Logic Probe.

Source: http://www.josepino.com/?logic_probe   Tags: electronics circuits homemade

If you work with digital circuits, TTL or CMOS, you may know sometimes is not easy to diagnose a problem or know the reason of an unexpected behavior in the circuit. That is the function of the Logic probe, It helps to monitor the signal on every connection. As the pulse injection is included, it also can help to send some pulses to the circuit. This is a perfect tool when developing circuits.

I built this one on 1989 when I was at the school learning about digital electronics. I saw one that does the same thing, monitors and injects pulses, with a cost of a few hundred dollars. I built mine for only $7 USD. I don't have any picture because that was more than 15 years ago, but I remember clearly that I used a toothbrush case. My friends were astonished how perfectly it worked.
Taken from http://www.josepino.com
Here is the schematic of my Logic probe with Pulse injection: [Corrected, Thanks Dave!]

How this works:

The power for the logic probe is taken from the circuit to monitor. As a CMOS is used, it works with 3V to 15 Volts. The tip is connected to the point in the circuit that needs to be monitored. To inject pulses, just close the switch. Pulse injection is used when working with latches, flip-flops, etc.

Here is another version of the same circuit that shows when the signal is "1", "0" or "P":

I don't use a logic probe anymore because I have enough computers to use simulators, however, soon I will start working with digital circuits and I will build my own Logic probe with pulse injector.

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