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Can You Drive an led from a gate output?
So, if you want to drive an LED from a gate output you can not also use that gate output as a logic signal. Your second solution is the more correct one. Now, if you no longer care about maintaining valid logic levels where the gate output is connected to the LED then you can source considerably more than 4mA from the gate output.
What happens to the current when an LED is turned on?
Limiting current into an LED is very important. An LED behaves very differently to a resistor in circuit. Resistors behave linearly according to Ohm’s law: V = IR. For example, increase the voltage across a resistor, the current will increase proportionally, as long as the resistor’s value stays the same. Simple enough.
Which is the output of an AND gate?
The equations output can be easily explained using the AND Gate Boolean Equation, which is Q = A x B or Q = AB. Hence, for an AND Gate the output is HIGH only when both the inputs are HIGH. A transistor is a semiconductor device with three terminals that can be connected to an external circuit.
Can You source more than 4mA from a led?
Now, if you no longer care about maintaining valid logic levels where the gate output is connected to the LED then you can source considerably more than 4mA from the gate output. The manufacturer doesn’t provide min/max values for this situation because that’s not really how the gates are supposed to be used.
Which is the correct way to drive an led?
The right way to control an LED is with a discrete transistor or using an integrated circuit that is specifically designed for driving high-current loads. Thanks for contributing an answer to Electrical Engineering Stack Exchange!
How much logic is needed to drive an led?
If you’re using old LEDs you might need up to 20 mA and then it is better to use an additional transistor to make that current flow as logic gates aren’t designed to supply such a current. You need to remember that the HC gates cannot supply more than 4mA and provide valid logic levels at the same time.
How to measure source currents of series 74LS?
Get your circuit working with the output HI and the LED on. Have the LED connected between the gate output and the resistor and the resistor connected between the LED and ground. Then measure the voltage across the resistor and make a note of it. Then measure the voltage at the gate output relative to ground and make a note of it.