Calculator & Measurement

Logic level tester

The tester shows on a seven-segment, common-cathode display whether its input is logic 1 (H on the display) or logic 0 (L on the display). An undefined level is indicated as ‘n’. When the input is low, T1 is switched off and T2 and T3 conduct. This results in a high output at IC1a and a low one at IC1b, whereupon the segmented’ lights. Together with the always actuated segments ‘e’ and T., this causes an ‘L’ to be displayed.

When the input is high, T1 is on and T2 and T3 are off. The output of IC1a is then low and that of IC1b is high. Apart from segments ‘e’ and ‘f, ‘b’, ‘c’ and ‘g’ also light, Which causes an H to be displayed.

When the input is undefined, or open, all transistors are on (because of R1, R2, and R3). The Outputs of IC1a and IC1b are then both high, which causes segments ‘a’. ‘b’ and ‘c’ to light together with ‘e’ and `f; an ‘n’ (no level) is then displayed.

The buzzer is not essential, merely useful; it may thus be omitted. If it is used, it should be a type of an integral oscillator. The type of the components used is not critical. The display may be any seven-segment, common-cathode type. ICI may be a standard 7400 or an LS version of this or the open-collector version 7401; HC and HCT types are less suitable because these cannot provide the required output current.

The switching thresholds of the tester are at 1 V d 3 V, which is not standard, but in practice, they do well. The 3 V threshold

may be lowered slightly by increasing the value of R4. The input impedance of the tester is 5 k so that it does not affect the circuit on the test.
The current drawn by the tester is determined by the type of display: in the prototype, it was about 60 mA.


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