A common circuit in electronics is the square-wave, astable multivibrator (one-shot), which is useful for various purposes, such as timing circuits and audible alarms. The most common way to generate the desired square wave is to use the inexpensive 555 timer. The need sometimes arises for a square wave with fixed frequency but variable pulse width or vice versa. It's difficult to satisfy these requirements with a conventional 555-based astable circuit. Figure 1 shows a modification of the basic 555-based astable circuit. You can use the circuit to generate stable, variable-pulse-width or variable-frequency signals, which are independent of each other by means of individual dedicated controls. The Pin 3 output of the 555 charges and discharges C1. D1 and D2 provide individual paths for the charging and discharging operations, respectively. The two timing potentiometers, P1 and P2, control the RC1 time constant during the charging and discharging cycles.
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| Figure 1. | You can independently and noninteractively control pulse width and frequency by adjusting two potentiometers. |
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When Pin 3 of the 555 is high, the capacitor charges through R2 (a component of P1, whose value depends on the wiper position). When C1 charges to two-thirds VCC, Pin 3 goes low, and C1 discharges through the combination of D2, P2 (resistance R1), and P1 (resistance R3). When the voltage across C1 reaches one-third VCC, the Pin 3 output again switches high. The process of alternately charging and discharging C1 continues; the result is an output with a desired pulse width and frequency. Because the forward resistance of the diodes is negligible, the pulse width equates to
The pulse period (reciprocal of frequency) is
Thus, the pulse width is independent of P2's wiper position, and the frequency is independent of P1's wiper position.
