Driving 2 relays from 1 PIC

[Marmoset123]

Hi,

I've built this relay bypass system from Coda Effects and it works like a dream, but I'm trying to work out how to modify it to switch 2 relays (for a stereo reverb I'm making based on the FV-1 chip).

How should I go about doing this? I'm out of my league here

I tried just wiring it up to both relays but the relays don't switch. My guess is I don't have enough current at the moment but I don't know how to resolve this.

Also I am using EC2-5NU relays as I couldn't get the NA5W-K. They seem to work fine for switching 1 channel though.

Thanks for the help.

Jamie




Here's the Coda Relay Bypass schematic


and the details for my relays (it's the 5v version)

[potul]

Hi

You ALWAYS need a relay driver. It's a bad idea to drive the relay directly from the PIC

You can use a dedicated IC or just build it using a transitor.

Mat

[Marmoset123]

Thanks for the reply. Would something like a ULN2003 work?

If I want to have 2 relays switch on and off at the same time can I just attach 1 output of the PIC to 2 inputs of the ULN2003? Or will in need a microcontroller with more outputs?

Cheers

[antonis]

just build it using a transitor.

What Mat said..
(adding "s" between last word's "i" & "t" ..)

[potul]

Thanks for the reply. Would something like a ULN2003 work?

Yes, an ULN2003 will work. But if you only have 2 relays, maybe you would prefer to build it using discrete components. Like this:



There are multiple variants around the web.... just look for "relay driver"

If I want to have 2 relays switch on and off at the same time can I just attach 1 output of the PIC to 2 inputs of the ULN2003? Or will in need a microcontroller with more outputs?

If you need the 2 relays to always turn on/off together, yes. You can attach one output of the PIC to two inputs of the ULN2003.

[potul]

just build it using a transitor.

What Mat said..
(adding "s" between last word's "i" & "t" ..)

haha, good catch. 

[antonis]

Using Zener for less than 30mA (140mW) relay "kick-back" should be an overkill, don't you agree Mat..?? 
(an ordinary diode like 1N4001 should be just fine..)

Also, OP should be warned for phase invertion of the above driver..

[GibsonGM]

Would a relay driver such as the above still function if the transistor was set up as a buffer - taking the output from the emitter, and therefore the output would not be phase inverted? 

[Marmoset123]

Thanks for all the replies, just breadboarding it up now

[antonis]

Would a relay driver such as the above still function if the transistor was set up as a buffer - taking the output from the emitter, and therefore the output would not be phase inverted?

Placing "would" between "above" & "still" and omiting question mark at the end of the paragraph, should form a true & correct statement..!!

The "synchroneous" relay activation depends on relay's NO/NC type..

[GibsonGM]

Where is the Duck, it's Riddle Time!!    

[R.G.]

Just to add to the possibilities, a small signal MOSFET, something like a 2N7000, can be driven with its gate connected directly to the PIC's pins. In this setup, the MOSFET doesn't need any other gate components as its gate is always driven to 0 or V+ by the PIC, and protected by the PIC's pin protection setup.

The logical polarity of the driving signal on PIC projects can be changed with a single instruction in the programming. This makes the internal polarity of the "on" signal in the PIC a lot less of a concern. It's a single instruction to flip it to what you want.

[antonis]

Just to add to the possibilities, a small signal MOSFET, something like a 2N7000, can be driven with its gate connected directly to the PIC's pins. In this setup, the MOSFET doesn't need any other gate components as its gate is always driven to 0 or V+ by the PIC, and protected by the PIC's pin protection setup.

I'm not aware about PIC internal protection (actually, I know nothing about PICs other than they can be programmed.. ) so I can't say if direct coupling is safe or not..

But for "delicate" CMOS logic, a coupling resistor (of 1k, say..) could prevent MOSFET Drain-Gate capacitance to couple the coil's inductive transients back to CMOS driver..

[potul]

Using Zener for less than 30mA (140mW) relay "kick-back" should be an overkill, don't you agree Mat..?? 
(an ordinary diode like 1N4001 should be just fine..)

Also, OP should be warned for phase invertion of the above driver..

sure, I just copy-pasted the first image I found referring to a transistor relay driver. In fact I have never used a zener here, always standard diodes.