TTL probe

This TTL probe is helpful when troubleshooting a serial connection. It provides an optical indication of the line state and traffic on the line. 

The TTL signal has two states: 0 volt or 5 volt. The TTL probe displays the two states using LEDs, and you can see the states changing. The maximum baud rate on the line should be 9600 baud (lower is better). 

The bit length in a 9600 baud signal is 104 microseconds, and with 10 bits being sent, the flashing will last about 1 milliseconds. The flashing can be detected by the naked eye. 

Figure 1. Circuit diagram.

If the input of the probe is between 0 V and 2 V, the green LED is on (red LED off). If the input is between 3 volt and 5 volt, the red LED is on (green LED off).

The TTL probe is powered by 5 volt DC. This voltage can be taken from the measured circuit, or from a external PSU. Even a 4.5 volt battery can be used.

Figure 2. Veroboard.

All 8 components are fitted on a piece of Veroboard. This is my preferred way of building electronic circuits! The dent on LED1 faces away from T1. The dent on LED2 faces T2.

The circuit was copied from DIY Logic Probe: Step by Step Guide.

Best 73 from OZ1BXM Lars
My homepage: oz1bxm.dk