LOGIC_PROBE_

Published:2009/7/17 1:34:00 Author:Jessie | From:SeekIC

The figure shows the schematic for a versatile logic probe. The zener diode clamps the input signal just above the TTL inverter's 2.2-V trigger voltage. Zener diode D1 can be left out if the probe is going to be used only on TTL circuits. The logic probe is based mainly on inverters IC1-a and IC1-b, which are sections of a 7404 integrated circuit. Depending on whether the input is high or low, the inverters enable AND gate IC2-a or IC2-b, each of which is a section of a 4081. Each AND gate is connected to an oscillator, one low-frequency, the other high. When an AND gate is made high, it passes the frequency of its oscillator to the piezo buzzer (BZ1). Whenever a high is at the input, the buzzer will produce the high tone; whenever a low is at the input, the buzzer will sound the low tone. With switch S2 in the PULSE position, if pulses are present at the input, a yellow LED will light and the buzzer will sound at the frequency of the pulses. The CMOS/TTL switch selects the voltage from the circuit under test or the voltage from the 78L05 regulator. Note that when this switch is in the TTL position, it can work only with 5-Vdc, but when the switch is in the CMOS position, it can work with from 7.5 to 35 Vdc.



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