One LF411 8 pin Operational Amplifier (Op Amp) ~$4
One PNP transistor model 2N3609 ~$2
Three resistors of values 10k, 1k, 150Ω <$1
A load of your choosing (3 LEDs in the above picture)
Power Supply. In this example, we used a +15 V and -15V commercial power supply.*
*Note: We were also able to power this circuit with two 9V batteries (one for each the + and the -), but circuit produced a current of only 5.4mA with a similar maximum load. Also, the negative terminal battery became very warm quickly, so the power supply is highly recommended. Power supplies can be made by converting an ATX power supply from an old computer such as this instructable by abizar.
Step 1: Assible the Circuit
The Op Amp has 8 pins; the first pin is designated by a small, indented circle. The rest of the pins are enumerated around counter-clockwise. The connections shown to the V- terminal is pin 2, V+ is pin 3, and the point of the triangle connected to the transistor is pin 6 . In addition, pin 4 should be connected to -15V and pin 7 to +15V (not shown in circuit diagram).