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    Preset potentiometer

    This is a component that can be used as a potential divider.  Inside, it consists of a circular resistance track with a rotating contact that slides along the track.  One resistor connects between terminals A and B, the other between terminals B and C.  Terminal B forms the junction between the two series resistors.

    Using a potentiometer as a potential divider instead of two fixed resistors allows the output voltage Vs to be adjusted continuously by rotating the sliding contact.  This alters both resistors in the potential divider at the same time (as the value of one increases the other decreases).

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    PCB Design

    In the complex and difficult task of designing a printed circuit board, it is very easy to become so preoccupied with the electronics that you forget to think about what the pcb will look like and how it will fit your intended enclosure.  Problems may only become evident when the pcb is assembled.  The following are examples of such errors.  Look carefully at these to try and avoid making the same mistakes.

    JK Connector height

    1 These vertical mounting JYK connectors look small on a pcb overlay which, being 2 dimensional, do not show their height.  A horizontal mounting connector is much bigger on the overlay but only the height of an IC in its socket.  These tall connectors are a problem in a case where push button actuators need to operate the push button.

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    Battery Internal Resistance

    Equivalent circuit for a battery

    The terminal voltage of the ideal power supply is called the electromotive force (emf) in recognition that this drives current around the circuit – it gives electrical energy to the moving charge.  It is measured in volts using a voltmeter.  (This is in contrast to the potential difference across components – also measured in volts – that is a measure of how much electrical energy is converted by the component.  Or how much electrical energy has been given up by the moving charge as it passes through the component.)  The diagram above shows the equivalent circuit for a 6V battery connected to a motor.

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    The phrase op-amp is short for operational amplifier.  The term derives from its use in analogue computers where it performs mathematical operations (calculations).  It can be used for addition, multiplication and integration.  It is also possible to use them for subtraction, division and differentiation.

    The symbol for an op-amp is the amplifier (triangle) symbol with two inputs.  Confusingly, the inputs are labelled with a + and – symbol.  These do not refer to the power supply but to the way the input is mapped to the output.  (The inverting input inverts the signal, the non-inverting input does not!)

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