Question
Asked February 20 2015

Community Question

Hey guys, what's the best and safest way to trigger 12V Relays from the Digital pins? Thanks.:P

I tried a Tip120 Circuit with a 1K resistor, didnt work, then with a 2.2K resistor, didn't work. I have an LED hooked up to the digital pin so I know it works, the LED lights up.
February 21 2015
Doombot, On your cct, never enable D0 D2 or D1 D3 at the same time else you will see smoke as these will be short circuits. A lot of homebrew H bridges use an (logic) invertor on these lines so this can never happen.

Likewise a 1 1 (on B1 and B2) of the first cct will also cause much smoke, its easy to accidentally do this with a simple coding error, the invertor method averts this.

Tony
February 21 2015
@Toymaker

I'm pondering your advice and I don't understand how these would smoke, I have tried this on relays triggered with manual switches in place of the Tip120's.

I'm not very experienced at how Transistors work. Please elaborate.*confused*

Here:


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How is it shorting out? Switching D0 and D3 closes the forward circuit and switching D1 and D2 should close the reverse circuit. Of course I won't switch everything on and assume it's "brake".

This is not going to be for continuous rotation, I'm only using this for short spurts since I'm using these motors for the shoulders.
*tired*
February 21 2015
@Toymaker
Oh, I think I understand your post now. Thanks.;)
February 21 2015
Doombot, it just that if say two of the vertical power transistors are allowed conduct at the same time then its a short cct. People put inverters in the control lines to stop this, they can be transistor or logic IC inverters.

Tony
February 21 2015
Doombot, I have quickly sketched a circuit for a typical transistor inverter for an H-bridge to show the idea, I hope it helps.

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Tony