Non contact AC voltage tester

[Karthik rajagopal]

Hi all,
I recently made a non contact AC voltage tester with IC 4017. It works well . In the video that I saw the person told that the clock input of this ic would detect the emf produced which causes the LED to blink at a higher rate. My circuit works just as he said but it also detect DC voltages that are produced in AC to DC adaptor. How is that able to detect DC voltages? If so it detects then why only the DC current produced in adaptors and not in normal DC batteries? Please help not able to understand the logic behind.
The video is :

 

[Bluejets]

Those things have a fairly broad pickup....they can detect AC over the wiring.

 

[Karthik rajagopal]

Those things have a fairly broad pickup....they can detect AC over the wiring.

Do you mean the ac voltage is not completely converted to DC? Please explain.

 

[Alec_t]

AC-DC adapters are not usually well screened, so stray radiated EM fields are present which can be picked up by your body and by nearby wires, such as the one you are holding in the pic.

 

[Karthik rajagopal]

AC-DC adapters are not usually well screened, so stray radiated EM fields are present which can be picked up by your body and by nearby wires, such as the one you are holding in the pic.

Thank you for the reply.

 

[davenn]

AC-DC adapters are not usually well screened, so stray radiated EM fields are present which can be picked up by your body and by nearby wires, such as the one you are holding in the pic.

yup as a result, any reading you got would be totally unreliable

 

[Karthik rajagopal]

yup as a result, any reading you got would be totally unreliable

But I have heard many saying all microcontrollers need a very precise DC voltage for them to function. Then how are they able to work with voltages from AC to DC power supply?

 

[Alec_t]

The power supply to a microcontroller needs to be well decoupled (using low-impedance capacitors) so that stray EM fields are 'shorted to ground' and don't adversely affect the micro's performance.

 

[Karthik rajagopal]

The power supply to a microcontroller needs to be well decoupled (using low-impedance capacitors) so that stray EM fields are 'shorted to ground' and don't adversely affect the micro's performance.

Does the value of capacitors used in decoupling differ for different IC 's?

 

[Alec_t]

In general, yes. ICs drawing heavy current will normally require greater decoupling than those drawing light current.

 

[BobK]

Typically, you need a 100nF ceramic cap near each pair of power pins.

Bob

 

[Karthik rajagopal]

Thank you for the answers.