I'm building a circuit that contains a 555 timer wired to operate in monostable mode.
In the final project, the trigger to the monostable will be driven at around 60Hz, with the output driving some strobing LEDs; the 555 is used to ensure that the pulse width is wide enough.
I don't have my input signal yet, but I wanted to try and tune my output to around 8ms, so I pulled the TRIG pin low with the expectation that I'd be able to fiddle with the potentiometer on THRESH.
After much frustration, I finally discovered that the voltage on VCC is all over the place (I have a scope). The circuit is powered by 2A power supply (MW122A); the LEDs each draw 20mA and I have 6 in 3 parallel pairs (so around 60mA of draw).
My question: should I expect to see a huge amount of noise on my VCC pin if I hold TRIG low for a 555 in a monostable configuration? Or should I be looking elsewhere? The chip is specced to source or sink up to 250mA, so I don't see how I can be pulling it out of spec.
I've added a schematic on CircuitLab
Below are some scope shots.
The probe is set to 10x.
The Vertical scale is set to 2V with 5x magnification.
Horizontal is 0.2us.
Normal Power line
Problematic power line
Any suggestions?
Best Answer
Your schematic shows no decoupling capacitor across the supply. High-current spikes, like the 555 generates on transitions, require a supply that is low-impedance at high-frequencies. The inductance and resistance of the cable connecting to your supply is too high for it to supply those high-frequency currents.
Try adding a capacitor of a hundred or so microfarads across the supply, as near to the 555 power pins as you can get.
Your scope shot is shows noise with a period of 3.5 * 200 nS, or about 1.4 MHz. This is a higher frequency than any noise generated by the 555 circuit. It could be residual switching noise from your power supply, but is most likely sloppy scope probing. If you are using the alligator-clip-on-a-wire ground lead on the scope probe, you have made a little loop antenna which will show you any magnetic field in the vicinity.
To see how the 555 operation is affecting the power you need to be triggering synchronous to it. Connect one channel to the LED and trigger off of it, then look at the supply with the other channel.