How to switch resistors?

Hi, i am a software guy trying hardware.

I have a camera with two pin interface, camera looks for 82k resistance between these two pins to "know" some device attached. 39k parallel to 82 to focus 12k parallel to both 39 and 82 to shoot.

I thought a circuit like this will help, proteus simulation proved that, however when i tried it physically it did not worked. What am i missing?

circuit is here.

Do you have a page or PDF or other reference where the values are mentioned? NB does the camera work when the resistors are "switched" manually? Did you try just a 39K resistor? and just a 12K resistor?

A lot of people build this cable with switches and using with their fuji cameras. Most of the designs are based on Bob's page. Bob opened his RR-80 shutter cable and found these values. This is his page: He bought a ready cable and added a phototransistor. I can not verify if third resistor is 12k or not, can you look at it?

I am willing to build an intervalometer for time lapse photography. Arduino will trigger the camera in desired intervals. Switching these resistors will replace this mechanical version.

Viewing to your circuit again (not having a hardware background either) , the idea of your sketch is to do a digitalwrite(pin, HIGH) to switch the 39K and 12K resistor in parallel. I think you need two additional resistors (100K?) at the base of the transistors to pull it down to GND level so the transitors will block when you do digitalWrite(n, LOW). might do the trick.

From the pictures on Bob's page I cannot see if the 3rd resistor is 12K but he seems confident. You might find the right values by using a 100K potmeter and see at which value the camera acts.

Have you seen Rob

Switches in my circuit will be replaced by digitalwrite(2,HIGH) and digitalwrite(3,HIGH) as you mentioned. I will try 100kOhm resistor. Thanks.

I have seen that page but that circuit uses relays, i think it is quite overkill for this job, i really want to do this with transistors or optocouplers.

Transistors do not switch resistors but when i try optocouplers, it works. Any comments on this one?

Just another thought, the Arduino and the other device must share/connect their GND. Did you?

Your circuit has no base resistors on the transistors, that couldn’t help and may damage the Arduino.

Transistors aren’t really my bag but what does D1 do? And why is the D1/Q1/R1 junction tied to GND?

I reckon FETs would be better here.

Also in the real world HM1 will be applying a voltage when measuring resistance, how much I can’t remember but it would have to be the right way around.


I have added 100k resistors on bases and tried that way as robtillaart previously recommended but it did not helped.

In theory, arduino digital pin 3 will just add 39k parallel to 82k and arduino pin 2 will add 12k with 39k to 82k resistor. Diode there is for this.

That ground on D1/Q1/R2 junction is arduino ground. I thought transistor will work that way.

This diagram may be completely wrong, i need an advice for correcting it.

I think Rob was talking about pulling the bases down to GND with 100k, that's one thing but I'm saying you need resistors in series between the AVR output and the base (say 10k for starters). Otherwise you are affectively trying to drive a diode to GND.

I'm pretty sure the diode has to go, as does pulling the junction to GND.


FWIW, and I'm prepared to be corrected because I'm not a transistor/FET guru, but no gurus are responding so untill they do I would do it something like this.

Maybe with a couple of 100k resistors from the gates to GND to ensure they turn off when the Arduino is powered down.

The parasitic diodes may cause issues depending on the exact nature (and polarity) of the gadget you're connecting to, but you can often just turn the FETs around to fix that.

Also note Rob's questions

does the camera work when the resistors are "switched" manually? Did you try just a 39K resistor? and just a 12K resistor?

Transistors do not switch resistors but when i try optocouplers, it works.

Don't know what this means without a schematic.


@Rob: camera works if i switch resistors manually, it focuses at 82//39 and shoots at 82//39//12

Erm how about using a digital pot?