Hey guys. I have this project that I'm working on. I'm a mechanical engineer but I'm not that knowledgeable when it comes to electronics.
I want to use a transistor as a switch to short circuit a very low voltage. The system I'm using has a 0.25V signal. When you hit the button, it connects that 0.25V signal to the rest of the circuitry and notifies the board that the button has been pressed.
I'd like to mimic this when my arduino and a transistor. Is that possible? Could an NPN transistor short the 0.25V?
My plan was to hook up the arduino to the base pin on the transistor and then the collector and emitter would be between ground and the 0.25V
To give you a qualified answer, you should tell us what you have that has 0.25v over it's switch. It sounds very low (not that it matters, but it might matter)
Your plan would probably work, but you need to remember that the Arduino's GND need to be connected to one side of your switch as well for the circuit to work.
In case the low side of the switch is not ground in your equipment, this could spell problems or even disaster. If we a talking about something portable, battery powered like a remote control, you are all good.
Anyhow, we've come to the conslusion that the transistor will do the job, but technically, it's a rather terrible solution.
The solution you are looking after is called an optocoupler or opto-isolator
It's an component that comprises of an infrared LED and light sensitive transistor, all enclosed in the same housing. If has no electrical connection between it's input and output, and offers a great isolation (often 1500V or more) - this means that your arduino-circuit never "touches" your system you are hooking into, and there is very little chance for something to blow up or get destroyed.
Here's a handy little guide on what those devices are and how they can be put to use.
That's funny you should mention those. I was actually just playing around with some photoresistors and LEDs. I was going to do it that way until I thought of the transistor idea.
So if I'm understanding you correctly, these will basically be exactly what I had before but in one convenient case? So when I send a signal from my arduino to one side of the opto-isolator, the other side of the opto-isolator will go to a very low resistance?
As for what I'm doing, it's just a little side project that I'm tinkering with. It's a battery operated circuit using very low voltages. There's very low risk in this system.
I could not find a spec for the part you referenced (I believe it is by uxCELL) but I found the same part by Sharp. PC817 4-pin optoisolator.
The spec I read stated that the voltage drop when the output transistor is "on" was between 0.1 and 0.2 volts. I'm not sure that would work if you really need to switch 0.25V
Couple of questions. I ask because you are new at this and I might misunderstand you meaning of different descriptions.
When you say "short circuit" what do you mean?
The 0.25V you are trying to switch, is it the only voltage available to pass on to your circuit?
I'm assuming the arduino will replace the "button" you reference and react to some sort of signal.
Am I close?
BTW There is a part that does not have the voltage drop characteristic however when it is "on" it has a resistance of between 200 to 330 ohms (depending on the version of the part)
See: ON Semiconductors H11Fxx
Hey thanks for your help. Essentially I'm just trying to recreate a switch. Right now this system uses a mechanical push button switch. When you push button, the contacts are closed and the board sees the 0.25V. I'd like to mimic that with an arduino so that I can program it to essentially "push the button" electronically.
I was planning on using a photoresistor and an LED. I had the code already done and I was 3D printing an enclosure for this so that light couldn't leak in. but I was still not happy with that system because I was worried that if light leaked in, it would falsely "push the button".
So the optocoupler seems like a good solution. When the arduino gives it power to two of the terminals on the optocoupler, the resistance drops very low on the other two terminals, essentially creating a short circuit, or a button push. (Assuming I'm understanding it all correctly.)
I'm not sure if the very low voltage will be a problem. But that's the idea anyway.
BTW, I found an old piece of circuitry that I kept because it had a bunch of capacitors and resistors on it. It had an 813 optocoupler on it. I hooked up my multimeter to one side and my arduino with a 1k-ohm resistor in series. When the arduino goes high, the resistance dropped to 85 ohms on the other side of the optocoupler. That's plenty sufficiently low resistance for what I need. I went ahead and bought those optocouplers from amazon. I think they're almost identical to the one I found.
mach_zero:
BTW, I found an old piece of circuitry that I kept because it had a bunch of capacitors and resistors on it. It had an 813 optocoupler on it. I hooked up my multimeter to one side and my arduino with a 1k-ohm resistor in series. When the arduino goes high, the resistance dropped to 85 ohms on the other side of the optocoupler. That's plenty sufficiently low resistance for what I need. I went ahead and bought those optocouplers from amazon. I think they're almost identical to the one I found.
Awesome, use the "813" - it's fine for your project.
