556 7556 question

Balok · Post by **Balok** » Thu Sep 27, 2007 1:01 am

I'm a bit puzzled by something. Where on the 556 and 7556 do pins 3 and 11
CONTROL VOLTAGE (1) and CONTROL VOLTAGE (2), respectively, go? The circuit works with them hanging in the breeze, but shouldn't they go high or low? Can't find a reference to what they are even for. Controlling voltage might be a good guess. What say you gurus?!

macfrank · Post by **macfrank** » Thu Sep 27, 2007 9:09 am

Nate, those are the equivalent of the Control Voltage on the 555. Most people leave them open, but this can cause erratic operations if the pin picks up stray voltage from somewhere. Most datasheets show the pin bypassed to ground with a 0.01uF capacitor.

Frank

Balok · Post by **Balok** » Thu Sep 27, 2007 12:01 pm

So they're what's called the BYPASS on the 555 then. That makes sense. OK Frank, thanks for that bit of knowledge.

Nate

macfrank · Post by **macfrank** » Thu Sep 27, 2007 2:15 pm

Balok wrote:So they're what's called the BYPASS on the 555 then. That makes sense. OK Frank, thanks for that bit of knowledge.

Nate

Really? I've always seen it described as the "control Voltage". Here's a good page describing the 555 in detail. I've used the CV input to modulate the output, but it's a generally useless pin for making LEDs flash.

It's probably safe to leave this pin disconnected in an astable circuit; the worst that'll happen is that you'll get one or more pulses that are not the correct width. In a monostable circuit, or when you need an accurate waveform, you really should bypass the pin to ground.

Frank

Balok · Post by **Balok** » Thu Sep 27, 2007 2:34 pm

Yeah the CMOS cookbook calls it the bypass. Thanks Frank. I'm just flashing in the astable mode. Hopefully I can get enough current through each side to run 14 25mA LEDs. We'll see. I have my doubts.