# Time delay and polarity switching

Discussion in 'General Electronics Discussion' started by john2k, Sep 11, 2012.

1. ### john2k

185
3
Jun 13, 2012
Was hoping someone can point me in the right direction and help me out.

I want to build some kind of circuit that would allow the following:

I have a 12V trigger power source which I want to use as a trigger for a relay. When the 12V trigger source is active, I want the relay to activate and provide the main 12V source for 3 seconds in a certain polarity. Then wait until the 12V trigger is turned off and then allow main 12V source through for another 3 seconds but in reverse polarity.

any suggestions and ideas of how I can achieve this would be very much appreciated.

i have come across the following time delay circuit, wondering if it can be used

many thanks

2. ### Harald KappModeratorModerator

11,813
2,752
Nov 17, 2011
What happens after the first 3 seconds have passed? Does the output stay active at the initial polarity or does the output turn off?
What happens after the second 3 seconds have passed? Does the output stay active at the reversed polarity, does the output turn off, or does it revert to the initial polarity?
Perhaps it would help if you could sketch a simple timing diagram.

Also: You have a constantly available 12 V source plus an additional trigger (at 12 V too)? Otherwise, if you were to use the same source for trigger and power supply you would not be able to output 12 V in reverse polarity after the trigger is turned off.

If you're going for a relay based solution, I think you'd need the following:
A first monoflop (3 seconds) triggered by the rising edge of the trigger pulse. This monoflop activates a first DPST relay to output 12 V at initial polarity. After the 3 seconds have passed, the monoflop will fall back and the output will turn off.
A second monoflop triggered by the falling edge of the trigger pulse.This monoflop activates a second DPST relay to output 12 V at reverse polarity. After the 3 seconds have passed, the monoflop will fall back and the output will turn off.

And no, the circuit you show us will as such not do what you want - provided I understood your problem correctly.

3. ### john2k

185
3
Jun 13, 2012
Once the relay is activated and 3 seconds of power has been provided, the output needs to turn off (even if the trigger 12V is still live)

The second 3 second output is when the 12V trigger is no longer active and before the relay completely shuts-off the power, i would like it to provide power in reverse polarity for 3 seconds and then completely shut-off.

For Example:

+12v trigger becomes live ===> activates relay ===> power is provided from secondary 12V power source for 3seconds ====> power cuts-off (even though +12v trigger may still be on)

THEN

+12v trigger is turned off ====> de-activates relay ===> before relay is de-activated, 3 seconds of 12V power from secondary source is provided in reverse polarity ====> then everything powers-off.​

and then it can loop through the process of trigger on and trigger off

Yes, I have a separate 12V power source for powering and a different 12V trigger source which is switched on and off

Many thanks

4. ### Harald KappModeratorModerator

11,813
2,752
Nov 17, 2011
So my monoflop solution would be one way to do it.

5. ### wingnut

242
9
Aug 9, 2012
What about using a PIC or other microprocessor?

6. ### Harald KappModeratorModerator

11,813
2,752
Nov 17, 2011
Yup, that's the high end monoflop. Plus you need an additioonal voltage regulator for the PIC. Although component count may be less than a discrete solution.