The Clark zapper has been used by thousands of people with good results often being reported. There have been two common complaints with the basic Clark zapper design. First, it uses a frequency that may not have as good penetration as some other frequencies. As Dr. Clark points out, it does not penetrate the gut very well. Second, the voltage is quite low, often around 2 volts under load. This design uses 727 Hz and a higher output voltage (lower resistance in the output).
There is a third problem that Dr. Gary Gear noticed. The zapper does not produce a square wave under load. The wave looks square on a scope until a person is attached. Then the corners go very round. Dr. Gear solved the problem by further reducing the output voltage to less than 2 volts. This helped keep the wave more square. Dr. Bill Biagioli came up with a better solution. Put a 10uf tantalum capacitor in parallel with the output resistor at pin 3. The positive side goes toward pin 3. This keeps the wave very square under load and the difference can easily be felt.
There has also been discussion of duty cycle in zappers and other frequency devices and reports that longer duty cycles or pulse lengths have produced better results than a 50% duty cycle. With that in mind, a 75% duty cycle has been chosen for this zapper design.
Be sure to use a socket for the 555.
I have also made several zappers using 2128 Hz. For these I used two 9 volt batteries in series with 2 diodes in series to drop the voltage slightly to avoid damaging the 555. Here is a diagram for that one. It is also 75% duty cycle.
WARNING: I recently got a call from a lady who used a 2000 Hz 10.5 volt zapper with nearly 100% duty cycle made by a company in Canada. She told me that the result of using the device was neurological damage that has not cleared up. I doubted that the zapper itself could cause damage and asked her about the details. She had used the unit 40 minutes a day from the first day. The result, she says, has been fairly serious problems that have lasted for a couple years. It appears that she had killed something like candida or Lyme spirochetes so fast that toxins had caused damage. These are research devices only. They are not approved for use on humans. If they are used, they should be used with caution, starting out with 1 minute per day and slowly increasing.