Oscillation circuit that generates an oscillating signal by charging and discharging a capacitor between two reference voltages employs several constant current sources, with the number of constant current sources that are connected to the capacitor at any one time being controlled so that the oscillation frequency can be changed in a step-wise fashion, whereby the circuitry is relatively simplified compared with known step-wise changing oscillators.

An adjustable frequency oscillator 2 is provided by a comparator timer 4 having a potentiometer 6 at its output 8 for concurrently changing both charging current and threshold trip voltage to control the frequency of oscillation of the comparator output. The oscillator is ideal for proximity switch applications, particularly photoelectric type proximity switches, for timing a delayed output signal following a given sensed condition.

A tunable electrical filter is disclosed wherein an RC (resistor-capacitor) network comprising a plurality of switchable capacitors among the components determining the time constant of the filter provides a given minimum value of filter time constant and, therefore, a given minimum bandwidth, when all the capacitors are continuously connected in the network. Each capacitor is coupled to the network through the output means of an electronic switching means. A pulse generator connected to the input means of said switching means provides a train of pulses having an adjustable ON-time to OFF-time ratio. When all the capacitors are periodically switched by the output of the pulse generator, the effective filter time constant increases from said given minimum value as the ON-time compared with the OFF-time decreases. The pulse generator is provided with control means for adjusting the ON-time to OFF-time ratio. Operation of the control means will, therefore, enable the filter to be tuned within a predetermined range.

A monolithically integrated microcomputer clocked at a processor clock rate includes a clock generator in the form of an RC oscillator being synchronizable by external signals for controlling at least one functional unit operating asynchronously with the processor clock rate. The RC oscillator has a frequency-determining resistor and a frequency-determining capacitor being monolithically integrated. The frequency-determining capacitor is formed of a plurality of switchable capacitors to be interconnected to make a total capacitor with a variable size. Registers are each connected to a respective one of the capacitors for defining a switching state of the switchable capacitors. A central processing unit is connected to the registers for adjusting the frequency of the clock generator by setting the registers.