555/556 Problem

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SgtWookie

Joined Jul 17, 2007
22,210
You need the capacitors between the 4069 outputs and the 556 trigger inputs.

You also need the pull-up resistors and diodes as you have shown in your schematic.

I don't know what else to tell you.
 

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CitrPug

Joined Feb 20, 2011
83
Thanks SgtWookie. You help is appreciated. Not sure why one side would work but the other wouldn't.

Is it worth trying a BJT 556?
 

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CitrPug

Joined Feb 20, 2011
83
I've further probed into the side of the timer that isn't working.

The capacitor charges on triggering, reaches 2/3 and then discharges. It would seem that there is no output though. It is remaining low.
 

CDRIVE

Joined Jul 1, 2008
2,219
MOS 555's don't drive (source) relay coils well. They can sink far more current than they can source. Do you have protection diodes across the coils?
 

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CitrPug

Joined Feb 20, 2011
83
I see, yes I have the protection diodes in place.

I think I might try a BJT 556 in that case then. Is the CMOS 4069 still ok or would I be better off using something non-CMOS?
 

CDRIVE

Joined Jul 1, 2008
2,219
The MOS 555 will have a hard time driving those coils. Unless someone has a reason that you shouldn't use BJT-555's, I'd say go for it.
 
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CitrPug

Joined Feb 20, 2011
83
Well I bought some BJT 556s. NE556N

Something has changed because I've just connected it up to be greeted with crackling, a soft pop, and some smoke from what looked to be from the 556. All I did was swap the CMOS 556 for the BJT one.

I'm a bit nervous about connecting it back up again now.
 

CDRIVE

Joined Jul 1, 2008
2,219
Are you sure that you don't have it based 180° from 12 o-clock? Doing so would place the GND pin in the Vcc position and the Vcc in the GND position. Another thing to check is your output pin connections. I remember you saying that the MOS chip wouldn't drive your relay coil. Are you sure that you don't have a short there?
 

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CitrPug

Joined Feb 20, 2011
83
Orientated the right way and no short on the relay.

One side of the CMOS chip would drive the relay. The other side wouldn't.

EDIT: I take that back. I just tested the back EMF diodes, they had failed short circuit it would seem. Not sure why but I'll test it with new ones later
 
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CitrPug

Joined Feb 20, 2011
83
Schematic on page 2 doesn't show the relays but the diodes are round the right way for EMF protection across the coils.
 

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CitrPug

Joined Feb 20, 2011
83
Quick update:

Replaced all the diodes and have tested with a new BJT IC (just in case the short had killed the first one)

Both sides work as intended! I did buy some 4.7uF poly caps and 1M resistors but seems they won't be needed. Didn't realise the poly caps would be as big as they are!

I am truly grateful for all the help received. I really am.
 

Thread Starter

CitrPug

Joined Feb 20, 2011
83
It is curious and I did wonder. I did have a couple of incidents when probing in that my multimeter croc clip touched another component and shorted so I put it down to that.

I forgot to mention I have made one change too. I read that a diode in series with the coil is recommended on a 555/556 when driving a coil. It was added in front of the coil. The explanation was to stop a 'glitch' (back EMF?) feeding back into the IC and possibly triggering the timer again. Whether they are needed or not I don't know but I put them in as I could see no harm in doing so. Both these newly added diodes, and the reverse EMF diodes (including the one that failed) are 1N4004s.
 

Thread Starter

CitrPug

Joined Feb 20, 2011
83
I'm going to stop saying it works from now on!

I was testing it again the other day and on first connecting to the supply noticed that the output was high for a very long time, more than the 4/5 seconds the timing network is set up for. Now this could be leakage from the caps, I do have some poly film caps to try to see if that makes a difference.

I also noticed that I could hear two clicks when the relay de-energised. Testing showed that the timer is operating both relays when it should only be operating one at a time. One timer for one relay and second timer for second relay.

Any clues on this?
 
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