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Multi-Function Test Probe Schematic Circuit Diagram

Multi-Functionality with Two ICs:

This circuit, comprising only two integrated circuits (ICs), demonstrates versatility by performing three distinct test functions, selectable through switch S1. Centered around a clock generator, IC1a-1c generates a rectangular signal with a frequency of 3 Hz. In position A of switch S1, T1 utilizes the clock signal to energize the probe. This buffer stage can deliver a current of up to 100 mA through the limiting resistor R7. Practically, this allows the probe to inject a 3 Hz signal into any digital circuit, effectively overriding all logic outputs. The detection of short circuits becomes swifter as only such anomalies can cause the signal to vanish. However, it’s essential to note that this method represents a rudimentary test, short-circuiting all outputs connected to the examined track simultaneously, whereas manufacturers typically allow this to occur for only one output at a time.

Logic Level Detection:

With switch S1 set to position B, the tester transitions to detecting logic levels. When the probe is unconnected, the clock at 3 Hz is present at the input of IC1d, causing DI to flicker rhythmically. Upon applying a logic level to the probe, it suppresses the clock; if the level is low, the LED extinguishes, and if it is high, the LED remains continuously lit. The switching thresholds of the IC are between approximately 1/3 and 2/3 of the supply voltage, making it suitable for the CMOS 4000 series and the HCT family. While standard TTL, LS, and HCT ICs are compatible, it’s crucial to note that the thresholds may not be optimal for these variations.

Multi-function test probe Schematic diagram

High-Frequency Signal Compatibility:

The tester is designed to effectively handle input signals with frequencies ranging from 30 to 40 MHz. When switch S1 is set to position C, the input signal is directed to the clock input of shift register IC2. Following eight clock pulses at the probe, the persistently high level at inputs A and B of IC2 reaches the OH output, illuminating LED D2. Due to the constant resetting of IC2 by the 3 Hz signal, the LED flickers as long as a clock is present at the probe, given that its frequency exceeds 25 MHz. The overall circuit draws a current of 10 mA when both LEDs exhibit flickering. It’s imperative to ensure a power supply capable of providing around 100 mA to guarantee satisfactory operation of the pulse generator.

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