I am building an electronic pendulum. I want to use a 2N2222 to power the drive coil when the pendulum bob interrupts an IR beam. The pendulum bob is magnetic and moves in reaction to the drive coil. I am trying to modify this circuit as follows

The 100 ohm, the 1K ohm, and the red led will be removed and an air core coil will be substituted.  The coil consists of a plastic sewing machine bobbin fully wound with 30 gauge magnetic wire. Resistance 10 ohms, inductance unknown. The coil is bridged using an led to suppress the back EMF caused by the collapsing field. 

The circuit as shown works BACKWARDS. The 2N2222 turns on when the photodiode is illuminated and turns off when the photodiode is not illuminaed. 

For simplicity the source of the ir beam is not shown. 

How can I change the circuit to make the 2N2222 turn on when the ir beam is interrupted and off when the off when the beam is not interrupted.

enter image description here

  • \$\begingroup\$ It should be possible to detect the magnet approach and zero crossing, to trigger a repulsive pulse to the magnets polarity. As used in a desktop "perpetual motion kinetic toy". That pulse signal can trigger your counter. \$\endgroup\$ – Optionparty Dec 28 '14 at 0:33
  • \$\begingroup\$ You could put a mask between the PD and light source and have the drive turn on when ever you want during a cycle. You need some sort of gate/ trigger so you get it in just one direction. (A second mask and PD...) \$\endgroup\$ – George Herold Dec 28 '14 at 4:10
  • \$\begingroup\$ For better timing stability keep the pendulum "Q" as high as practical. For thermal stability a "Quartz" rod would make for a lower price than "Invar". \$\endgroup\$ – Optionparty Dec 29 '14 at 14:42

Different technology - superb result.

This does not use the technology you asked about but performs your required task done superbly. As noted in the on-site text, the pulse can be provided as the magnet approaches or leaves the inductor by changing polarity. Properly designed such a circuit can use "just enough" energy to keep something in motion for very long periods. The circuit is shown here operating from 9V but should work easily enough on 2 AA or AAA torch cells (2V to 3V+ range) or from 1 cell (1V - 1.5V+ range) with more care.

This "magnetic kicker
on the utterly utterly utterly superb http://www.sparkbangbuzz.com website

will 'kick' a pendulum or other device my detecting a moving magnet and, with due amounts of fiddling and fun, producing a pulse to repel it at 'just the right time".

enter image description here

Warning :-) !!!:

is a "Black Hole" that contains more mental filter opening and mind blowing ideas that any one website has a right to have in one place. Exposing a suitably impressionable young person to this website may transform them into an engineer for life. (Adult oversight recommended - some of the items described could kill - but an adult could get highly enthused as well).

  • \$\begingroup\$ Exactly what I had in mind. Didn't know what it was called. Thanks for posting the web site link. \$\endgroup\$ – Optionparty Dec 29 '14 at 14:24

Something like this should work


simulate this circuit – Schematic created using CircuitLab

In this case, the resistor needs to be large enough that the voltage across the diode must be less than ~ 0.5 volts without the transistor connected, and the pendulum needs to block the light to the diode pretty completely. So when the diode is illuminated it keeps the base voltage below turn-on, and when blocked it allows the resistor to drive the transistor on. Depending on how sharp the edge of the pendulum's shadow is, and how big the coil, you may not need D2, but it's probably a good idea on general principles.

Please note that this circuit, while about as simple as it gets, is not suitable for a battery. Since the resistor/diode draws current when the pendulum is off-center, and that's most of the time, it will drain any battery fairly quickly. If you must have a battery-powered circuit, that will take a bit more doing.


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