555 makes handy voltage-to-time converter

J Jayapandian, IGCAR, Tamil Nadu, India -- EDN, 2/1/2001

The circuit in Figure 1 is a simple, low-cost voltage-to-time converter using the ubiquitous 555 timer chip. You can use the IC's monostable multivibrator as a voltage-to-time converter by connecting the analog-voltage input to the charging resistor, R, instead of connecting R to V_CC. With this modification, the timer chip's output-timing cycle, t_P, is proportional to the input voltage, V_IN. When you apply an input voltage, the voltage across capacitor C charges exponentially according to the formula V_C=V_IN(1–e^t/RC), where RC is the time constant of the circuit, with C in farads and R in ohms. During one time constant, the voltage across the capacitor changes by approximately 63% of V_IN. The output timing of the monostable multivibrator is t_P=1.1 RC. By keeping RC constant with fixed R and C values and varying the input voltage, V_IN, you obtain variable output timing. The output pulse width in this circuit is inversely proportional to the input voltage.