# Dual Power Supply (Two 9V)

I am Not Good At This, and I am trying to breadboard this circuit here:

http://www.instructables.com/id/DIY-EEG-and-ECG-Circuit/

But I am stuck on the power supply. The instructions state:

Regarding power: the easiest way to power the circuit is with 2 9V batteries. To feed your op-amps -9V to 9V of power, connect one battery the correct way, and one backwards. That is, connect the positive lead of one battery to your positive power supply line and its negative lead to GND (ground). With the other battery, connect its positive lead to GND, and its negative lead to the negative power supply line. To "set" GND, you will eventually connect an electrode from your leg directly to the GND line. This will ensure that "0V" is your leg's voltage (unaffected by any head activity), and that all readings will vary from there.

Which is fine, until I can't actually figure out what rails to attach the two 9Vs to. I think I get the idea that GND happens between the first + and the second - terminal, i.e.

But I can't wrap my brain around how to actualize this. I put the two possible configurations I can imagine on this image below, with the given schematic that I can't entirely understand:

I've been testing it wired as the 2nd configuration, and it Mysteriously Does Not Work. Are the batteries hooked up wrong? If so please help me wrap my brain around how this works and I'll see if my chips are fried or not. Hopefully not.

If the batteries are hooked up right, don't worry about trying to troubleshoot my other problems. I can test it indefinitely if i'm at least assured the power is right.

Here's an example of how one person did it:

But I can't be sure how those wires on the right-hand side end, and I'm not experienced enough to guess.

Thanks very much!

The important thing here is to ensure that the polarity on the op-amp chips is correct.

Figure 1. TL084 pinout.

simulate this circuit – Schematic created using CircuitLab

Figure 2. The TL084 consists of four TL081 opamps in the one package with a common supply.

Your figure (1) is the correct setup.

A few notes:

• The supplied schematic doesn't give the power pin-out so that makes your task a bit more difficult as you have to look it up.
• Note that I have followed the schematic drawing convention of showing the more positive supply voltage on top and negative at the bottom with GND in the middle. The chips are physically the reverse in your layout.
• It may help to understand that because this is an audio circuit that the signals need to swing above and below 0 V. To do this we need the dual supply. In my Figure 1 BAT1 will supply the current for positive-going output signals and BAT2 will sink the current for negative-going output signals.
• Just commented on the other one, this makes a lot of sense to me, especially with the diagrams. Yes, the chips are reversed in the layout, and I had no idea it wasn't standard until I read some more. Inner rail bottom should be +9v regarding the physical layout presented, or at least I wanted it to be. Thank you so much also! Commented Oct 27, 2017 at 21:31
• I appreciate the clarification! Excellent explanation, thank you. Very clearly labelled. Commented Oct 27, 2017 at 21:35

In your option 1, the two outermost power rails will be Ground - they are connected together by the vertical black jumper at the right of the circuit.

The inner top power rail will be the -9 V supply, while the inner bottom rail is +9 V.

• That sounds exactly like what is necessary, thank you. Looks like I had it completely backwards. I need the inner top rail to be -9v and inner bottom to be +9v, as you said. I will switch to configuration 1. I wish I could give you both the checkmark. Commented Oct 27, 2017 at 21:29
• We'll give you some rep and you can upvote Peter's answer. Commented Oct 27, 2017 at 21:33