Control Double Coil relay (SET/RESET) with 2 different DC source

[elskardio]

Hi Guys,

I need to control a double coil relay with 2 different continuous DC source. (both at 5v)

One DC signal sets the relay and the other one reset.

I have to figure out a way to make a small pulse every time one of the DC signal is switched in.

I tried a simple RC time delay but the capacitor won't discharge properly and the relay always stay in one state.

Here's a ruff schematic.

Any help/suggestions on this would be appreciated.

Cheers

 

[EthanSpaulding]

What is your relay part number? Are you triggering it from a microcontroller? You may be able to trigger it just capacitively coupled (220uF is good starting point) from your microcontroller or buffered from a hex-inverter.

 

[elskardio]

What is your relay part number? Are you triggering it from a microcontroller? You may be able to trigger it just capacitively coupled (220uF is good starting point) from your microcontroller or buffered from a hex-inverter.

Hi,
It’s triggered from the GPI outputs of an Avid Sync HD.
2 independent GPI at 5v each.

 

[ccaudle]

Just to make sure I understand, GPI_DC1 and GPI_DC2 are asserted continuously high once they are turned on.
Is there anything to make sure only one is asserted at a time, or could both possibly be high at the same time?

Do you have space or cost limitations? The old standby for for something like this would be to use a 555 timer circuit to generate a known width pulse based on an input enable signal.

 

[elskardio]

Just to make sure I understand, GPI_DC1 and GPI_DC2 are asserted continuously high once they are turned on.

Yes. When Protools is recording GPI DC 2 is High (SET) and when it stops GPI DC 1 is High. (RESET)

The old standby for for something like this would be to use a 555 timer circuit

I was looking at monostable 555 circuits. But I have a positive trigger and it stays High continuously (instead of the momentary switch usually used in those circuits)

I guess it could be modified...

 

[ccaudle]

I have a positive trigger and it stays High continuously (instead of the momentary switch usually used in those circuits)

It doesn't actually stay high continuously, you already pointed out that it goes low in the different states. Think of it as a momentary switch for a veeerrrryyy long definition of "momentary."
The 555 doesn't actually care how long there is between input transitions, it just starts the output cycle when the input goes low.

Which gets to your second point, your outputs are high. You can either put an inverter on the signals, or just swap the signals, so that when GPI_DC2 goes low it triggers the reset coil, instead of triggering the set coil when it goes high. Just switch DC1 and DC2.

Just watch out for the corner case where you start recording then change your mind right away and hit stop. You want to make sure that it isn't possible to start the second astable if the first astable cycle is still in progress.