Author Topic: Logic porbe pulse detection  (Read 1663 times)

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Online ataradov

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Logic porbe pulse detection
« on: August 03, 2015, 01:39:28 am »
Just as a quick mental exercise I decided to create a logic probe with the following limitations:
1. Show "1", "0", undefined (exact thresholds are not important).
2. Show pulse signal (LED lit for ~500 ms on any level transition).
3. Use a single LM339 for everything. Any passives are allowed.

Schematic I came up with is attached. Exact component values are approximate and are subject for fine-tuning.

Additional limitation - indicator for "1" and "0" is a dual red-green LED and a mix of red and green is used to indicate an undefined state.

The problem here (apart from head-on approach) is that pulse LED remains lit for undefined state (since both LEDs need to be lit). Technically pulse detection here indicates transition of the "sum" signal to a positive value (== any of the LEDs turning off == one of the defined levels became active).

The logic level detection is OK, but is there a better way to indicate pulse without using additional ICs?

I have a version of the circuit that only tracks the positive edge of the "sum" signal. It is based on the small capacitor separating two parts with minor modifications to level shifting resistors. But it misses short transitions, and quite fiddly even in the simulator.

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