Need Circuit that switches between ground and no connection (open). Needs to be adjustable

Thread Starter

Cam2363

Joined Feb 2, 2018
15
Title says it all. I need a circuit that acts like flipping a switch where when its on, it outputs ground and off, nothing.

It needs to be adjustable, like a 555 timer clock, but that doesnt work because it switches between power and ground, not ground and open.

If im confusing, ask and ill clear up what im confused about.

Thanks!
 

ElectricSpidey

Joined Dec 2, 2017
770
Depends on just how "open" you need.

If you drive a NPN transistor with a 555 you can get a connection to ground (minus the CE voltage) and a high impedance condition (open). (Or a MOSFET)

But if that is no good just drive a relay with a 555.
 

Thread Starter

Cam2363

Joined Feb 2, 2018
15
Depends on just how "open" you need.

If you drive a NPN transistor with a 555 you can get a connection to ground (minus the CE voltage) and a high impedance condition (open). (Or a MOSFET)

But if that is no good just drive a relay with a 555.
Could you find me a schematic that does that?
 

Tonyr1084

Joined Sep 24, 2015
3,879
The 555 has an open collector at pin 7. When active, pin 7 is ground. When not active, it's open. How much current do you need to pull to ground?
 
Last edited:

ElectricSpidey

Joined Dec 2, 2017
770
Pin 7 is not "open" when being used by the chip for timing purposes.

Here is all you need, as per what you specified, I didn't include any of the details on the 555 because you didn't say what you needed it to do. (astable...mono...etc)

When the 555 output is high the transistor connects output to ground.
When the 555 output is low the transistor is turned off and the output is "open".

Symbols.jpg
 

Tonyr1084

Joined Sep 24, 2015
3,879
Pin 7 is not "open" when being used by the chip for timing purposes.
It is if you're using it as a one shot. Pin 3 could be used as a discharge port. Just put a diode in series and pin 3 can discharge a timing cap just as easily as pin 7.

Only thing I don't know for sure is how much current pin 7 can sink. And the TS hasn't stated how much current he wants to switch - so - - - .
 

wayneh

Joined Sep 9, 2010
16,124
Problem is we're not given enough information to adequately help this post.
I think he has another thread on the topic and is looking to simulate a tachometer signal. So he needs the timing. I thought of pin 7 but couldn’t recall without looking it up how you might use that while still running in astable mode.
 

AnalogKid

Joined Aug 1, 2013
8,220
First, "nothing" is called an open circuit.

What are you switching to ground?
AC
DC
765,000 Vac at 1200 A (when I'm driving in from somewhere, I know I'm close to home when I drive under these bad boys)
voltage
current
frequency
phase of the moon
-
-
-
anything?

ak
 

sghioto

Joined Dec 31, 2017
1,038
I think he has another thread on the topic and is looking to simulate a tachometer signal. So he needs the timing. I thought of pin 7 but couldn’t recall without looking it up how you might use that while still running in astable mode.
You can by using the output pin 3 as the discharge for the timing cap. Pin 7 I recall can sink 20ma max, similar to open transistor output in the LM393
SG
 

Tonyr1084

Joined Sep 24, 2015
3,879
You can by using the output pin 3 as the discharge for the timing cap.
Didn't I say that in post #6 ? ? ?

Looked at a data sheet but didn't find the sink current for pin 7. But if you use pin 3 as a discharge you have to block the high output, hence, use of a diode.
 

sghioto

Joined Dec 31, 2017
1,038
Yes you did. Apparently I misread wayneh 's post, I thought he was asking a question.:D

I could never find any data for pin 7 either, but most circuits show at a minimum a 1K resistor from pin 7 to supply in timing circuits so that's at least 15ma.
SG
 

Thread Starter

Cam2363

Joined Feb 2, 2018
15
It isnt very much current at all. It just needs to act like im flipping a switch connected to ground. So ill have to try what
ElectricSpidey said in post #5. That should work, if not ill try to figure out how to do it with pin 7


 

Tonyr1084

Joined Sep 24, 2015
3,879
It isnt very much current at all.
How much is "it isn't very much current at all"? 20 mA isn't very much current, but what you think isn't much current may be more than pin 7 can handle. Well, you could find out the hard way and see if your chip blows out.
 

Thread Starter

Cam2363

Joined Feb 2, 2018
15
How much is "it isn't very much current at all"? 20 mA isn't very much current, but what you think isn't much current may be more than pin 7 can handle. Well, you could find out the hard way and see if your chip blows out.
I could always do that. But i dont thing it was very much at all. Probably a few mA in not exactly sure. If i blow up the chip ill use a different one.
 

Dodgydave

Joined Jun 22, 2012
8,571
I could always do that. But i dont thing it was very much at all. Probably a few mA in not exactly sure. If i blow up the chip ill use a different one.
Pin 7 has an Open Collector output, and has a Sink capability of 200mA, if you need more current use a slave transistor .

555-Pinout-2.gif
 

Tonyr1084

Joined Sep 24, 2015
3,879
I could always do that. But i dont thing it was very much at all. Probably a few mA in not exactly sure. If i blow up the chip ill use a different one.
Something recommended early on was to use an NPN transistor (base) connected to pin 3 through a resistor. The emitter tied to ground and the collector of the NPN becomes your "OPEN" source or "GROUND". Now you're not putting the chip at risk, but rather, an insignificant transistor which can handle a lot more power than pin 7 or even pin 3 if it were sinking only. But then again, you could forego the transistor and just use a diode tied to pin 3. Now you have a ground source when 3 is low and an open when 3 is high. But now you're limited to the chip's ability to sink to ground. And you'll still need the resistor between the diode and pin 3.

If it's as low a current as you suggest I'd still opt for the reliability of a small NPN transistor hooked up in the manor I described.

[edit] OK Dave beat me to the post. Since he seems to authoritatively state that pin 7 can sink up to 200 mA then maybe if you KNOW you have less than that then you're good to go.

Have at it.
 
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