I am having an issue with a circuit I am working on.

I am running some logic (2x4017s a 4081 and a 40106) from an 8 volt regulated supply (coming from a 7808), this in turn is being powered by a 12v transformer through a 1 amp diode bridge.

The circuit is designed to flash a light every 2 seconds.

My issue is....

When I have my oscilloscope probe attached to the circuit it works perfectly, my flash rate is steady and I get a good trace on the scope. As soon as I remove the probe however everything just stops.

I am presuming this is a grounding issue but I have built 2 of these units and have exactly the same problem on both. I have gone through my circuit designs and cannot see where the issue might be coming from. My most recent thought was that my 0.1μF decoupling caps on each IC might be too big (I am clutching at straws now), but I dont see why it would happen.

Any suggestions would be greatly appreciated.

 

Well... by looking at your schematic I see the problem in the area of...

oops, I have to guess...

The scope ground is typically connected to the AC neutral ground. If your circuit works only when the scope is connected you have two faults: your supply is not isolated from the AC line, and the supply is not connected to your circuit. That's how the scope ground alone makes this all work.

Get a volt/ohm/meter and start checking. With the circuit off (and the scope unconnected) you can use the ohms scale to find the open, or turn it on and use the voltmeter to see where there is voltage (on the power return) where none should be.

Don't trust your connections, you may think things connect but it is still an open from a bad solder joint or such.

 

Well, certainly the decoupling caps are not the problem

What are you using for an oscillator to flash the LED? Is it possible that your oscillator only oscillates with the scope connected to it?

Can you post a schematic?

 

Do you have your dc ground (8 volt supply) connected to your ac (120volt power) ground

 

You didn't say where you attached the scope probe, but I can tell you from experience that the capacitance of the probe has led me to find a problem...right after I realized that the probe is not "ideal". Most of this discussion is about the fact that the scope probe becomes part of the system when you attach it. The probe has both resistance and capacitance to ground.