Transistor switch NOOB

Using Arduino General Electronics
#1

Hi guys

I've really enjoyed the last year of programming arduino and completed a few nice projects. Programming skills are improving however my electronic circuit skills are lacking. On this project I have an existing OEM circuit with two push button switches that I would like to tap into and control from an arduino. Programming side is easy. I've made a transistor switch in the past for an LED but where I'm a little stumped on this one is the conditions. I've measured the voltage across the botton and it's 3v, when I try measure the current when the button is closed it does not measure anything and the electronic circuit does not register a button push at all due to my tester. I left this till the last part of the project and up until now I've just controlled them with a relay module. Understanding this is an overkill and not wanting to avoid my electronic skills shortfall could someone assist me in the correct transistor circuit and the calcs to correctly size the transistor and resistors. I assume this has almost no current one the button is pushed so how does this affect the calculations. I understand that there will be a certain current flow from the arduino to the circuit with a transistor, can I minimize this or avoid it with low side transistor switch?

#2

You need to include more information. A sketch of the circuit would prove handy as well. What are you controlling with a transistor? What is the function of the button?

#3

It can be a little "risky" connecting to an unknown circuit.

Are you trying to activate/simulate the switch, or are you trying to read it?

If you want to activate/simulate the switch a relay is often the best solution because a relay is an electrically-operated, electrically-isolated switch.

A transistor can be used to switch voltage & current on & off in certain applications but it's NOT the same as an actual switch and it's NOT always a replacement for a switch. And, it's not electrically isolated and that can cause all kinds of issues with an "unknown" circuit.

I've measured the voltage across the botton and it's 3v, when I try measure the current when the button is closed

You might be confusing voltage & current. ...In the old water analogy, voltage is water pressure, and current is the flow of water. Resistance is a valve or small pipe that resists water flow. But, don't take this analogy to far... An with an "open" water pipe you have zero resistance and water flows-out all over the place. An open electrical circuit has infinite resistance and no current flows.

#4

Hi guys. Some answers. Yes I'm trying to activate the switch. The switch is a simple surface mounted momentary push button in a electronic circuit that is very similar to MP3 player.

The buttons function is switching it on and off.

Just had a thought. If my measurement are correct which I think they are there is only 3v and almost no current flowing at the button....maybe and optoisolator would work in this instance. The arduino on one side of the isolator causing the other side to go closed or open circuit? Would be safer than a transistor?

Regards Brian

#5

You probably measured the current wrong, it doesn’t make much sense for there to be no current.

Did you put the current meter in series with the button or in parallel? You cannot measure current the same way you measure voltage.

#6

Easier way to do it, just remove the button from the circuit and use the current meter to bridge the contacts. That low impedance of the meter will act close enough to a closed switch.

#7

Still a little foggy about what you are trying to do. What I’m reading leans to me say you want to bypass a momentary on/off switch of a device with the arduino, and you are wanting to use a transistor to do it??