Nuts and Volts - February 2018

monostable pulse width.

Mims Circuit 19

Circuit 19 is titled as a Three-State Tone Generator; refer to Schematic 10. Both sections of the 556 are wired as astable multivibrators. The components of Section 1 cause it to operate at a frequency of about 2.1 Hz with close to a 50% duty cycle, while those of Section 2 cause it to operate at about 960 Hz with a 66% duty cycle.

With the switch in position 1, the output of Section 1 is used as a gate for Section 2. Section 2 will output its 960 Hz signal only while the output of Section 1 is high; about 230 ms. With the switch in its center position, the output of Section 2 will be a continuous signal. When in position 3, the output of Section 1 will affect the charge and discharge of C2, so that Section 2 will output two different frequencies. The difference between the frequencies is affected by the value of R3. The smaller the value, the larger the effect.

You can do the same thing using two of the PIC 555 replacements. However, program NCO_ 3_State ToneGen.asm (also with the article downloads) uses a single PIC16F18313 to emulate the same circuit with somewhat more flexibility; see Schematic 11.

Pins 5, 6, and 7 are an astable with separate on-time and off-time controls, and a frequency range of about 0.1 Hz to 100 Hz. To get this low frequency range, I used one of the timers to generate a 200 Hz interrupt. In the Interrupt Service Routine (ISR), I count the number of interrupts in order to determine when to turn the pulse on and off. Initially, I wanted to use one of the PWM systems but was unable to get the low frequencies I wanted and still maintain a higher CPU clock frequency.

Pins 1, 2, and 3 are an astable which uses the NCO system to generate a square wave with a frequency range of about 7. 5 Hz to 8

kHz. The source code in the downloads is well documented and should be easy to modify if you have a need to change any of the operating characteristics. The switch and its three positions duplicate the operation of SCHEMATIC 10. 556 three-state tone generator.