Introduction: Randomly Reacting Pendulums

About: Tinkerer from childhood on. After my retirement, together with my wife, fully committed to creative production. I prefer simple solutions for non-existing problems.

The aim of this project is to cause a continuous swing of 2 pendulums. I discovered a nice interplay between an active and a passive pendulum. They move in a cloud of permanent-magnetic, electro-magnetic and gravity force fields. The weight of the pendulum is a magnet hanging horizontally from a needle. A sharp steel point has very low friction at the magnetic suspension point.
To count the runtime of the pendulum I use a 6 digit lcd module as day counter. When dark the counter adds one step. If the pendulum stops the counter resets. This gives me a true record of the ‘swing time’. A solar panel, a voltage regulator and a super capacitor provides energy for the 'everlasting' power supply.


  • Wooden baseboard 14 x 18cm
  • Strip aluminium 10 x 1 x 630mm
  • 3 neo magnets 10 x 10 round
  • Mattress needle 25cm 10inch
  • Electronic parts; see diagram
  • Trumeter 7000as 6 digit counter

Step 1: Video

Step 2: Construction

The pendulums are simply constructed. A wooden board, a bow of aluminium strip, a mattress needle, a piece of glass and 3 magnets. The bow is connected with messing screws to the board. The only iron part is a 10 inch mattress needle with a sharp point. Make this on length. Magnets are of the 10 x 10mm round type. The magnet weight is connected to the needle with a copper plate . Connect the glass plate with second glue below the top and put the magnet with double sided tape on top. Make four small stick-on feet to the bottom plate.

Step 3: Electric Circuit

As coil pulse driver I use my simple 2 transistor circuit. Variable resistor RV is set for a clean pulse. The LED lights up by the back EMF. NPN transistor 2N3904 is reversed connected; this works fine, try it! I have extended this circuit with a day counter. I use a low power Trumeter 7000AS as totaling counter with reset and up/down direction function. The count input C is connected with the solar panel and is negative edge triggered. By night the voltage falls below the 0.7 threshold and the counter will add one step. Reset at input R happens also on the negative edge.

In the active state the pulse circuit feeds a positive pulse (via C 100nF and the schottky diode) at C 470nF. Transistor T3 is in conduction and T4 is closed.

When the pendulum stops the basis of T3 becomes low and shall close this. After C 100uF is charged T4 is in conduction and this will reset the counter. The circuit uses as little as 30uA, inclusive day counter. The supercap will charge even in cloudy conditions and indoor light. The 3V regulator is an ultra low power SMD type.

Step 4: Conclusion

The duo pendulum project belongs to my investigation with micro- and nano powered moving devices. Before this I had to make many prototypes. It is important to make reliable electrical and mechanical connections. That sounds simple but is not. Double check is necessary. The active pendulum reacts jerky because of hidden magnets. There is no restpoint; the pendulum starts instantly. Watching a pair of pendulums 'dancing' is pure fun.