Digital Electronics: Principles, Devices and Applications

370 Digital ElectronicsV CC3TriggerI/pC 1R 1D24, 85556, 7Rv o150.01C(a)TriggerI/P0V CCAtpin-2V CC+0.70(b)Figure 10.12555 monoshot triggering on trailing edges.the trigger waveform. Figure 10.12(a) shows the monoshot configuration that can be triggered on thetrailing edges of the trigger waveform. R 1 –C 1 constitutes a differentiator circuit. One of the terminalsof resistor R 1 is tied to +V CC , with the result that the amplitudes of differentiated pulses are +V CC to+2V CC and +V CC to ground, corresponding to the leading and trailing edges of the trigger waveformrespectively. Diode D clamps the positive-going differentiated pulses to about +0.7 V. The net result isthat the trigger terminal of timer 555 gets the required trigger pulses corresponding to HIGH-to-LOWedges of the trigger waveform. Figure 10.12(b) shows the relevant waveforms.Figure 10.13(a) shows the monoshot configuration that can be triggered on the leading edgesof the trigger waveform. The R 1 –C 1 combination constitutes the differentiator producing positiveand negative pulses corresponding to LOW-to-HIGH and HIGH-to-LOW transitions of the triggerwaveform. Negative pulses are clamped by the diode, and the positive pulses are applied to the base ofa transistor switch. The collector terminal of the transistor feeds the required trigger pulses to terminal2 of the IC. Figure 10.13(b) shows the relevant waveforms.For the circuits shown in Figs 10.12 and 10.13 to function properly, the values of R 1 and C 1for the differentiator should be chosen carefully. Firstly, the differentiator time constant should bemuch smaller than the HIGH time of the trigger waveform for proper differentiation. Secondly, thedifferentiated pulse width should be less than the expected HIGH time of the monoshot output.