background power causing problems

[sid]

I've made a small circuit using a 20x2 which works well enough until I switch on a couple of 240ac > 24/12vdc transformers which are close by, then circuit starts acting up.
If I disconnect all wiring into the circuit so the only connection is the 5v & 0v the circuit stops playing up, but as soon as I touch the circuit it acts up again.
If I turn off the transformers then the circuit is as good as gold regardless of whether anything is connected up to it.
So it's obviously stray emf from the transformers and the close proximity of my picaxe that's causing the problem. Unfortunately because the circuit is to be used close to the transformer I can't relocate the circuit any further away from the transformers than it already is, so the question is.....
Is there a way to shield the inputs on the axe from this unwanted interference?

 

[1968neil]

fit a surface mount 100nf capacitor directly between the 5v and 0v pins on the chip that should help

 

[BeanieBots]

The first thing to do is check that all your chips and regulator are properly decoupled in the manner that has been discussed so many times over the last few days and countless times in the past.
If you still have problems, then you will need to consider ferrous shielding. Put simply, enclose your circuit within metal shielding. To cut out magnetic effects as well, the shielding needs to be of ferrous material. Same as the type you will find in the back of a mobile phone.
Any lines going in/out of the shielding need to be capacitively coupled to the shield which in turn should be connected to PICAXE 0v.

 

[Rbeckett]

BB beat me to it. A faraday cage will help eliminate outside EMF in addition to decoupling thouroughly. Google Faraday cage and have a read, it addressses your issue almost exactly.
Bob

 

[sid]

Thanks,
The circuit is well decoupled so I will try placing the circuit inside a metal box.

 

[Dippy]

Is it a nice neat PCB or hairy/messy breadboard or stripboard?

The crappier your circuit layout the more likely it is to be vulnerable to RFI/EMI and any transients kicking around.

Input wires can act like antennas.
Ideally you make a low impedance route to ground for the noise.
Hence decouple/bypass capacitors.
This can apply to inputs as well as power supply as described above.

All this can appear a bit of a Black Magic Art.
In fairness, it is often trial and error when you have a number of potential noise sources.

And often it is better to suppress the source rather than the poor little board.

 

[John West]

Thanks,
The circuit is well decoupled so I will try placing the circuit inside a metal box.

Note the difference between a "metal" box and a "ferrous" box. Aluminum, for instance, is not ferrous.

If the interfering fields are electrical in nature an aluminum or copper box works fine. If the interfering fields are magnetic fields inducing electrical spikes within devices then a ferrous (iron/steel) box is the best bet.

E fields (electrical) and H fields (magnetic) are NOT the same and each must be dealt with somewhat differently.

 

[sid]

Thanks all for the input, I have since rebuilt the board with decoupling capacitors on each input and then placed the whole thing inside a tobbaco tin, which although made from cheap tin seems to have done the trick. The board is held in place with a square of blue tac which also insulates the board from the tin and then the boards earth rail is connected to the tin via an earth strap as suggested.

 

[sid]

meant to add photo

 

[John West]

I'm guessing those electrolytic caps are on all of the signal lines as well as +5 to Gnd. The ones on the signal lines might have been better off as .01uF or .1uF or so.

The large electrolytic values will substantially slow down signals. That's OK if you expect only very slow signals, (on the order of seconds,) but it might become a problem if you are looking for faster incoming pulses, or if the source of the input signal is high impedance.