The 741 is the most famous operational amplifier and due to its characteristics it can be used in a great number of practical circuits, including a bio-amplifier. A version using the 741 is shown in Figure 1.
This circuit uses a voltmeter or a multimeter with a scale of 0 to 1 volts as an indicator. Digital or analog multimeters can also be used.
The reader can also use a 50 to 200 µA galvanometer in series with a 10 k ohm resistor and a 100 k ohm trimmer potentiometer. The trimmer potentiometer can be adjusted to have 1 volt of the full scale.
Choosing the appropriated value for R1, the circuit can reach end scales of 1 millivolt, 10 millivolts, 100 millivolts, or 1 volt.
The reader can add a switch to select the values for R1 according to the following table or simply use the best value for the application he or she has in mind.
| R1 | Full Scale |
| 1k | 1 mV |
| 10 k | 10 mV |
| 100 k | 100 mV |
| 1M | 1V |
As the reader can see, the 741 acts as a x 1,000 amplifier due to the ratio between P2/R3 and R1. P2 is used to adjust the correct gain of the circuit, compensating for the tolerances of the used components.
The circuit is powered from two 9-volt batteries, but because the current drain is very low, the life of these sources will last for several weeks. We don’t recommend the use of power supplies plugged into the AC power line for safety considerations, as in other projects that involve the interaction with people.
P1 is used to adjust the zero offset and the electrodes, as in other versions of the bio-amplifier and in biofeedback projects.
IC-1: 741 integrated circuit operational amplifier
M1: 1 V voltmeter (see text)
R1: See table
R2: 100 k ohm x 1/8 W resistor, brown, black, yellow
R3: 47 k ohm x 1/8 W resistor, yellow, violet, orange
R4: 2.2 k ohm x 1/8 W resistor, red, red, red
R5: 1 k ohm,x 1/8 W resistor, brown, black, red
P1: 10 k ohm, trimmer potentiometer
P2: 1M ohm potentiometer, lin or log
S1: DPDT switch
B1, B2: 9 V batteries and connectors
Other: PCB or solderless board, wires, electrodes, etc.
