Bi-Color LED Driver

The circuit described here that senses the charge of a control circuit in a sound effects generator and lights a Bi-color LED red if the charge is positive or green if the charge is negative is the one circuit in all of my electronic construction that I designed completely from scratch.

The sound effects generating integrated circuit at the core of the Sound Effects Generator unit is complex and capable of producing a wide range of sounds. It can generate two phases of a single sound like a train fading into the distance with an intermittent whistle from the train. One control circuit can begin the fading of the train sound and the second control circuit turns the whistle on and off. Seeing the state of these control circuits as on or positive (red color) or off or negative (green color) could help adjust the sound effect. I did not want the Bi-color driver circuit to affect the circuit it was sensing so I wanted it to have a very high signal input resistance. In addition, the circuit had to get current to run in two directions to light the LED red or green and I had available only a 9 volt dc power supply that ran the rest of the unit.

The circuit I designed worked perfectly. Before I installed it, I tested it by touching the input lead with my right hand and touching the negative side of an AAA battery with the other hand. The LED lit green. When I touched the positive end of the battery with the other hand, the LED lit red. Just the few electrons added to me or removed from me by the small 1.5 volt battery were enough to drive the LED. When I just touched the input lead with no battery, the LED lit orange. The LED was flashing red and green at high speed (and looking orange) as the driver sensed electromagnetic alternating currents I was intercepting like a radio aerial.

This is what the circuit looks like:

Figure 1. Bi-Color LED Driver Circuit Diagram

If you’re not familiar with electronics and circuit diagrams, this will not have any meaning for you so I’ll describe the circuit component and interaction as simply as possible to start: the transistors Q1 and Q2 are switches that respond to the charge at the input. If the charge is negative, both switches are off and current flows from right to left and lights the green LED. If the charge is positive, both switches are on and current flows from left to right and lights the red LED.

To describe the circuit in some more detail, the circuit reads from left to right and starts with a signal coming in to the circuit. If that signal is negative, it turns off the first transistor (Q1) and that turns off the second transistor (Q2). When Q2 is off, the left side of the Bi-color LED (which is next in line) is at 7 volts. The right side of the Bi-color LED is more negative, since it is being held at 5.1 volts so electrons (current)flow from right (negative) to left (positive) and the green LED lights.Conversely, if the input signal is positive, it turns on the first transistor (Q1) and that turns on the second transistor (Q2). When Q2 is on, the left side of the Bi-color LED is at 3 volts. The right side of the Bi-color LED is held at 5.1 volts and is less negative than the left side so current flows from left to right and the red LED lights.

That’s it in a “nutshell”. What follows is a more detailed description of the “nut”.

This circuit does not affect the control circuit it is sensing because Q1 is a Field Effect Transistor (FET). A transistor is a triode, a three layer solid state device where an input in the middle (here called the gate) controls the flow of electrons from the source at the bottom to the drain at the top. Q1 differs from Q2, a standard transistor in that the FET has a very sensitive input that’s shielded from the other parts of the transistor but the distances between these parts are very small. So a tiny charge at the gate has a big effect on the very narrow channel between the source and drain. If the input to this FET is negative it prevents current flowing from the source to the drain and Q1 is off. (Conversely, a positive gate in an N-Channel FET will allow a large current to flow from source to drain.)

Q2 is a PNP transistor. A transistor is a triode, a three layer solid state device where an input in the middle (here called the base)controls the flow of electrons from the emitter at the top to the collector at the bottom. PNP refers to the three layers of the device which are relatively Positive, Negative and Positive. (There is an NPN transistor where the layers are Negative, Positive and Negative respectively.)

If Q1 is off, the charge at the input of the base of Q2 is positive since Q2 input is “seeing” the positive 9 volt line through a variable resistor. When the base of a PNP transistor is positive, no electrons flow from the emitter to the collector so, like Q1, it’s emitter is “seeing” the positive 9 volt line through a variable resistor or a voltage a bit less than 9 volts,specifically 7 volts.

The right side of the Bi-color LED unit is held at 5.1 volts by a solid state component called a Zener diode. A diode is a two layer solid state device that has a positive layer and negative layer and permits current flow in only one direction. It has many uses including as a rectifier that converts alternating to direct current. A Zener diode has the ability to set a specific voltage above it when it is placed in a circuit with its negative side facing the positive side of the circuit. An ordinary diode would simply act like an open circuit and prevent current flow but a Zener diode in that configuration, “leaks” enough current to set a precise voltage above it.

Using a Zener diode set to 5.1 volts (a voltage between zero and the 9 volts powering the system) allowed me to get current to flow in both directions through the Bi-color LED depending on whether the voltage on the left side of the LED unit was closer to zero or 9 volts, specifically, 3 volts or 7 volts. I limited the voltage difference between the left and right sides (using the resistors in line with the transistors) to 2 volts (7 minus 5.1 for green or 5.1 minus 3 for red) in order to optimize the brightness of the LEDs. 

Note: In the circuit diagram, the arrow inside Q1 that is pointing to the left at the “G” (for gate) should be pointing at the right on the right side of that line, as this is the correct symbol for the N-Channel FET.

As drawn, it would indicate a P-Channel FET.