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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 CircuitLab Schematic

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 VCC with TRIG left high

Problematic power line VCC with TRIG pulled low

Any suggestions?

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  • \$\begingroup\$ Let me know if there's any more information I could offer; this is my first time posting in this part of StackExchange \$\endgroup\$
    – Dancrumb
    Commented Feb 18, 2013 at 4:01
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    \$\begingroup\$ Tell us more about the power supply. It would also be helpfun if you could post a picture of the schematic of what you actually built. Make sure to check that the picture that you make does indeed match what you built. Sometimes in preparing such documentation you discover a wiring error in your circuit. \$\endgroup\$ Commented Feb 18, 2013 at 4:08
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    \$\begingroup\$ Do you have a filter capacitor on your circuit board? Or a decoupling capacitor on the 555 supply pins? Try adding a >100uF capacitor across the supply rails on your circuit board, and also a 100nF-1uF ceramic capacitor across the supply also but as near to the 555s power pins as possible. As mentioned the schematic (and scope shot) would help. \$\endgroup\$
    – Oli Glaser
    Commented Feb 18, 2013 at 4:32
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    \$\begingroup\$ 555s have big current spikes on the transitions, so the bypass / decoupling caps are an important part of the circuit. \$\endgroup\$
    – markrages
    Commented Feb 18, 2013 at 5:00
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    \$\begingroup\$ what are the vertical and horizontal setting for your scopeshots? \$\endgroup\$
    – markrages
    Commented Feb 18, 2013 at 5:56

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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.

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  • \$\begingroup\$ Thanks; I've ordered some 100uF caps and I'll let you know how it goes \$\endgroup\$
    – Dancrumb
    Commented Feb 19, 2013 at 5:52

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