Nuts and Volts - February 2012

To complete the eight light nonsense circuit, you could duplicate Figure 1 or use slightly different resistor values such as: 1.1 meg, 910K, 750K, and 510K. Having the resistor values close in value gives the illusion of randomness. If the frequencies are very different, the pattern is easily observed.

■ FIGURE 2.

RANDOM NUMBER GENERATOR

QI need a source of noise to make a random number generator. Articles on the Internet indicate two sources of evenly distributed static are resistor noise and reversing a PN junction A design using the latter involves two 2N3904 transistors with their bases tied together. One is reversed, the other acts as amplification. It worked on a breadboard, but on a PCB I got no more noise out than went in. I think the breadboard injects a little EMI in the bases. Can you suggest a circuit? Preferable 5V or 3.3V in and static suitable for an MCU to read with its ADC.

because the A/D
will average
noise that
exceeds its
bandwidth which
will limit the
range of
numbers. Have
you considered a
pseudo random
number generator? A real time clock
can be used to seed the generator
and it will generate a different
sequence of numbers each time it
runs.

CODE PRACTICE OSCILLATOR

QAre there any variable capacitors that will work in a code practice oscillator? If so, who has them and what is the cost? The variable capacitor must work with common resistors in an audio oscillator; for example, 47K ohms that will change the pitch of the tone. Hope you can help me.

— Victor Davis — Geoff Clark

AYears ago (1970s), I built a noise generator using a reversed biased base-emitter to check the sensitivity of CB radios. If I could hear the noise of the noise generator, the radio had sufficient sensitivity. That told me that the signal level at the source is in the microvolt range. The noise generator had an attenuation of 40 dB, so the noise at the transistor must have been hundreds of microvolts. You will need a little less than 80 dB of gain to get up to volts.

One way to get white noise is to use a sensitive radio receiver connected to the noise generator, and turn up the gain. You will want to band limit the noise

AMy first thought when I saw the circuit (Figure 2) was: I don’t know how this works. I simulated the circuit in LT Spice and, lo and behold, it is a sine wave oscillator.