Transistor to act like a dry contact.

Thread Starter

ulms

Joined Mar 19, 2024
50
Hello, I've got a device that works on a dry contact switch closure. A voltmeter across the input of this device reads 3 volts. I flip a toggle switch and the input is sensed. I have another device that I would like to control it with but it is a transistor output. Without installing a relay is there something I can do with a 2n2222 transistor? Thanks
 

Tonyr1084

Joined Sep 24, 2015
7,943
Welcome to AAC.

A 2n7000 or 2n7001 MOSFET would be more suitable for this sort of switching. Which one you use depends on whether you're high side switching or low side switching. A schematic of what you're trying to do would be most useful.
 

Thread Starter

ulms

Joined Mar 19, 2024
50
Welcome to AAC.

A 2n7000 or 2n7001 MOSFET would be more suitable for this sort of switching. Which one you use depends on whether you're high side switching or low side switching. A schematic of what you're trying to do would be most useful.
IMG_20240319_104852.jpg
 

DickCappels

Joined Aug 21, 2008
10,224
Per TonyR, a MOSFET would be a better choice than a bipolar. If a 2N2222 is all you can get your hands on, connect the switch input to the emitter, connect the collector to ground (common) or the - power supply (when properly drawn the 2N2222 will look upside down with the emitter switching the signal and the collector connected to where the emitter would be). To wit:

1710860704606.png

Take the output directly from the emitter. RC represents the pull-up in the device you are controlling (and in the drawing, the output is specified incorrectly - it should be between the emitter and collector).
 

Thread Starter

ulms

Joined Mar 19, 2024
50
Per TonyR, a MOSFET would be a better choice than a bipolar. If a 2N2222 is all you can get your hands on, connect the switch input to the emitter, connect the collector to ground (common) or the - power supply (when properly drawn the 2N2222 will look upside down with the emitter switching the signal and the collector connected to where the emitter would be). To wit:

View attachment 317947
Thanks I will try this, I just didn't want to destroy what's ever in that box on the left in my diagram.
 

Thread Starter

ulms

Joined Mar 19, 2024
50
I am a recently retired construction worker who just picked this up as a hobby. It took everything I had to make a 2n2222 transistor work. Don't be throwing mosfets at me not yet anyway
 

DickCappels

Joined Aug 21, 2008
10,224
While you were responding, I added a clarifying note:

Take the output directly from the emitter. RC represents the pull-up in the device you are controlling (and in the drawing, the output is specified incorrectly - it should be between the emitter and collector).
 

Thread Starter

ulms

Joined Mar 19, 2024
50
I am a recently retired construction worker who just picked this up as a hobby. It took everything I had to make a 2n2222 transistor work. Don't be throwing mosfets at me not yet anyway
The laffy face emojis didn't send?
 

Thread Starter

ulms

Joined Mar 19, 2024
50
Incidentally how are you defining the term "dry contact'?
As the contacts of a magnetic relay, the circuit works when I energize the coil of a relay. I just wanted to eliminate the relay. Incidentally I'm not getting the above to work with my transistor and I'm quite confused by Dicks circuit and correction note!
 

MaxHeadRoom

Joined Jul 18, 2013
28,758
As the contacts of a magnetic relay, the circuit works when I energize the coil of a relay. I just wanted to eliminate the relay. Incidentally I'm not getting the above to work with my transistor and I'm quite confused by Dicks circuit and correction note!
Just that 'Dry-Contact' usually infers a contact that although may carry current does not switch when current is present, as in enable etc.
 

Thread Starter

ulms

Joined Mar 19, 2024
50
I think I tried that first. I'll revisit it later thanks.
I'm pretty sure that's the first thing I did. I will try it again. But now I'm thinking some kind of inverter shorting situation if you know what I mean. Maybe that's what Dick's schematic was supposed to be. He drew it upside down and that just put the whammy on my head. I only know enough about this stuff to start fires and burn out components.
 

sghioto

Joined Dec 31, 2017
5,420
Short the red and blk wires on the device with your meter set for DC current to get a measurement.
Q1 may need more base current, try changing R1 to a 1K resistor.
 
Last edited:

Tonyr1084

Joined Sep 24, 2015
7,943
An Opto-Isolator would do the same thing. But it isolates the signal from the response.

Basically it's just an LED inside a chip device facing a photo transistor. Apply power to the LED and it closes (so-to-say) the contacts. All you need to do is not exceed the rating of the Opto.
 
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