I am using a voltmeter to test the output voltage of the SN74LS08N AND gate. The inputs are simply jumper wires that I can conveniently plug into a voltage source (5V), GND, or otherwise leave it hanging. The output of the gate I hook it up to the voltmeter, whose COM port is hooked to my circuit's GND. When I plug both input jumper wires into GND, my voltmeter reads 0V as expected. When I plug both input jumper wires into 5V, my voltmeter reads 5V as expected. However, if I leave the jumper wires hanging, the voltmeter hovers around 2.5V. Why is this the case? I am suspecting this has to do with noise interference from the environment.
The datasheet shows the internal circuit:
In theory, open inputs should behave as if high. But high-impedance inputs can easily pick up noise, and the circuit was not designed to work with open inputs.
A DC value of 2.5 V does not make sense; maybe the output is oscillating.
Please note that the base current of the two high output transistors must be able to flow out of the output, so a voltmeter alone will not show the behaviour as in a TTL circuit. (With a small load, the high output voltage usually is near 3.4 V.)
TTL has some rules:
Open inputs are logic high. This is by design, as driving an input high just reverse-biases the input diode.
Outputs have “nice” voltages only when pulled via 20k or less to VCC.
A logic low is between 0 and 0.8V while a logic high is anything above 2.0V.
Assuming the output you’re measuring is not oscillating - and it shouldn’t be - you’re seeing a valid HIGH level. With a pull-up resistor it would go up a bit higher and perhaps look “more convincing”, but as far as TTL goes, 2.0V and up is a valid high and that’s that.