IES65482B2 - Signal amplitude control circuit - Google Patents

Abstract

An electrical or electronic appliance 10 has an amplifier 14 which provides a signal whose amplitude determines the level of an output from the appliance, for example, the volume of a radio or the voltage or current applied to patient electrodes of a medical appliance. The amplitude of the signal from the amplifier 14 is determined by the setting of one of a pair of ganged sliding contacts 13 on a respective resistor Rv2 of a slide tandem potentiometer 12, the other sliding contact 13 engaging the other resistor Rv1. To ensure that the amplifier signal amplitude is not set too high when the appliance is switched on, the setting of the other sliding contact 13 on the resistor Rv1 is monitored by a microprocessor uP via a transistor Q1, and if the amplifier signal exceeds a predetermined threshold the microprocessor holds the amplifier 14 disabled until the potentiometer is re-set to bring the signal amplitude below the threshold.

Description

SIGNAL AMPLITUDE CONTROL CIRCUIT , The present invention relates to a signal amplitude control circuit for an t electrical or electronic appliance.

It is a well known problem with electrical and electronic appliances that circuit settings are often not checked before applying power to the appliance. A common way of altering circuit settings is to use a potentiometer connected to a manual control element such as a dial or slide to set the amplitude of a signal. This signal may be a volume setting in a radio or a treatment setting in a medical appliance. The use of a previous high setting, or a high setting to which the manual control element was turned while the circuit Was off, can result in danger or annoyance when power to the appliance is subsequently switched on.

It is an object of the present invention to mitigate this problem.

According to the present invention there is provided a circuit for controlling the amplitude of a signal which is set by a manual control element in an electrical or electronic appliance, the circuit comprising means operative upon powering of the appliance to determine if the setting of the manual control element would result in a signal amplitude which exceeds a predetermined threshold value and if so to hold disabled a part of the appliance circuitry which provides the signal until the latter is re-set to bring the signal amplitude below the threshold value.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing which is a schematic diagram of an electrical or electronic appliance including a signal amplitude control circuit. ΐ Referring now to the drawing, an electrical or electromc appliance 10, for example a radio or a medical treatment appliance, includes a plurality of amplifier circuits 14. Each amplifier circuit 14 provides a respective signal whose amplitude determines the level of an output from the appliance, for -2example, the volume in the case of a radio or the voltage or current applied to patient electrodes in the case of a medical appliance. Although two amplifiers 14 are shown in the embodiment there may be only one or there may be three or more, depending upon the nature of the appliance 10.

Each amplifier circuit 14 has a respective control circuit 11 which comprises a manually adjustable slide tandem potentiometer 12. An example of a commercially available slide tandem potentiometer is marketed by Radio Spares under stock number 136-137. The slide tandem potentiometer includes two separate electrical contacts or taps 13 which are ganged together to slide in tandem along respective resistors Rvl and Rv2. Moving the slide contacts 13 along the resistors Rvl and Rv2 causes the resistance between each respective contact 13 and a corresponding terminal 15 at the end of the respective resistor Rvl and Rv2 to vary proportionally.

The terminal 15 on each resistor Rv2 and its associated sliding contact 13 are connected to the respective amplifier circuit 14 so that by adjusting the position of the contact 13 along the resistor Rv2 the amplitude of the signal provided by the amplifier circuit 14 can be adjusted, for example to set the radio volume or the medical treatment level.

In each amplifier circuit 14 the sliding contact 13 associated with the resistor Rvl is connected to the base of a transistor Q1 through a resistor R2 while the terminal 15 of Rvl is connected to ground. The sliding contact 13 is also connected through a resistor R1 to a D.C. power supply line Vcc. The resistor R1 and the variable resistance of the portion of the resistor Rvl between the sliding contact and ground form a potential divider which biasses the base of the transistor Q1 so that if the resistance between the terminal 15 and the sliding contact 13 exceeds a certain value the transistor Q1 will turn on by the voltage drop across the resistor Rvl between the sliding contact 13 and ground. Clearly, this value is related to a predetermined threshold value of the amplitude of the signal provided by the amplifier circuit 14.

The collectors of the transistors Q1 are tied together and connected to Vcc through a resistor R3 in a wired-or configuration, and they are also -3connected to an input pin I/P on a microprocessor uP. Further, the microprocessor uP has a group of output pins O/Pl.. .O/Pn which are » respectively connected to the amplifier circuits 14. The pins O/Pl...O/Pn are driven by the microprocessor to enable or disable the respective 4 amplifier circuit 14 depending upon the binary signal level at the output O/Pl.. .O/Pn. In the present case it is assumed that if an output O/Pl...O/Pn is high the respective amplifier circuit 14 is enabled.

When power is applied to the appliance 10 (i.e. when the appliance is switched on), the microprocessor uP initially holds the amplifier circuits 14 disabled while it checks the state of the input pin I/P. This I/P pin will be pulled low if any transistor Q1 turns on when the appliance is switched on, indicating that the amplitude of the signal which would be provided by the amplifier circuit 14 if it were enabled would be above the aforesaid predetermined threshold value. In such a case, the microprocessor signals in any suitable manner that the potentiometer setting(s) should be lowered and holds the amplifier circuits disabled until this is done.

When the settings are lowered so that all the transistors Q1 are turned off, as determined by the input pin I/P going high, the microprocessor enables the amplifier circuits 14. Thereafter the microprocessor relinquishes control of the amplifier circuits 14 so that the signal amplitudes provided thereby are then free to be adjusted as desired, including above the predetermined threshold value.

Naturally, if the initial check of the state of the input pin I/P determines that none of the amplifier circuits 14 would provide a signal whose amplitude would be above the aforesaid predetermined threshold value (input pin I/P high) the amplifier circuits are immediately enabled and the microprocessor relinquishes control as before.

Claims (5)

1. A circuit for controlling the amplitude of a signal which is set by a manual control element in an electrical or electronic appliance, the circuit comprising means operative upon powering of the appliance to determine if the setting of the manual control element would result in a signal amplitude which exceeds a predetermined threshold value and if so to hold disabled a part of the appliance circuitry which provides the signal until the latter is re-set to bring the signal amplitude below the threshold value.

2. A circuit as claimed in claim 1, wherein the manual control element is a potentiometer and the part of the appliance circuitry is an amplifier circuit controlled by the potentiometer.

3. A circuit as claimed in claim 2, wherein the potentiometer is a tandem potentiometer and wherein the resistance value selected by the tap on one resistor of the potentiometer controls the amplifier and the resistance value selected by the tap on the other resistor of the potentiometer is used to determine if the signal from the amplifier would exceed the said threshold value.

4. A circuit as claimed in claim 3, wherein the resistance value selected by the tap on the other resistor forms one side of a potential divider such that the voltage at the tap depends upon the resistance value selected by die tap, and wherein the tap is connected to the control input of an electronic switching device which changes state when the voltage at the tap exceeds a predetermined value.

5. A circuit as claimed in any preceding claim, wherein the means to determine if the amplitude of the signal set by the manual control element would exceed the threshold value comprises a microprocessor which also disables the said part of the appliance circuitry.