The low dropout voltage regulators from the LM2931 series are not just useful for powering microcontroller systems — they can also act as a low-frequency (practically) random noise generator. The random noise could be used in the system for making an ADC (analog to digital converter) produce random numbers, or a digital port purposely ‘go haywire’ depending on the noise level applied to it, or for other purposes you clever programmers may have in mind.
The output voltage of an LM2931A-5.0 is between 5.19 V and 4.81 V. The same parameter for the LM2931-5.0 is specified as 4.75 V to 5.25 V. The ‘Z’ suffix indicates a TO-92 package. Both ICs also supply typically 0.5 mVrms worth of output noise across a 100 µF capacitor within the frequency range from 10 Hz to 100 kHz. When amplified more than 200 times, a noise voltage of typically 100 mV can be obtained, which should be enough to randomly toggle a few bits in a 10-bit ADC with a least significant bit (LSB) equivalent to 0.5 mV. These bits could be used individually or collectively to ‘construct’ larger random numbers.
The schematic shows a power supply providing an output voltage between 4.5 V and 5.5 V and a maximum output current of 80 to 100 mA. Diode D2 lifts the output voltage of the LM2931Z-5.0 IC by 0.6 to 0.7 V. Diode D3 eliminates (approximately) the voltage lift produced by D2. Still on diodes, D1 protects the regulator from reverse voltage.
The output noise of the regulator IC is branched off via C7 and R2 and amplified more than 200 times by T1 and T2. The noise generator. The random noise could be used in the system for making an ADC (analog to digital converter) produce random numbers, or a digital port purposely ‘go haywire’ depending on the noise level applied to it, or for other purposes you clever programmers output resistance of the amplifier is relatively low and the output signal could be used to directly drive the input of an ADCs. The gain of the transistorized amplifier may be changed to requirement chiefly with resistor R3. Components D3, C5 and C6 reduce the impact of the load (typically a microcontroller system) on the noise seen by the amplifier input.
Jumpers J1 and J2 select the minimum load capacitance between D3 and the regulator output, to strike a compromise between a stable operation of the regulator on the one hand and maximum noise output voltage on the other. The jumpers are set or soldered at the final stage when the equipment is tested. Although the circuit will also work with other voltage regulators like the 78L05, it should be remembered that much lower noise levels may be available, forcing the amplifier gain to be raised considerably.