MINUTE_MARKER

Published:2009/6/19 1:54:00 Author:May | From:SeekIC

The figure shows the schematic of a minute marker. The output of transformer T1 is 12.6 Vac at 60 Hz, which is rectified by D1 and regulated by IC4, and LM7805 regulator, to provide 5 Vdc for the circuit. The unrectified ac is bandpass-filtered by R1, R2, R5, C1, and C2. Resistors R2 and R5 also form a dcvoltqge divider, which biases the input of Schmitt trigger IC3-a to 2.5 V. The Schrnitt trigger generates a 60-Hz square wave, which is fed to the input of IC1, a CE4040 12stage binary counter.The outputs of the counter are a 4081 quad AND gate (IC2), and the decoded output is fed back to the reset input of the counter, which resets the counter when the desired count is reached.The pulse from IC2-d is inverted by Schmitt trigger IC3-d, and passed along to the output pulse generator. The output pulse is generated by two Schmitt triggers cross-connected as an RS flip-flop (IC3-b and IC3-c). The output of the flip-flop is fed to 3, R4, and C3, whose values set the output pulse duration. The output pulse duration (T) can be approximated by the formula T=1.2×C3×(R3+ R4).A positive or negative-going pulse is selected by 51, and buffered by the remaining AND gate (IC2-a).



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