need help on noise protection

[black_wing]

i created a project for my subject in my school and found out that it was prone to noise comming from other devices especially cellular phones. when ever i try my device on my home, it works perfectly, but when i brought it to our school in a room with other computers and aircons and with some people using their cellular phones, my device didnot work well, can anybody tell me how to create an easy to build shielding for my device? my current casing is an acrylic glass because they required me to use a see through casing... thank you very much..

 

[audioguru2]

Hi Black Wing,
I make many audio things and a few are RF. I keep the internal wiring compact by using Veroboard (never a breadboard) and use shielded audio cable on high impedance inputs. They don't pickup interference when mounted in plastic boxes.

My latest Sound Level Indicator project has a very high gain so even though its mic is close to the circuit's input I used a short piece of shielded audio cable to connect it.
Attach your schematic and a pic so we can see where the interference gets into your project.

View attachment 38368

 

[black_wing]

>>audioguru

here are the immages of what i have made...

the device will be connected to the computer via the parallel port.. it accepts input by connecting it to the human body by the use of an electrode..

 

[audioguru2]

Hi Black Wing,
An amplifier for ECG pickup uses an Instumentational Opamp specially designed with true differential inputs and the gain setting resistors with extreme accuracy built-in to avoid hum pickup.
Your circuit has a low 10k input impedance to high frequecies and is -3dB down at 160Hz on its inverting input and has a high impedance and very good low frequency response on its non-inverting input. It also doesn't have nearly enough gain for an ECG.
I don't know why the output is filtered so heavily with the transistors.

This is a very good differential inputs very high gain ECG amplifier that uses an instumentational opamp:

View attachment 38386

 

[black_wing]

>>audioguru

by the way sir.... the circuit is not for ECG...its for EMG..

the circuit's original design is to generate a sound when you contract your voluntary muscle...i just replaced the two 250Kohm pot with two 1Mohm pot. i also removed the Q3, R13, and the speaker...

i could not cound the website where i found the pdf file, but i still have it...i just dont know where to uplod it so i can link it from here

 

[audioguru2]

You need 3 opamps to make a true differential amp.

View attachment 38388