Visible electric current circuit

Thread Starter


Joined Feb 7, 2009
I'm planning to build a variant of this circuit as a mini project for my design class.

I need help analyzing it. Firstly, I haven't quite understood the operation of the precision full wave rectifier in the circuit. When I simulate it on Multisim, I don't get a full wave rectified output across the diodes. Is it a full wave rectifier? This is the designer's explanation of the circuit

Secondly, what is the function of the transistor 2N3904 in the circuit? Thirdly, it's output goes to a series of NAND gates, whose function I am unable to figure out.

I will greatly appreciate any help you guys can provide me.


Joined Apr 20, 2004
Take a look at the connections between the diodes in the feedback for the first half of the TLO72 and the inputs of the other half. The second half is a difference amp. It is what keeps the output at one polarity.

You can't drive LED's right off logic - or don't if you want the IC to last. Then 2N3904 switch current through the LED's.


Joined Nov 13, 2008
I think the transistor he is asking about is the one connected to the pot.
As I see it, it is part of the direction circuit.
"The polarity signal is level-shifted and is used to control the fwd/rev direction of a shift register."