[sdiy] 555 osc lock-up???

[Scott Gravenhorst]

"George Hearn" <georgehearn at btinternet.com> wrote:
>I'd agree with all the responses, to avoid lock up, and un-necessary noise
>coupling via the power rail:
>
>1. Use CMOS 555's (7555?)

7555 is the correct number, tho sometimes there are letters in front of 7555.

>2. Small impedance in the power supply path of 555 each chip, could try
>100R, with large 10u and small .1u decoupling for each chip separately.

The 100R resistor can be replaced with an LED.  LEDs work better than small signal
diodes for this.  The performance is a bit better than a 100R resistor.

>3. Separate ground returns.
>
>I don't tend to use 555's for exactly the problems you are having.  They
>have a tendency to talk (shout) down your power rail!  George
>
>-----Original Message-----
>From: synth-diy-bounces at dropmix.xs4all.nl
>[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Paul Perry
>Sent: 29 December 2009 12:44
>To: synth-diy at dropmix.xs4all.nl
>Subject: Re: [sdiy] 555 osc lock-up???
>
>Yes, using CMOS 555s will help, and separate power supplies.
>But in the absence of separate supplies, it might be worth trying to
>decouple the 555 circuits from the rest of the circuit and each other
>by running the power to them via a 220 ohm resistor with a .1
>ceramic and 10u electro bypass on the 555 side.
>
>Also, the coupling may well be (probably is!) via a shared ground
>return for the two 555 sections.. try to get a direct path back to the 
>power supply ground for both of them.
>
>paul perry Melbourne Australia
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-- ScottG
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-- Scott Gravenhorst
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