Actually we are making a working model of nerve impulse and so.. we want to produce -70 mv and +30 mv so that we can show that deflection in a voltmeter. Please help..

Why do you want to produce 70 millivolts?

A simple circuit such as the attached should do it for you. The 7805 IC is a voltage regulator - it produces a constant 5V DC output. That 5 volts going to the the pot and 120K resistor form a simple voltage divider that produces from 0 to 5/120 = 41mV. You can replace the 1K pot with a 1.6K resistor to get 67 mV output, or replace it with a 750 ohm resistor to get 31 mV output.


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