Let current flow when signal is low?

Other Hardware General Electronics
1

So long story short I'm looking for something like a relay that is on until powered, Except a big relay is kind of overkill. Something like a transistor that lets current flow without input and stops flow whilst receiving.

The idea of this is so that i can use an Arduino to go between a PWM signal and a fan, So that I can read the PWM along with some other stuff and create my own PWM but if for some reason the Arduino is off or broken the fan will function off of the original signal.

I did try and google a few different things but I'm obviously using the wrong search terms.

2

https://www.jameco.com/z/GS-SH-205T-Electromechanical-Relay-DPDT-2A-5Volt-125-Ohm-Non-Latching_139977.html

Drive the relay with a transistor.

.

3

Find a relay with 3 output terminals: NC, COM and NO.

COM is the common pin between the other two. One wire goes to that.

NC stands for "normally closed". That is, when the relay is off, this contact is "closed" which connects it to COM. Your other wire goes to that. Now the current will flow when the relay is NOT energised.

4

So just to clarify:

  • If the Arduino is active, it will read an existing PWM signal and some other signals, and construct a new PWM which goes to the fan
  • If the Arduino is inactive, the existing PWM will go direct to the fan, and the other signals are ignored

Is that correct?

5

larryd:

https://www.jameco.com/z/GS-SH-205T-Electromechanical-Relay-DPDT-2A-5Volt-125-Ohm-Non-Latching_139977.html

Drive the relay with a transistor.

.

Thanks! I didn't know you could get relays this small, Definitely a much more feasible option now.

MorganS:
Find a relay with 3 output terminals: NC, COM and NO.

COM is the common pin between the other two. One wire goes to that.

NC stands for "normally closed". That is, when the relay is off, this contact is "closed" which connects it to COM. Your other wire goes to that. Now the current will flow when the relay is NOT energised.

Is there something else that could do this? Not the biggest fan of electro-mechanical components and surely there is something that can do this when a transistor does the opposite?

kenwood120s:
So just to clarify:

  • If the Arduino is active, it will read an existing PWM signal and some other signals, and construct a new PWM which goes to the fan
  • If the Arduino is inactive, the existing PWM will go direct to the fan, and the other signals are ignored

Is that correct?

Correct! Sorry my explanation wasn't so great, Little bit scatter brain at the moment.

6

So just to clarify:

If the Arduino is active, it will read an existing PWM signal and some other signals, and construct a new PWM which goes to the fan
If the Arduino is inactive, the existing PWM will go direct to the fan, and the other signals are ignored

Is that correct?

SnipeYa:
Correct!

Then it's not just a simple matter of a relay is it?- needs some kind of logic to switch the existing PWM to the fan if and only if the Arduino's out of play. (You need another Arduino to see if the first Arduino's there or not :wink: )

So I'm not really sure what the relay of which you speak is going to do: I need a diagram.

7

SnipeYa:
Something like a transistor that lets current flow without input and stops flow whilst receiving.

That would be a depletion-mode FET - not sure its easy to find one rated for significant power
and low on-resistance though, tend to be small-signal components, but some do exist.

Not sure about the practicalities of a depletion mode device switching a through-signal (as opposed
to just switching to ground)

8

What device are you talking about? What fan? What is creating the PWM?

9

I think you can use MOSFET with pull-up (or pull down) to keep it conducting. When Arduino takes control it pulls the gate to other direction.

10

Or,

  • Arduino PWM drives one input of a two-input NAND gate

  • Main PWM drives an input of a second NAND gate

  • Arduino also drives a retriggerable one-shot (555?)

  • One-shot Q output drives 2nd input of Arduino NAND gate and through an inverter (4th 74HC00?):

  • drives the 2nd input of the main PWM gate

  • both of these NAND gates together drive a third two-input NAND which in turn drives

  • a solid state relay which runs the fan

While Arduino provides PWM signal one-shot keeps Q output high. If Arduino goes away one-shot times out and switches to the main signal.