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 ICM and a low one at lC lb, whereupon segment ‘d` lights. Together with permanently actuated segments ‘e’ and ‘t` , this causes an “L’ to be displayed. When the input is high, This on and T2 and T3 are off. The output of Cla is then low and that of ICH, is high. Apart from segments ‘e‘ and ‘t` , ‘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 Rl, R2 and R3). The outputs of ICM and ICH, are then both high, which causes segments A ‘a’, ‘b‘ and ‘c’ to light together with ‘e’ and ‘f; an ‘n’ (no level) is then displayed. The buzzer is not essential, merely use ful; it may thus be omitted. If it is used, it should be a type with integral oscillator.
The type of the components used is not critical. The display may be any seven segment, common-cathode type. IC] 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 and 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 kQ, which has no effect on the circuit on test. V The current drawn by the tester is determined by the type of display; in the prototype, it is about 60 mA.
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 ICM and a low one at lC lb, whereupon segment ‘d` lights. Together with permanently actuated segments ‘e’ and ‘t` , this causes an “L’ to be displayed. When the input is high, This on and T2 and T3 are off. The output of Cla is then low and that of ICH, is high. Apart from segments ‘e‘ and ‘t` , ‘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 Rl, R2 and R3). The outputs of ICM and ICH, are then both high, which causes segments A ‘a’, ‘b‘ and ‘c’ to light together with ‘e’ and ‘f; an ‘n’ (no level) is then displayed. The buzzer is not essential, merely use ful; it may thus be omitted. If it is used, it should be a type with integral oscillator.
The type of the components used is not critical. The display may be any seven segment, common-cathode type. IC] 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 and 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 kQ, which has no effect on the circuit on test. V The current drawn by the tester is determined by the type of display; in the prototype, it is about 60 mA.
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