Logic Level Indicator Input Voltage
Figure 1. The simple logic level indicator.


  However I cannot find out what voltage I should have at the input for the LED to light up, ...


The logic level indicator is simply an inverting buffer driving the LED. When the input is 'high' the output will be 'low' and current will flow from the +5 V to the inverter output. (The inverter will 'sink' the current from the LED.) When the input is low the output will be high and there will be no voltage across the LED so it will turn off.

A few things:


The inverter chip needs to be of the same logic family as the circuit under test so that logic levels (switching voltages) match.
Additional inverters could be used in parallel to increase the current sinking ability.
The inverter chip needs to be connected to +5 V and GND.




Figure 2. Logic level switching voltages.

Digital logic became the standard because there are only two states to worry about - '0' or '1'. That means we can allow for some deviation away from 0 V for a logic 0 and some deviation from +5 V for a logic 1. This is important because if a gate is driving multiple other gates the loading will cause the voltage to droop somewhat. Each logic family has its own switching thresholds due to the internal construction.

What Figure 2 is showing you is:


(1) The VIH is the minimum voltage that is guaranteed to be detected as a '1'.
(2) For your TTL LS family this is 2.0 V and any voltage above 2.0 will give a logic 1.
(3) The VIL is the maximum voltage that is guaranteed to be detected as a '0'.
(4) For your TTL LS family this is 0.8 V. Any voltage below that will give a logic 0.


The chart also gives other parameters that you can read up on.