Calculator & Measurement

Logic Level Tester Schematic Circuit Diagram

Logic Level Display on the Seven-Segment Tester:

This tester employs a seven-segment common-cathode display to indicate whether the input is logic 1 (displaying ‘H’) or logic 0 (displaying ‘L’). An undefined level is represented as ‘n’. When the input is low, T1 is turned off, allowing T2 and T3 to conduct. This leads to a high output at IC1a and a low output at IC1b, causing the segmented display to illuminate. Combined with the always active segments ‘e’ and ‘T,’ this configuration displays an ‘L.’ Conversely, when the input is high, T1 is on, turning off T2 and T3. This results in a low output at IC1a and a high output at IC1b, illuminating segments ‘e,’ ‘f,’ ‘b,’ ‘c,’ and ‘g,’ displaying an ‘H.’

Handling Undefined or Open Inputs:

When the input is undefined or open, all transistors are activated due to R1, R2, and R3. Both outputs of IC1a and IC1b are then high, causing segments ‘a,’ ‘b,’ and ‘c’ to light up alongside ‘e’ and ‘f,’ displaying an ‘n’ (no level).

Optional Buzzer Integration:

The buzzer, while not essential, adds utility to the tester and can be omitted if not needed. If utilized, it should be an integral oscillator type, and the specific component type is not critical. The display can be any seven-segment common-cathode type, and IC1 may be a standard 7400 or its LS version or the open-collector version 7401. However, HC and HCT types are less suitable as they cannot provide the required output current.

Setting Switching Thresholds for the Tester:

The tester’s switching thresholds are set at 1 V and 3 V. A configuration that may deviate from the standard but performs effectively in practice. If a lower threshold of 3 V is desired, it can be achieved by slightly increasing the value of R4. The tester’s input impedance is 5 k, ensuring it has minimal impact on the circuit under test.

Adjusting the Current Draw with Display Type:

The current drawn by the tester is contingent on the type of display used. The prototype registered an approximate current of 60 mA. The flexibility to tailor the switching thresholds and the low input impedance contribute to the versatility and adaptability of the tester in various testing scenarios.

 

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