Dodgy Chinese flip flop...or my circuit?

[rockrockmcrock]

Right I intend to get this sorted today one way or another (after a busy week). Wish me luck

 

[rockrockmcrock]

Ok attached are some photos +new circuit and the behaviour is still very odd, so any thoughts on the following -


Behaviour of this circuit is odd:

(1) triggering the flip flop (Tg to ground) with the LED attached works perfectly.

(2) triggering with relay and led attached results in a flash of the LED and quick click on and then off of the relay. The circuit behaves the same way of the LED is removed.

(3) triggering initially without the relay and then connecting the relay into the circuit later throws the relay as intended and it holds with no buzzing. Relay pulls 117mA which is about right. Triggering the flip flop again turns the relay off as it should, however trying to trigger back on with the relay still connected results in same behaviour in (2) above.

So....Noise in the circuit and a poorly designed flip flop or circuit?
Any help appreciated, thanks! Rrmr

 

[Alec_t]

What are you using as your 12V supply?
The supply to the flip-flop IC should be decoupled by capacitors, e.g. at least 100uF in parallel with a 100nF cap very close to the IC supply pins.
The ground pin of the IC should go directly to the supply negative terminal via one wire. The ground pin of the relay should go directly to the supply negative terminal via a second wire. That way (i.e. a 'star ground' arrangement), the ground currents of the IC and the relay are kept separate.

 

[rockrockmcrock]

Thanks, much appreciated.

the power supply at present is a regulated 12v. However in practical application (portable air compressor) the supply will vary between 10v min and 14v as it will be battery supplied from a 12v lead acid battery. The flip flop and circuit has been rated/designed to cope with these worst case voltages.

Space is an issue here hence distances between components is in mm's. If space wasn't an issue I'd have given up by now and just gone the old fashioned route and whacked 4 car relays together.

Given the spec of the relay (see first post) and the circuit diagram given I was under the impression that suppression was on board the PCB. Hence no isolation by me beyond ensuring grounds are not shared on the cheese board.

Is this the most likely solution then? I feel I am going round in circles here and am getting to the point of not wanting to mess with someone else's pcb that is unknown in design and just chuck it and design/build in my own.

 

[Alec_t]

I was under the impression that suppression was on board the PCB.

Perhaps it is, but it's not obvious from the pic. Can you post a schematic of the board?

 

[rockrockmcrock]

The flip flop or mine? I don't have one for the flip flop I'm afraid

 

[BobK]

Try adding a pull up resistor on the trigger input. 1K for testing. I doubt this is the problem, but who knows, if there is no pullup on the board, the input might be floating between button presses and subject even to noise from the relay activating.

Bob

 

[rockrockmcrock]

Thanks guys. Well I got up this morning with the intention of isolation the flip flop. Before I did that I recheck the circuit in operation - seems the tests last night before I posted (or the single test this morning) have blown the flip flop. So as this is the last one I have, I give up and will just design my own. Annoying as I could have done that in the first place and not wasted a week on this hunk of junk. Grrrr........

Moral of the story: Chinese promises are dodgy: DON'T buy Chinese electronics from eBay, unless you've found an independent review of it elsewhere. Sad thing is I know this and they still hook me occasionally!

So if you see something on Ebay advertised as Bistable Flip-Flop Latch Switch Circuit Module Button Trigger Power-off Memory - suitable for driving relays - think carefully before buying it - hopefully this thread might save another mug some pain!

Thanks again guys.