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CIRCUIT NOTEBOOK Interesting circuit ideas which we have checked but not built and tested. Contributions will be paid for at standard rates. All submissions should include full name, address & phone number. Biofeedback for stress management This circuit connects to a pair of skin electrodes on your scalp and gives you a ‘stress level’ reading. If you can see how your stress level varies immediately, it will help you to determine when you are particularly stressed and also what you need to do to relax. It measures the magnitude of the alpha waves produced by your brain, which have signal frequencies in the 4-11Hz range. A small amount of these waves (in the microvolts range) reaches the surface of your skin and can be picked up using electrodes. These signals are fed to a TL071 JFET-input op amp to be amplified to a level that can be read out on a digital voltmeter. A small bias current (less than 750µA, varying depending on skin resistance) is applied through the skin electrodes. siliconchip.com.au The AC alpha waves are coupled to the op amp input via a 1µF MKT capacitor, and they are DC-biased to around -26mV by the resistor network following that capacitor. Higher-frequency signals are filtered out using a 1kW/1nF RC low-pass filter plus a series ferrite bead. This filter also helps to eliminate any RF that is picked up by the electrode leads. The 1µF coupling capacitor and 39kW resistor form a high-pass filter with a -3dB point of 4Hz to eliminate very low frequency signals, below the range of alpha waves. The gain of this op amp stage is adjusted using trimpot VR1. It can be up to about 1000 times (1MW ÷ 1kW). The signal is then fed through a 15kW/1µF RC low-pass filter which has a -3dB point of 10.6Hz. This filters out most Australia’s electronics magazine signals above the 11Hz maximum frequency of alpha waves. The signal is then rectified by diode D3, which also reduces the reading by about 0.2V, eliminating noise from the results. A DMM set to read millivolts connected across the output terminal therefore gives a reading proportional to your alpha wave activity, with VR1 correctly adjusted. The circuit runs from a pair of 9V batteries which produce an approximately ±8.4V split supply after reverse polarity protection diodes D1 & D2. 100µH chokes reduce RF pickup from the battery leads while 47W resistors and 100µF bypass capacitors provide further supply filtering. David Strong, Penshurst, NSW. ($105) April 2021  61