PN2222 transistor can't solve my problem

Other Hardware Motors, Mechanics, Power and CNC
1

I am enjoying the Arduino experience, and I am now trying to use an Arduino to simulate a simple switch closure using a transistor. I have tried 2222 transistor following all sorts of forum conversations, but I think I now know that my voltage being controlled is too high.

Background: I have a piece of electronics that sends 6.55Vdc from one pin, which goes out into the world, thru a switch, and then comes back to another pin. Switch open or switch closed can be sensed by the receiving pin, thus determining the circuit's "state".

I wish to replace the original switch with an Arduino-powered "switch" driven open or close by the state of a digitalOutput pin. I started using PN2222 with a 230 Ohm resistor. I can confirm that my PN2222 transistor theory worked by using a LED to test following a thread I found here.

When I moved to the "real" circuit, I have now gotten stuck.

I believe my error is that when I move to my real scenario, 6.55Vdc is greater than my 5Vdc to my source pin, the PN2222 won't work. Unfortunately I cannot reduce the 6.55Vdc circuit I am controlling to bring it below my PN2222's source pin voltage. I cannot tamper with the original circuit at all since it will break the products function.

I am hoping to find something solid state and simple, for which I can make a pigtail. I envisioned one wire from digitalOutput, thru a current limiting resistor, then to the base pin of a PN2222. On the other side two wires (one in to the collector and the other out from the emitter) that connect to the terminals of the product. Simple. Functional.

If anyone can point me in a direction I would be grateful. I am hoping to not have to use dry contact relays, which is the only other idea I have at the moment.

2

Your idea will work, but a common ground for both circuits is required. From your description, it sounds like that is not the case. Connect the Arduino ground to whatever serves as ground on the sensing device (which could be the input pin and hence the emitter of the transistor).

3

Maybe you would be better off with a MOSFET.

When I moved to the "real" circuit, I have now gotten stuck.

Can you elaborate ? (ie: nothing happens, works but not very good etc. )

4

I was thinking that I had a common ground plane, but I didn't. I was pulling 5V from digitalOutput of Arduino. Arduino powered from Mac's USB. Other device was powered from wall wart. No common ground between the two. Right!! (Forgot the two respective AC/DC transformers isolate AC side from DC side, hence no common ground.)

Tied ground together, both my PN2222 and my N7000 mosfet work like a champ.

So now the lsat question becomes which is preferred, PN2222 or N7000?? I see on the N7000 packaging that it is static sensitive... but I also here the mosfet is more like a true switch??

Thanks for the seeing what I couldn't on this one. I have been selling the product I am tinkering with for 15 years, I have been playing with ways to use an Arduino to simulate real world behaviors for about 15 minutes.

5

but a common ground for both circuits is required.

(FYI, that's a given. That's why I didn't mention it. I'm glad someone else thought of bringing that up.

6

Admittedly a freshman error. Sometimes one needs another to point out something one should be able to see.

Thanks.