Nuts and Volts - June 2014

The Pump Oscillator

The pump oscillator schematic in Figure 6 employs the CMOS ICM7555 timer chip. The circuit is configured to provide a square wave frequency given as:

R should be on the order of 2. 4 megohms for a pump frequency of about 3 Hz, and C fixed at 0.1 µFd. R is made up of a standard 2. 2 megohm resistor in series with a 500K ohm potentiometer to trim the frequency. The circuit is simple, so the pump oscillator was constructed on perfboard with point-to-point wiring and mounted in the lower half of the case where the battery compartment is located. Leave long leads to connect to the relay actuator to allow access to the oscillator trim pot for adjustment of the correct frequency.

Frequency Adjustment

The bandwidth, Dƒ, at the half power point or sharpness of resonance is defined as: The Q factor can be measured by releasing the pendulum at a given angle and recording the time, t, for the angle to decay to about one-third its initial value. The Q factor is then calculated from the relationship: The factor Q of the pendulum is on the order of 150, so the pump oscillator frequency is quite critical and must be set at exactly twice the pendulum frequency. The pendulum period can be easily obtained with a stopwatch by measuring, say, 10 full cycles and dividing the elapsed time by 10 to obtain one period. Alternately, an optical tachometer could be used to measure the period of the pendulum, and a frequency counter to set the pump oscillator frequency.

I have used a small silicon photovoltaic cell taped on the table directly below the pendulum and connected to a sensitive oscilloscope. The photocell is illuminated from above the pendulum with a bright flashlight, so a voltage pulse from the pendulum shadow is cast every time the pendulum swings over the photocell. There will be two shadow pulses generated for every swing of the pendulum because the pendulum has two zero crossings per cycle of a sine wave. The pump oscillator trimpot R2 is then set, so the complete time of one cycle of the pump oscillator is equal to the time between any two shadow times of the pendulum.

This completes the construction of the parametric amplifier model. It’s called an amplifier rather than an oscillator because the pendulum must first be excited with a signal (such as a large push) to start it swinging. A large perturbation will allow the pendulum to continue to oscillate long enough for the phase to slip into sync and execute a swing of about ± 2”.