Separating the NE555

JAMESveeder · Joined: Nov 16, 2010 Posts: 34 Location: tucson

Okay, so I've come to a problem that I can't seem to figure out.

Basically, I'd like to have two 555 chips running at the same time, but not modulating each other. IE two square waves into a mixed signal, not one modulating the other's frequency.

Is this possible? Is there anyway to isolate the two?
Any help is greatly appreciated!

Tim Servo · Posted: Thu Mar 31, 2011 4:26 pm Post subject: Separating the NE555

Hey James,

The 555 is notorious for pulling a lot of current when it triggers, creating a short 'spike' in the power supply. There are two things you need to do that can help minimize or even eliminate this:
1) Use a decoupling cap or even two on the power supply going to each 555. Put the caps as close as possible to the chip. If you use two caps for each chip, use two slightly different values like .001uF and 0.1uF. Otherwise, just use a single 0.1uF next to each chip between the power pin and ground.
2) Use the CMOS version of the 555. This is known as the 7555, and this chip pulls MUCH less current when it triggers than the standard version. This is a common fix for synths that have a 555-based VCO (like the PAiA Fatman) to keep interference or phase locking to a minimum.

You might also want to keep the power supply lines for each 555 separate, joining them only at the main power input to the board. Hope this helps.

Tim (short and spikey) Servo

JovianPyx · Posted: Fri Apr 01, 2011 8:18 am Post subject:

Personally, I wouldn't use a bipolar 555 timer in a music device. As Tim points out, they crowbar the supply lines when the output (pin 3) changes state. The 555 timer is really an industrial timer meant to, um, time things. Like a toaster or a microwave oven.

The 7555 timer IC will do everything that a 555 will do, it is pin for pin drop in compatible and will NOT crowbar the supply which leads to softsynch and other problems.

7555 timers are a far better choice for generating sounds IMO.

Clack · Posted: Fri Apr 01, 2011 9:41 am Post subject:

I noticed on the EDP Wasp that they put 10R resistors on the +v rail and Gnd (its Single supply) does anyone know if thats the reason they did it? so that it sucks less current on a trigger

(in master oscillator PDF on the left)
http://www.elby-designs.com/pixie/pixie-about.htm

JAMESveeder · Joined: Nov 16, 2010 Posts: 34 Location: tucson

Ahh thanks for the reply, guys. Tim, I'll definitely go ahead and give your suggestion a try. Hopefully that will help a bit. And Jovian, I wish I would have figured that out before I ordered the NE555s, I'd rather not wait another 3 weeks for a low voltage 555 chips unless I have to.

I'll see what happens with your suggestion too, clack. Maybe a combination of both.

So much to learn, thanks for the suggestions guys. I'll tell you how it goes.

inlifeindeath · Joined: Apr 02, 2010 Posts: 316 Location: Albuquerque, NM

Feel free to correct me but IIRC, typically smaller value resistors on the power line protect the chip from incorrect pwr supply hookup. I believe it will fry the resistor (also sometime a ferrite bead) before it will fry the chip.

JovianPyx · Posted: Fri Apr 01, 2011 12:13 pm Post subject:

JovianPyx · Posted: Fri Apr 01, 2011 12:18 pm Post subject: