Inverter output at high question

Thread Starter

samyang

Joined Jul 24, 2022
35
Hi. Can someone please explain the output at pin 2 when the inverter (SN74LV06ADBR) is high? I thought it toggles between 0 and 3.3V, but a friend told me that when the output is high, the voltage at pin 2 is not 3.3V, and the inverter has a high impedance so it is at some unknown voltage with very little current. Please see the first attached photo below. However, when the output is low, the voltage at pin 2 is 0V.

2. The goal of the new circuit is to turn on an LED when SERVO_ON_LED_N is 0 (the output at pin 2 of the inverter) and turn off an led when the output at pin 2 is high and at high impedance. Please see the second attached photo( the inverter and the 1k resistor is the same as in the first attachment) I understand how the LED will turn on when the signal is 0V at low, but how will it turn off when the output is high at the inverter? I would love it if someone could explain how that proposed schematic would turn the LED on and off at the right time based on the previous schematic. Thank you so much.

Update: the last photo is a more zoomed-out one.
 

Attachments

Papabravo

Joined Feb 24, 2006
19,023
The 06 inverter has an open collector output. This means there is only one transistor in the output stage and it pulls the output very close to GROUND when it is on. When that single output transistor is off the output voltage is determined by the pullup resistor. The output of the 06 inverter will be the value of whatever supply that resistor is connected to. It is likely to be the 442Ω, but we really can't tell because you keep giving wrong or incomplete information.
 

Papabravo

Joined Feb 24, 2006
19,023
What more information should I supply to be more clear? Thanks.
I told you what we need to see. We need to see what the resistors connected to the gate output are connected to. One of them should be the pullup resistor that determines the output high voltage of the open collector gate. The purpose of the other resistor remains obscure for the moment.
 

eetech00

Joined Jun 8, 2013
3,330
What more information should I supply to be more clear? Thanks.
Hi

The SN74LV06A has an OPEN DRAIN output. This means the drain pin of the output NMOS transistor is not committed to a voltage and is connected directly to the output pin of the device. Therfore, the output pin "floats" unless it is pulled high (a +voltage), (usually) thru a resistor. The internal NMOS transistor source is connected to the -supply pin (ground). A logic "High" state is not the same as a "high impedence" state. A high impedence state for this device means the output is floating with its resistance so high that it behaves as if not connected at all.
 

BobTPH

Joined Jun 5, 2013
5,510
What is driving the input signal? If it can sink several ma, you can connect an LED between +3.3V and the output through a suitable resistor and it will be on when the output is 0.
 

MisterBill2

Joined Jan 23, 2018
12,745
IT seems that you are asking for an explanation, and so my question is if the circuit is functioning,and if the LED is switching on and off as hoped for. If it is working then certainly the explanation given is correct.
 

BobTPH

Joined Jun 5, 2013
5,510
I now see that you have posted the circuit in question. The output is through the inverter in and a 1K resistor. A high efficiency red, yellow, or green indicator LED can be connected from +3.3V to the output and it will light when the output is zero.
 

Thread Starter

samyang

Joined Jul 24, 2022
35
I now see that you have posted the circuit in question. The output is through the inverter in and a 1K resistor. A high efficiency red, yellow, or green indicator LED can be connected from +3.3V to the output and it will light when the output is zero.
Thank you all for your help. Why exactly does the led turn on when the output is zero? What is the output of the inverter at its high state?
 

Thread Starter

samyang

Joined Jul 24, 2022
35
To clarify, I understand that the output of the inverter is zero at the low state, but what is output out of the inverter at its high state?
 

BobTPH

Joined Jun 5, 2013
5,510
There is no output in the high state. Therefore the LED is off. That is what high impedance means, basically, no connection.
 

Thread Starter

samyang

Joined Jul 24, 2022
35
There is no output in the high state. Therefore the LED is off. That is what high impedance means, basically, no connection.
Thank you for the clarification. What words or symbols on the datasheet revealed that it had high impedance at the high state? Thanks.
 

eetech00

Joined Jun 8, 2013
3,330
Thank you for the clarification. What words or symbols on the datasheet revealed that it had high impedance at the high state? Thanks.
Like I wrote in post #5. The fact that it is an OPEN DRAIN output implies that if the output is not pulled up it will be in a high-impedence state when the input is low.

BTW-Read the datasheet
 
Last edited:

Papabravo

Joined Feb 24, 2006
19,023
I meant to post the following simulation a day or two ago, but I was having a problem with connecting a symbol to a subcircuit library. That problem is resolved, and I can post the simulation of an open collector gate without a pullup, with a modest pullup, and with a large pullup. Using an open collector gate means making a determination on the size of the pullup resistor. Too big and it slows down the rising edge, too small and it can dissipate excess current pulling the output low.
1659656274561.png
 

k1ng 1337

Joined Sep 11, 2020
651
Get yourself a handful of shunts resistors, I use values 0.1Ω and 1Ω to find the current of my circuits. Use KVL and KCL to work out what's on and off, when and why. I like to monitor the voltage across components as well as the current through them with a microcontroller. For linear circuits, this provides 2 of the 3 criteria for Ohm's law. Also, use LTspice, it is YOUR FRIEND.
 
Top