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Cheap Noise Generator

Cheap CircuitsCheap Circuits wrote 09/03/2020 at 09:53 • 3 min read • Like

This article is about a cheap noise generator that I made.

I found a cheaper solution to the circuit constructed in this article:

https://www.instructables.com/id/Random-Number-Generator-2/

I used an old black box from a very low frequency (VLF) noise generator, disassembled the circuit and used the parts to make a cheap audio noise generator:

Step 1: Design the Circuit

I designed the circuit with two simple BJT transistor feedback bias amplifiers:

The input signal comes from EMI (electromagnetic interference) in a loose 20 cm (in length) insulated wire connected the amplifier input.

You might be able to increase the circuit gain if you change Rc1 and Rc2 values from 1 kohm to 10 kohms. However, reducing the collector biasing current can also reduce the transistor current gain. It all depends on the transistors that you use. 

Different transistors have different current gain due to production tolerances. Transistor current gain can also changes with temperature and aging.

Step 2: Simulations

I used the old PSpice simulations software that has a quick user interface.


Transient:

Frequency:

You might not be able to obtain such as high bandwidth with general purpose transistors. 

Step 3: Make the Circuit

I did not include the RC power supply filters in my circuit to reduce the cost of my circuit because I have high current/low internal resistance power supply.

You do not need a high power resistor to drive the load. I did not have any other resistors in stock. 

I also omitted Cb1 capacitor that is only needed for preventing the input for affecting the biasing of the first transistor Q1.

Step 4: Testing

I connected the output to USB oscilloscope:

The noise generator was generating noise even when I was not touching the yellow input wire with my fingers.

I noticed that the power supply needs to be at least 3 V. When I reduced the power supply to 1.5 V (the minimum supply voltage for bipolar junction transistors), the noise signal magnitude fell to a very small value, because reducing supply voltage also reduces the transistor current gain.

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