Discrete LED Fading Circuit Using 5 Transistors


Circuit Description:

This idea uses only descrete parts and no ICs to fade a pair of red LEDs. The circuit employs 5 transistors, 8 resistors, 2 red LEDs and one capacitor. The ramp output is not as linear as the circuit using the 1458 opamp, but the visual effect is fairly close. See the scope picture below that illustrates linearity. It's about the same as you would get using a 555 timer. The circuit is basically the same as the Schmitt Trigger Oscillator in the misc section, with the addition of two NPN transistors in a darlington arrangement to buffer the ramp output at the top of the capacitor. The transistors can be almost any general purpose NPN, PNP types such as 2N3904, 2N3906. Ramp time (up or down) is approximately 3/4 RC so that a 100uF cap and 20K resistor yields about 1.5 seconds. The LED peak current is about 20mA. This is worked out from the peak ramp voltage of 6 volts minus the 600mV E/B drop on each of the transistors (Q1 and Q2) and also minus the LED voltage of 1.5. This leaves 6 - 0.6 - 0.6 - 1.5 = 3.3 across the resistor in series with the lower LED. The resistor value will be R = 3.3 / 0.02 = 165 ohms, or 150 standard value. Since the collector current of Q1 is very close to the emitter current, the second LED can be placed in series with the collector of Q1 without any additional resistor. Adjustments to points where the LEDs just extinquish and reach maximum brightness can be made with the 20K resistor at the collector of Q5. Lowering the 20K will increase the ramp amplitude and cause the LEDs to extinguish earlier for a longer period. It will also increase the peak LED current at the top of the ramp which can be readjusted with the 150 ohm resistor so the peak LED current remains at approx 20mA.

Scope display of the triangle waveform as seen on the capacitor

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