# TTL NOT-gate

Niles
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?

I_am_learning
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?

Niles
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?

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.

Niles
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.

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.

Niles
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?

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.