A common method is a relay with N.C. contacts wired in series with the 
coil.  It 'self-oscillates'. Then the supply voltage to the coil can be 
wound around the circuit under test.

Sometimes it helps to ground the noise-generator to the circuit.

One tester I made used a two pole relay and capacitor setup as a flying 
capacitor.  The charge side was line voltage through a variac feeding a 
  voltage multiplier then to a large resistor charging the capacitor. 
When the relay was made to toggle, the cap 'flew' from the charge side 
to the circuit under test side.  By varying the size of the cap and 
including a series resistor to apply the zap, as well as the variac to 
control the cap voltage, you can sneak up on the circuit being tested. 
This could produce spikes from zero up to a few thousand volts.  I 
connected this setup one at a time directly to each circuit node that 
went off the board.

Of course, it is a good idea to monitor the circuit with a scope to know 
that you are really injecting noise and also how much of a spike you are 
making.

peiserma@ridgid.com wrote:
> Most circuits have some inputs and outputs (external wires) connecting
> them to the real world. I want to create electrical noise and subject an
> unsuspecting PIC and its inputs to noise. 
> 
> I'd love to have a gizmo that can throw some noise into the system in
> the lab. It doesn't need to be fancy, just an effective noise-maker. I'm
> thinking a bank of relays with long leads to the relay coils (no coil
> suppression of course). The leads could then be wrapped around the
> inputs to the circuit. I also have in my lab a sample 120V DC motor; I
> could run that directly from rectified 120V AC, and switch rapidly
> on-off with one of the aforementioned relays...
> 
> Does anyone have a favorite way of doing a quick bench test like this in
> their lab? Something you've maybe constructed that you'd like to share?
> 
> 


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