Electronics Puzzler Solution: Bulls Eye

By Forrest M. Mims III

What's the missing component?

John's solution to the problem is a piezoelectric disk. This solution occurred to John when the piezoelectric buzzer in his watch announced the meeting. By the time he walked to the meeting room and opened the door, the idea gelled well enough to announce the solution to his boss.

How It Works

Figure 2 shows a piezoelectric disk connected directly to the leads of a red LED. When the disk is struck by a small rubber ball, it generates a pulse of voltage high enough to exceed the forward voltage of the LED. The LED then flashes. Polarity is unimportant. Test the circuit by tapping the piezoelectric disk with the end of a pencil, or place the disk on a hard surface and drop a light weight of 1 ounce or less onto it.

A practical implementation of this circuit will require tradeoffs when selecting a ball or object with sufficient mass for the piezoelectric disk to generate a voltage spike without damaging the disk.
Piezoelectric Disk
Figure 2. The mystery component in Figure 1 is a piezoelectric disk.


After the circuit was built and tested, a literature search revealed several U.S. patents use one or more piezoelectronic sensors to detect an impact. Those that were checked suggested the use of additional circuitry. The circuit in Fig. 2 requires only two components and no power supply.

Going Further

The voltage spike from a piezoelectric disk will easily drive a string of LEDs connected in series, Start with two and see what happens when more are added.

The piezoelectric disk generates a dual polarity spike. You can demonstrate this by connecting two LEDs across the disk, one in the forward direction and one in the reverse direction. Depending on the disk, both LEDs should flash when the disk is struck.

Additional circuitry can be added to cause the LED to stay on longer than for a very brief flash. It may be possible to devise an electromechanical device with a movable armature inside a coil that will also flash an LED. But this is more complex than a piezoelectric disk.

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1. Forrest M. Mims III, Light Emitting Diodes, Howard W. Sams & Co., 1973, pp. 118-119.

2. Ibid., Sun Photometer with Light-Emitting Diodes as Spectrally Selective Detectors, Applied Optics , 31, 33, 6965-6967, 1992.

3. Ibid., An inexpensive and stable LED Sun photometer for measuring the water vapor column over South Texas from 1990 to 2001, Geophysical Research Letters 29, 20-1 to 20-4, 2002.

4. Ibid., Five years of photosynthetic radiation measurements using a new kind of LED sensor, Photochemistry and Photobiology 77, 30-33, 2003.

5. Ibid., LED Sun Photometry, Optics & Photonics News 20 , 32-38 (2009).