Unfortunately not.

I've gone back to basics and made a standard monostable circuit using just one side of the 556. Using a pull up resistor and driving the trigger low, the 556 operates the relay as it should.

Next step was to add the inverter. Once this is connected and I provide a high pulse to the inverter, nothing happens. Seems the fault lies here.

If I'm honest, I'm now at a loss.

 

Use a volt meter and see if the output of the inverter is changing states with a change of state on the input.

 

Use a volt meter and see if the output of the inverter is changing states with a change of state on the input.

Yes, the output shows around 5v and drops to 0 when I input high. It climbs back up to 5v when I take the HIGH input away.

 

Maybe try a larger cap between the inverter and trigger input of the 556. Maybe a 0.1uF

 

Right, now I have some life whereas before I had nothing.

With the 0.1uF capacitor there it triggers as it should. I'm not sure if its because of another fault in the circuit or if its just a quirk of the breadboard set up but if I leave the trigger wire (which I touch to 12v to simulate the HIGH input to the inverter) alone and just dangling all is well, the minute I put my hand near it to touch it 12v it triggers the circuit.

 

Based upon what I think you mentioned earlier, the lines should either be +12 or ground. An open wire on the trigger input to the 556 will act like an antenna and anything can happen. Sounds like you have a WINNER!

 

As I'm interested in learning, can you explain why it won't work with the smaller cap yet will with the larger one?

I can't really simulate the conditions exactly so I guess it'll just have to be tested once connected before it is put into its operating position.

Many, many thanks for you help Bill. It is truly appreciated.

 

The larger cap will supply a trigger pulse to the 556 that lasts longer than it would with the smaller capacitor. More energy will couple through a larger capacitor than will couple through a small one.

 

Seems I was a bit hasty.

I'm now having problems again.

On connecting the power supply, the 556 powers up and the relay can be heard to click. It doesn't de-energise though. It's like the 556 is holding its output HIGH rather than timing out.

I'm only using one side of the 556 at the moment too. I've checked and double checked and all is connected right on the breadboard.

 

Check the voltage at the timing capacitor. Make sure it is rising to 2/3 Vcc.

 

It does, but very slowly. Takes a good few seconds to reach it if it makes a difference. It also continues to climb after 2/3 Vcc which I think is correct anyway.

This is without even triggering it. Just connecting the supply Vcc

 

One problem is solved and another rears its face.

I now have the relay energising on connecting the supply, the output of the 556 goes LOW 5 seconds later. To solve this I added a 10k pull-up resistor between Vcc and 556 trigger input. I didn't think I'd need it due to using a 4069 but I can see why I was mistaken.

Now, the 556 output would go LOW. Once it did I couldn't get it to trigger again by giving the inverter a HIGH input.

I connected my voltmeter to the output to see if it was going low and lo and behold, it starts working as it should. Take the voltmeter away and it stops triggering. What on earth is going on!?

 

http://www.kpsec.freeuk.com/555timer.htm#monostable gives a good description of how a oneshot works with a 555. Theory is the same for 556. Are you using one of the proto boards that has holes that you just push wires into to make connections. I've seen some where oversized wires were used once and the contacts lost the ability to make solid contact. Just touching with a meter made all OK.

 

Yes, using breadboard. All the connections seem pretty solid though.

I just can't understand why it worked properly when the volt meter was connected.

I'll have a read of that page, thanks for that.

 

I just took another look at the schematic you posted earlier. It already shows 10K between Vcc and trigger. Did you correct the drawing on the other post? Now I'm confused, too.
Also, I'm not sure I understand the diodes between the output of one side tied to the trigger of the other side of the 556.

 

The 10k resistors shown in the schematic 'belong' to the diodes. The diodes and resistors are supposed to form a circuit that prevents one timer from being triggered if the other's output is HIGH.

Looking at where they are though, they do appear to be connected the same way as pull-up resistors.

 

Hmmm,

Now, even having the voltmeter connected won't make it trigger.

It does show that the output of the inverter is not going LOW though. Can't work out why though.

 

Are you using electrolytic caps between the 4049 and the trigger input? If so, make sure the + side of the cap is toward the 556.

 

Nope, 0.1uF ceramic disc cap.

 

To repeat a phrase that one of my co-workers frequently uttered, "Something is causing it!" I would still be looking for loose connections or wires that may have pulled out without noticing.