Open-Collector to Push-Pull

Question

In a project I am using a 74LS05 to provide an ESP32 microcontroller with 6 open-collector outputs. Actually this is fine for 5 of the outputs, but for the remaining one I'd rather have a full push-pull driver, since the rise time I currently have is still too high (and I am already using a 470 ohm pull-up resistor).

I cannot use an ESP32 output directly since the output needs to be at +5V when high.

I'd like to keep the solution as simple as possible. I was thinking that maybe I could add an external BJT or MOSFET to "complement" the open-collector output but I have no idea of how to do that.

Any hints?

EDIT: Here's some additional info:

• I don't want to add a 74xx04 for just one output, the other 5 outputs are fine open-collector-style.
• I am using LS logic because the project is retrocomputing-related. I could switch to an HCT but would that change much?
• I don't know the exact characteristics of the input I have to drive. it's the right mouse button input of the Amiga Paula custom chip, which is notoriously hard to drive.
• I can take some measurements with a scope if that helps.
• On a different project I am driving the same input straight from an output pin of an AVR microcontroller and in that case the signal rises "immediately".

EDIT2: Here's an old scope screenshot showing how slow the signal is rising (blue). It needs to be high before the following clock rising edge (yellow). I can't remember what pull-up value I was using when I took this screenshot though, maybe 1k or 820 ohm.

Answer 1

You can convert a slow rising input signal to a fast rising output signal using the following circuit (in blue dotted box):

Here are the input (green) and output (blue) waveforms:

The circuit on the left is to simulate the slow rising input voltage. The capacitor on the right is just to show that it is able to drive the output high fast enough even with 50pF capacitive load.

Answer 2

Adding a MC74VHC1GT50 to an open-collector with pullup resistor output, will give a TTL level output which can source/sink 8mA. It is one 6 pin surface mount chip.

Answer 3

The analog characteristics of logic are computed by the. Vol/Iol and Vil/Iil output and input low impedance ratios. All TTL families differed in speed and power yet maintained a 10:1 fan-out ratio. This applies to both current and resistance.

The 74LS05 is an inverting Open Collector with an output saturation resistance of 25 Ohms maximum ( 400 mVmax/16mA) and an input Vil Resistance if 250 Ohms minimum (400mV/1.6mAmax). the input voltage threshold for all TLL is two Vbe drops or 1.3 but for noise power immunity, the thresholds were defined safely as “0” = <= 0.8V and “1” = >2.0V.

Similarly impedance for 3.3V logic has been designed for 22 Ohm drivers @ 3.0V and this lowers slightly at 3.6V.

Thus you can get superior performance driving LS logic directly from 3.3 V CMOS logic such as the ESP32 family. If for any reason your wires are too inductive, and ringing causes logic errors, this can be well-damped pulses with same risetime by adding 50 to 150 series from 3.3V logic to drive say a 240 Ohm ribbon cable or twisted pairs.

For open collector outputs or switch contacts from the joystick, you just need any suitable pullup R’s.

Thus there is no need to add redundant interface circuitry to drive a TTL Joystick from 3.3 Logic.