how to avoid interfering with the circuit behavior while measuring it?

[evsc]

hey all.

I am building these little circuit units, that are made up of half a Multivibrator circuit (see circled 'active circuit' in the picture). I want to connect several of them together, and at the same time be able to read out their behavior. The 'measurement circuit' in the diagram was created so an LED could indicate the circuit status with light. Additionally i also connect the units up to a microcontroller (Arduino MEGA in this case) to read out the voltage levels through an analog input.

I notice that whenever i connect more units to analog inputs, the overall circuit behavior changes slightly (f.e. the amplitude of the pulsation goes slightly up or down). As if the measuring takes avoid some current from the original circuit. How can i avoid this? How would i get my microcontroller-input probing to behave like proper oscilloscope probes? I am guessing that it might have something to do with input impedance, but my electronics knowledge is not substantial enough to make sense of it ...

any input would help!!
thanks!

 

[Sceadwian]

Use an opamp as a high impedance input from your circuit, and let it's output drive your tester. If it's the capacitance of the device you're using for testing that's affecting the circuit it might be a little harder to isolate it, so you'll want an opamp with very low capacitance inputs.

 

[flat5]

You might look for a cheap "logic probe".

 

[crutschow]

You might look for a cheap "logic probe".

"Logic probes" only read one or zero logic levels, as you might expect. The op is concerned about the loading cause by reading the voltage with the Arduino analog input. He wants more than just a logic level indication.

Using an op amp as Sceadwian suggested would be one way to minimize the loading. You would connect the op amp in a follower configuration (minus input connected to the op amp output, signal goes to the op amp plus input). For signal supply operation you want a rail-to-rail type op amp.

 

[evsc]

i just tried it with an opamp (LM358) and it seems to work. the interferences seem to have vanished. i'll have to do some more testing to really confirm it, but meanwhile i am very happy it!
so thanks a lot!