Perpetual Swinging Pendulum


Video Demo in WMV format.

Original document and schmatic can be viewed here

This project originally appeared in "Nuts and Volts" magazine August 2012
by David Williams. It was titled "Build The Mystery Solar Powered Pendulum"
and can be viewed at the link above. It has been adapted here to operate
from a couple AA cells. The circuit board, magnetic coil, and battery are
contained in a wooden cigar box, with balsa wood supports for the pendulum.
A neodymium magnet is placed at the base of the pendulum and held in place
by the attraction of a small flathead screw at the bottom of the pendulum.
The magnet can be removed by just pulling it off. The magnet measures 1 inch
diameter by 1/8 thick and has a clearance of 1/8 inch from the top of the box.
Smaller neodymium magnets can be used. The coil measures about 1.5 inch diameter
by 5/8 wide and was wound with 150 feet of #33 wire (approximately 900 turns)
and is located inside the box in the center of the top. The coil measures about
3.5 millihenries with 30 ohms of resistance. The height of the balsa wood
supports is about 10 inches spaced 8 inches apart which yields a pendulum
period of about 1 second. The video shows the pendulum starting up and gaining
amplitude. It should start from a stand-still, or you can give it a little
nudge to getting it going. Two modifications were made to allow the circuit
to operate on a 3 volt battery. R2 was lowered to 56K and a 330 ohm resistor
was added in series with the battery to simulate the higher impedance of
the original solar cell.

----------- An alternate circuit for the Swinging Pendulum --------------


Above is another approach to the Swinging Pendulum circuit using a dual op-amp and a few more parts. It operates on three 'C' cells (4.5 volts) at about 2 milliamps for a running time of about 5 months. The coil was wound with about 3000 turns of #34 copper wire for a total resistance of around 150 ohms. The pulse width generated as the magnet passes over the coil is about 25mS. The pendulum period is about 1 second so the duty cycle is about 5% The coil current is 4.5/150 or maybe 30 milliamps so the average coil current would be 30 * .05 = 1.5 milliamps. The LM358 draws about 1/2 millamp so the total circuit current is about 2 milliamps. The alkaline C cell batteries are rated at 7000 maH, so the running time would be 3500 hours or maybe 145 days, but I haven't measured it yet. In operation, as the magnet swings past the coil (L1), a single cycle (20 Hz) sine wave of about 600mV peak is generated at the collector of the transistor (2N3904). The op-amp responds to the negative half cycle and causes the output at pin 1 to move positive about 1 volt above the 2.2 volt reference at pin 6. This causes the output at pin 7 to move positive and turn on the transistor and supply a short (23mS) pulse to the coil and give the magnet a slight push to keep it going. The pulse duration is about R*C, or 470K * 0.05uF = 23 milliseconds. Notes: The magnet used is a neodymium type measuring 1/2 inch diameter by 1/8 thick. Larger magnets could be used. The gain of the first op-amp stage (pins 1,2,3) is the ratio of the feedback resistor (470K) to the capacitive reactance of the 0.05uF capacitor at 20 Hz or about 470/160 = 2.9. Other values of R and C could be used, but the ratio should be the same to maintain the same pulse width. The second op-amp (pins 5,6,7) is used as a comparator where the output switches high when the input at pin 5 is more positive than the (2.3) reference at pin 6. The reference voltage at pin 3 of the first op-amp is worked out from the supply voltage (4.5) divided by the total divider resistance (56K + 20K + 39K) times the 39K resistor. This would be (4.5/ 115K) * 39K = 1.526 volts. The reference voltage at pin 6 would be the same divider current (4.5/115K) = 39 microamps times the sum of the 2 resistors (39K ,20K) or about .000039* 59000 = 2.3 volts. This gives us a fairly good margin where the static state is 1.5 volts, and the reference to the next stage is 2.3, so the output at pin 1 must move about 800 millivolts to switch the second stage (pin 7) and turn on the transistor. Since the gain of the first stage is about 3, the minimum input voltage from the coil is about 800/3 = 267 millivolts. This is about the amplitude you can get by starting the pendulum about 3 inches or more from the coil. As it gets going, the pendulum will swing about 11 inches from the coil. A capacitor (.05uF) is used between pins 5 and 7 of the second op-amp to supress oscillations and provide positive feedback as the output switches on. The picture below illustrates the pulse waveform at pin 1 in red compared to the signal at pin 5 in green. The 2 waveforms are offset slightly for illustration, but are actually the same voltage level. Notice the oscillations at pin 1 are filtered out and do not appear at pin 5 of the second op-amp.

The picture below illustrates the RC waveform at pin 2 (green) compared to the output signal at pin 7 (red). The output at the collector of the transistor is the inversion of pin 7 and so provides a ground to the coil for about 25 milliseconds which pushes the magnet away from the coil for about 25mS and keeps the pendulum swinging.

Menu