Nuts & Volts - January 2005

Q&A

Many of you had problems with that circuit in that there was too much variation between the NE-2 lamps, making it unusable or impossible to calibrate the counter.

Well, I have done a little research since then and think I have found a better design that uses — of all things — a peanut can. The “Cheap, but Sensitive Radiation Detector” can be found at www.techlib.com/science / ion.html The design is by Charles Wenzel, whose designs I trust. Figure 9 shows the basic concept. His commentary is self-explanatory, so I won’t go into it here.

The second missing part of the equation was a radiation source. I found that on eBay under the guise of a Coleman lantern mantle. One mantel produces 1.5 mr/hr (about 750 counts per minute) and sells for about $5.00. You can also find them from time to time at K-Mart and camping supply houses. Look for the brand that says “Made in India.”

The radioactive ingredient in the mantle is thorium-232. By itself, thorium is only slightly radioactive, emitting only alpha particles that are easily blocked by a thin piece of paper.

However, thorium breaks down into two parts: a small part — the alpha radiation — and a larger part called the decay product. The decay products include radium and radon, which emit alpha and beta particles and gamma rays.

In fact, the longer thorium stands (half-life is 14 billion years), the more radioactive it gets — but that’s another story.

Frequency MHz

Number of Turns

Tap at Turn No.

Wire AWG

Coil Diameter

Winding Length

1. 8-3. 8

82

12

#26

1 1/4

1 9/16

3. 6-7. 3

29

5

#26

1 1/4

9/16

7. 3-14. 4

18

3

#22

1

3/4

14. 4-32

7

2

#22

1

1/2

29-64

3 1/2

3/4

#18

1

3/4

Table 1. Dip meter coil data.

Figure 9

A. Controlling the speed of an AC motor is fairly simple. It’s done all the time in hand drills, but what you’re trying to control is a flow of water, which has a lot of latency — resistance to change in flow. No matter how hard you try, the fountainhead won’t follow the fast-moving audio. The controller needs a low frequency filter to turn sporadic into rhythmic. Here’s what I’d do (Figure 10).

In this circuit, I’ve used the control voltage input (pin 5) of a 555 timer to PWM (pulse width modulate) the speed of the motor. With this design, the motor is never completely off — for good reason. Starting a water flow from a stall takes a lot longer than revving it up from a trickle, making your fountain slightly more responsive. It also takes care of the low frequency filter I mentioned earlier by using the inertia of the

Figure 10

+12V

115VAC Hot

Three Coins in a Fountain

Q. I want to hook up one or more submersible fountain pumps (typically 115 VAC, 0.8 A) to a circuit that will vary the flow of water in concert with the input from a sound source. Should the circuit change the voltage or current?