Designing a NOT-gate Circuit with LED Output

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Discussion Overview

The discussion revolves around designing a circuit that functions similarly to a NOT-gate but incorporates an LED output. Participants explore the implications of using an LED in place of grounding in a NOT-gate configuration, including current limiting and circuit functionality. The scope includes theoretical considerations, practical circuit design, and component selection.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning
  • Experimental/applied

Main Points Raised

  • One participant questions whether a resistor is necessary before the LED when modifying a NOT-gate circuit.
  • Another participant argues that using an LED changes the output voltage, suggesting that it may not function as a true NOT-gate.
  • Some participants propose that the circuit could serve as both an LED driver and a NOT-gate, depending on the design intent.
  • There is a discussion about the need for current limiting resistors for both the LED and the transistor, with varying opinions on the specific values required.
  • Participants discuss the design choice of using a discrete transistor versus a TTL gate for controlling the LED.
  • Some participants express uncertainty about the desired behavior of the LED during operation, whether it should remain on or flash with the input pulses.
  • One participant suggests that the circuit could be similar to an inverter, with specific recommendations for resistor values based on LED type and brightness.
  • There is a clarification on the necessity of resistors to limit current for the LED, transistor, and the driving electronics.

Areas of Agreement / Disagreement

Participants express differing views on whether the circuit can be considered a NOT-gate when an LED is included. There is no consensus on the exact resistor values needed or the best approach to design the circuit, indicating multiple competing perspectives.

Contextual Notes

Participants mention various assumptions regarding the circuit's functionality, including the role of the TTL gate and the specific requirements for LED operation. There are unresolved details about the exact current limiting needed and the implications of using different components.

Who May Find This Useful

Individuals interested in electronics design, particularly those exploring logic gate configurations and LED applications, may find this discussion relevant.

Niles
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Hi

I am trying to design something similar to a NOT-gate (very simple), and I have been following this figure: http://en.wikibooks.org/wiki/File:Electronics_TTLNOT.PNG. My question is the following: Say that I instead of grounding the lower branch want to send a current through it to e.g. a LED. Is it necessary for me to place a resistor before the LED?
 
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If you place a LED there, then you will not have a 'NOT' gate, because now, when A is high, A' won't be Low (0 V) but around 2.5V instead (depending upon LED forward voltage).
However, if you don't need the pure NOT function, then you can go ahead and place the LED, (no need for extra resistor), but make sure the top resistor is at least around 330ohm so as to properly limit the current.
 
I agree with "I am Learning". Depending on the values involved, the LED current in that circuit could be limited by either the base or the collector resistors. Niles, you probably should to tell us is this meant to function as a NOT gate or an LED driver or both?
 
It is supposed to function as an LED driver (or something similar), which can be turned on/off regularly by a computer that deliveres +5V for a few milliseconds periodically. But I guess this is equivalent to a NOT-gate, right?
 
Niles said:
It is supposed to function as an LED driver (or something similar), which can be turned on/off regularly by a computer that deliveres +5V for a few milliseconds periodically. But I guess this is equivalent to a NOT-gate, right?

Well yeah, if it's just a LED driver then there is no need for the logic output.

Do you want to make it yourself out of a discrete transistor or were you looking to make use of a TTL gate or open collector driver? There are lot of simple possibilities.

Regarding the "few milliseconds" bit. Do you want the LED to remain on between the millisecond pulses or just flash and then go off? Either way can be easily handled, but this is an important design aspect.
 
uart said:
Well yeah, if it's just a LED driver then there is no need for the logic output.

Do you want to make it yourself out of a discrete transistor or were you looking to make use of a TTL gate or open collector driver? There are lot of simple possibilities.

Regarding the "few milliseconds" bit. Do you want the LED to remain on between the millisecond pulses or just flash and then go off? Either way can be easily handled, but this is an important design aspect.

I'm going to use a discrete transistor. The TTL-gate will be controlling the base of the transistor (I may have misunderstood your question, I'm unfortunately not very experienced in these matters). The LED is turned on during the pulse, as the TTL gate delivers a square pulse. I basically just have to make the circuit, everything else is already made.
 
Niles said:
I'm going to use a discrete transistor. The TTL-gate will be controlling the base of the transistor (I may have misunderstood your question, I'm unfortunately not very experienced in these matters). The LED is turned on during the pulse, as the TTL gate delivers a square pulse. I basically just have to make the circuit, everything else is already made.

Well you could just use a circuit the same as that inverter if you like. Keep the emitter grounded and connect the LED in series with the collector resistor. You probably want a few k-ohms for the base resistor and a couple of hundred ohms for the collector resistor, though the exact value will depend on the type of LED you're using and the brightness required.
 
uart said:
Well you could just use a circuit the same as that inverter if you like. Keep the emitter grounded and connect the LED in series with the collector resistor. You probably want a few k-ohms for the base resistor and a couple of hundred ohms for the collector resistor, though the exact value will depend on the type of LED you're using and the brightness required.

Thanks for that reply. Is the reason to insert resistors before the base and collector simply to make it easy on the transistor in terms of reducing the current?
 
Niles said:
Thanks for that reply. Is the reason to insert resistors before the base and collector simply to make it easy on the transistor in terms of reducing the current?

Yes you need to limit the current for several reasons.

- For the sake of the LED.

- For the sake of the transistor.

- And possibly for the sake of whatever electronics is driving the transistor.
 

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