IES65481B2 - A selector circuit - Google Patents

Abstract

A selector circuit comprises a potentionmeter having a resistor R1 with two end terminals T1 and T2 and a movable contact V1 which engages the resistor intermediate its length to provide a variable resistance between the movable contact V1 and one end terminal T1. The movable contact forms part of an RC circuit with a capacitor C1. To determine the position of the movable contact V1 a microprocessor uP applies a fixed voltage to the movable contact V1 and determines the time taken for the voltage on the capacitor C1 to reach a pre-determined value.

Description

A SELECTOR CIRCUIT The present invention relates to a selector circuit.

According to the present invention a selector circuit includes a potentiometer comprising a resistor having two end terminals and a movable contact which engages the resistor intermediate the end terminals, the circuit further includes means for determining the position of the movable contact by measuring a quantity which varies according to the value of the variable resistance between the movable contact and one end terminal.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing which is a schematic view of a selector circuit.

Referring now to the drawing, a selector circuit 10 includes a potentiometer comprising a resistor R1 with two end terminals ΤΙ, T2 and a movable contact S connected to a further terminal VI. The terminal VI is connected to an output port O/Pl of a microprocessor 11, the terminal T2 is connected to a second output port O/P2 of the microprocessor 11, and the terminal T1 is connected to the positive terminal 12 of an electrolytic capacitor Cl via a resistor R2. The positive terminal 12 is further connected to a Schmidt trigger input I/Pl of the microprocessor 11 and the negative terminal 13 of the capacitor Cl is connected to ground. By way of example, a PIC16C54 would be a suitable microprocessor for use in the circuit 10. A 25kohm potentiometer, lOkohm resistor and a lOnF capacitor are suitable for use as Rl, R2 and Cl respectively.

The rate at which the capacitor Cl charges is determined by the time constant of the circuit 10 which in turn is determined by the magnitude of ’ capacitance and resistance within the circuit 10. The value of the resistance between the terminals VI and T1 is variable and is determined by (the selected potentiometer setting, i.e. the position of the movable 365481 -2contact S relative to the resistor R1. Thus, the value of the time constant of the circuit 10 is dependent on the potentiometer setting.

To calibrate the selector circuit, the output port O/Pl is switched to high impedance and the microprocessor applies a fixed voltage to the terminal T2 via the output port O/P2. The microprocessor 11 monitors the time constant of the circuit 10 in this configuration by recording the time DO taken for the voltage level at terminal 12 to rise to a pre-determined threshold value and trigger I/Pl. The time DO is determined by the equation:DO=C1[R1+R2] eq.l Having calibrated the circuit, a measurement of the position of the movable contact S is made by the microprocessor switching the output port O/P2 to high impedance and applying the same fixed voltage at terminal VI via the output port O/Pl. Again, the microprocessor records the time DI taken for the voltage at terminal 12 to rise to the threshold value and trigger I/Pl. The time DI is determined by the equation:D1=C1(RV1+R2) eq.2 where RV1 is the resistance between terminals VI and Tl.

Now the microprocessor combines the equations eq.l and eq.2 to calculate the proportion of the resistance R1 between the terminals VI and Tl and hence the position of the movable contact S.

It is well known for a potentiometer setting to be varied using a rotary dial (not shown) connected to the moving contact S of the potentiometer. The circuit above can be used to determine the angular position of the dial as a function of the resistance between the terminals VI and Tl. For example, the total range of angular movement of the dial can be divided into four contiguous ranges, and the dial can be designed with four detent positions each located approximately at the middle of a respective range. The microprocessor 11 can then determine into which of the four detent -3positions the dial has been set by making a relative comparison between DO and DI. This provides the equivalent of a l-of-4 selector switch. > The use of a potentiometer rather than a conventional selector switch or I using an analogue-to-digital convertor offers significant cost and size savings in a simple selector circuit. Costs are reduced further by the use of a relative comparison technique which eliminates the need for a potentiometer with high quality and tolerance ratings.

Claims (5)

1. A selector circuit including a potentiometer comprising a resistor having two end terminals and a movable contact which engages the resistor 5 intermediate the end terminals, the circuit further includes means for determining the position of the movable contact by measuring a quantity which varies according to the value of the variable resistance between the movable contact and one end terminal. 10

2. A selector circuit as claimed in claim 1, wherein the portion of the resistor between the movable contact and the one end terminal forms part of an RC circuit with a capacitor and the determining means measures the time for the capacitor to charge to a pre-determined voltage after a fixed 15 voltage is applied to the movable contact.

3. A selector circuit as claimed in claim 2, wherein the determining means includes a microprocessor which applies the fixed voltage to the movable contact and monitors the subsequent rise in voltage on the capacitor.

4. A selector circuit as claimed in claim 3, wherein in order to calibrate the circuit the microprocessor applies the same fixed voltage to the one end terminal and monitors the subsequent rise in voltage on the capacitor. 25

5. A selector circuit substantially as described with reference to the accompanying drawing.