# Indirectly Measuring Resistor Pullup Size

#### brightnight1

Joined Jan 13, 2018
91
If I have a simple circuit (below) with an unknown sized pull-up resistor and I’d like to know the size of the pull-up resistor without directly measuring it. Can I measure the current draw in this circuit and if the VCC voltage is know then use V=IR to calculate the size of the pull-up resistor? IS there a better way to indirectly measure the pull-up resistor size such as using a scope to see how fast the signal changes from a high to a low?

#### peterdeco

Joined Oct 8, 2019
484
A very common value is 10K. I prefer 47K. It's not that critical.

#### brightnight1

Joined Jan 13, 2018
91
I have a PCB where the voltage sits at VCC and when it gets pulled low an event happens. I'm hacking it to artificially pull it low but don't know if I can just connect to to ground to pull it low, or if I should be pulling it low with a resistor.

#### peterdeco

Joined Oct 8, 2019
484
According to your diagram above, the switch pulls it low directly to ground. How are you artifically going to pull it low?

#### brightnight1

Joined Jan 13, 2018
91
That's a simple diagram to help with my explanation. The real circuit is on a PCB board inside a camera and when the signal goes low the camera take a picture but same idea. I could not trace out the board to find the resistors but I planned on artifically pulling it low but shorting a wire or a wire and a resitor across the switch.

#### ElectricSpidey

Joined Dec 2, 2017
2,594
If you are just going to short a switch, why are you worried about any pullup values?

#### Irving

Joined Jan 30, 2016
3,496
Just connect a multimeter on the mA range across the switch, it'll act as if the switch has been closed and show the current pull down requirement - it'll be microamps probably.

#### brightnight1

Joined Jan 13, 2018
91
I'd like to have a complete picture of the circuit for down the road, never know when it will be helpful and don't want to reopen the camera again if I already have it open now.

#### Ya’akov

Joined Jan 27, 2019
8,167
They are Panasonic Gh4 and GH5 cameras. There are some schematics of remote triggers which is what I'm doing but I don't think I need the resistors they add in and would to build a complete picture of the circuit:
https://www.norwegiancreations.com/2018/07/controlling-a-camera-shutter-remotely-with-an-arduino/
Those resistor create a voltage divider. It appears the camera determines whether the signal is focus or shutter based on the voltage drop. Why do you think you can omit them, and why would you want to?

#### MrChips

Joined Oct 2, 2009
29,245
The way I would do it is as follows.

Measure the voltage at the input pin with nothing connected.
Put a variable resistor of about 100kΩ from the input to GND while monitoring the voltage.
Adjust the variable resistor until the voltage is 50% of the initial voltage.
Remove the variable resistor and measure its resistance. That value will be equal to the value of the pull-up resistor.

In other words, put a resistor from input to GND to bring the voltage down to half the open circuit voltage.

#### Kjeldgaard

Joined Apr 7, 2016
476
I have shown the image below earlier in a thread here on the site.
It's from my Canon EOS60D camera.

I used something similar to @MrChips method to find the values displayed for the camera's remote control socket, just with a pair of fixed resistors and Ohm's law.