Logic Probe

Although you can buy a commercial probe, here is one I designed to fit a probe case I had.

Specification.
Supply 0 and +5V @ 10mA to 0 and +12V @ 25mA
Input Z > 15K
Logic 0 0v to 1V
Logic 1 Supply less 1V
Single full level pulse detection >10MHz.

Circuit Description.
Both power rail transistors TR1 and TR2 are normally on, shorting out the HI and LOW indicating LED unless the probe is within 1V of ground or +ve rail. When that occurs the shorting transistor across the LED is tuned off and the appropriate logic 1 or 0 LED lights.

Pulses of Logic 0 from TR2 feed through the 10nF cap to diode detector to turn on T4 with the pulses. TR4 discharges a 4u7 Cap to hold on the pulse indicator LED. Pulses only of logic 1 from TR1 do the same for TR3, which turns on T4 to light the LED as before.

73 de John G8MNY @ GB7CIP

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