the accompanying drawing, which schematically illustrates the principal operating parts of one embodiment of the invention. The present embodiment is intended for use with a hot-air hand drier. It may be mounted within the drier, which preferably has a front panel including a touch button protected against physical damage. It is intended that when the hand drier should be operated, the touch button be firmly touched with, for example, a forefinger so that the control unit (to be described) starts the hand drier. The drier may run for a preset period, for example 30 seconds, to enable a user to dry his hands. After the predetermined operating time, the control unit may automatically switch off the hand drier which will then be ready for another operation. The control unit has line and neutral terminals L and N connected to an incoming main supply, preferably at 240 volts, 50 hertz. The line terminal is connected through the contacts of a relay to an output contact C which, when the relay is closed, supplies power to a motor for a blower fan and a heating element. The line and neutral terminals supply the primary of a double wound transformer T which provides a 24 volt A.C. supply to a rectifying bridge Di. The output voltage of the rectifying bridge is smoothed by a 100 microfarad electrolytic capacitor C1. A 1 K8 1 watt resistor R1 in series with a Zener diode D3 are disposed across the capacitor Cl to provide a regulated 7.5 volts for the switching network.A diode D2, in series with the capacitor C1, isolates the coil of the relay from the capacitor C1 and improves the regulation of the low voltage supply for the electronic circuit. 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 a repetitive pulse output at, for example, 1 mHz. The output from the oscillator is fed into an 82 picofarad capacitor C3. This is connected to the junction of a capacitative divider consisting of a 1 nanofarad capacitor C4 and a screened cable connected directly to the metal plate of the touch button. Normally the output signal of the oscillator passes through the capacitative network and switches a NAND gate B on and off, so as to forward bias a diode D4 when the output of gate B goes "high" and charging the associated 1 nanofarad capacitor C5. A 1 meg ohm resistor R5 holds the input to a gate C high; this gate is cross coupled with a gate D forming a bistable circuit which is initially reset via a capacitor C7 when the circuit is initially switched on. Immediately thereafter the capacitor C7 charges through the forwardly biassed diode D7 and a 1 K ohm series resistor R8. The cross coupled gates remain in a stable condition when the capacitor C7 is charged because the input from the gate C is held "low". The circuit is thereby rendered ready for operation. The capacitor C3 is matched to the stray capacitance of the touch button which is arranged such that the unbalance caused by the alteration of the stray capacitance associated with the touch button when it is touched by a finger will shunt the signal from the capacitor C4. If no signal arrives at the input of the NAND gate B, the 1 OOK pull up resistor R3 at the input of gate B will cause the output of this gate to go "low" and reverse bias the diode D3. The nanofarad capacitor C3 will discharge through the 1 OAK resistor R4 to earth, producing transient pulse across the capacitor C3 and thereby a negative going pulse at the input of gate C.This trigger pulse forces the output of gate C high and consequent rise in output through the 1 OK ohm resistor R6 switches on a transistor BOl 82L of which the collector emitter circuit is in series with the coil of the relay. The high at the output of gate C is also fed to the input of gate D; the consequential "low" output causes the discharge of the capacitor C7 through a 380K ohm resistor R7 and an adjustable resistor P1. A timing cycle commences and continues until the capacitor C7 charges to the lower threshold level of gate D to force the output thereof high. The output of gate 3 will then go low, to switch off the transistor and release the relay. The capacitor C6 isolates gate B from Gate C, ensuring that the touch button must be released and retouched before the timing cycle can be restarted. CLAIMS

1. A manually operable switching network disposed to provide an operating signal of predetermined duration after actuation, comprising a signal generator which provides a repetitive pulse signal to a trigger circuit via a capacitative voltage divider of which one part comprises a screened connection of which one end is available for approach by a digit of a human operator, the divider being arranged so that alteration of stray capacitance at the end of the connection by that approach is sufficient to impede the passage of the said signal to the trigger circuit, which thereupon provides said operating signal.

2. A hot-air hand drier including a motor driven fan 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 said end of the screened connection terminates in a touch button.