How to click a relay with a low signal?

Community General Discussions
1

I need to click a relay when a signal goes low. I have a couple relays on Walter, but they all click when signal goes high. These are pretty straight-forward, just a simple 5v relay, and a 2n222 NPN transistor to click it when a data-level signal goes high. --No Problem

In this case, I will have a steady data-level signal high, and I need the relay to click when it goes to gnd. Any thoughts? I, of course, do not want to use a DPDT relay and have it energized all the time.

2

PNP transistors work for
PNP transistors work for that. I’m sure you could use a 2n3906 or something similar.

3

If for some reason you can’t
If for some reason you can’t source any suitable PNP transistors you could use 2 NPN transistors, where the first NPN inverts the signal.
The NPN connected to the relay would be pulled up by a resistor (ie: on by default), but the second NPN would pull the signal down, keeping the first NPN (and therefore the relay) turned off. When the input signal goes low, the second NPN switches off, releasing the first NPN which now activates and turns on the relay.
Slightly more complex, since it requires 2 NPNs and 2 resistors instead of just 1 PNP and 1 resistor, but the gain is a lot higher making the device more sensitive. Not that it’s likely you’ll need to boost your input signal anyway, but it’s worth mentioning =D

4

Wait, wait…
Ok, another chance to learn something… Am I to understand that PNP transistors “click” when ground is applied to thier base? I feel I am missing something here… something I should already know.

5

I was about to go sketch up
I was about to go sketch up a diagram but OddBot has saved me the hassle =D

6

Thanks Guys
Yup, it seems so simple and clear now… Thanks

7

I’m going PNP
Just to double check… I am drawing up the schematic and want to double check the part. I have access to a 2N3906, a 2N2907 and a 2N4403. I would think one of these should work, could you give me a suggestion as to the best one to use. Or suggest another part all together. I just need to know what to add to my PCB.

8

Nevermind…
I just did a draw-up and found I need to click a 12v relay… I guess I am going NPN. I am just going to use a good-ol’ 2N2222.

9

I agree. I have like 200 of
I agree. I have like 200 of em, and mostly use them for MCU controlled relays, or MCU controlled things that require more than a few mA’s.

10

A relay is a coil, just like a motor is.
Remember to put in a flyback diode, to protect your transistor against induction currents.

11

Got it covered!
You got it rik, I have a 1n4001 stuck right in there on my new boards… I’m getting stoked, these boards are the last of what I want to clean up before the “Great Tear-Down” which leads to PAINT!!! Woo-Hoo PAINT!!

12

Make over show O?
Just don’t forget to take before, during and after pictures.