Nuts and Volts - February 2018

Stepped Tone Generator — took me a while to understand what was happening.

As it turns out, that although the graph is correct, the text is incorrect — the frequency rises and the step size (DF) increases as R3 is reduced.

To duplicate Mr. Mims results, I used a 50K pot for R3 to get better resolution. I set R3 so that the pulse width on pin 9 was about 200 µs and adjusted R1 such that the input and output frequencies were the same at 2 kHz.

I then rotated R3 through each of the step frequency changes and measured its resistance.

The results are shown in Graph 2 which closely matches Mr. Mims results. As you can see, the output frequencies are sub multiples of the 2,000 Hz fundamental.

Section 1 of the 556 is wired as an astable multivibrator with a frequency range of about 282 Hz to 12. 8 kHz. Section 2 is a monostable triggered by Section 1, with an output pulse width range of about 16 µs to 5 ms.

The operation of Section 2 is that of a frequency divider (see Part 2 of this series for a detailed explanation).

For any single setting of R1 (period), as R3 is changed, the output frequency will be constant until a pulse width threshold is reached (N*period), at which time its output frequency will jump to its next value.

Changing the period of Section 1 will change the output frequencies of Section 2 at which it jumps. You should also note that even though the monostable frequency does not change until the threshold is reached, the tone sound will change because the duty cycle of the pulse is changing. Using two PIC 555 replacement ICs will operate exactly in the same way if you set one to mode 4 or 5 (astable) and the other to mode 0 (monostable). Also, the program of Mims Circuit 21 (discussed later in this article) will yield similar results if all you want is an output that steps among several frequencies.