![]() We need close to the same times for on and off so that we have time to make the LED fade on and off in about the same amount of time (one second for each fade) so that it looks good. If I build the circuit and have a plug-in for only a capacitor, it works as desired with almost equal on and off (1 and 0) times. I can never get the off time of the square wave close to the same time as the on time. A quick note about the 555 chip: I always have problems with this chip when I use a proto strip to test the circuit. The next circuit uses a 555 timer IC for the square wave. Schmitt trigger inverter driven fade on-off circuit. I listed the C1 capacitor value I used to get close to this in Schematic 3. The desired timing was to fade on and then off in two seconds. The capacitors were selected by testing out various values, and then working from there to increase the values until a good-enough triangle wave was the output to drive the LED at approximately the required period/frequency. That worked fine for fading the LED on and off. In the end, I didn’t bother with a sine wave when I made it look something like a triangle wave. Then, I followed that with a simple resistor-capacitor filter circuit to make something like a sine wave. The circuit in this class led me to using one inverter of the hex Schmitt trigger inverter (SN74LS14/ CD40106BE) to generate the square wave. This is a membership site and costs money every year unless you join as a life-time member for ~ $500. Another site he talked about and runs is. In this class, he gave a very interesting use of an inverter. I signed up for it to see if it was good for use with my grandchildren. However, the first circuit I put together came from an idea inspired by a beginner’s class held by Oyvind Dahl (his free beginner’s course is at ). This led me to conclude that a simple transistor circuit could be made for the fade on/fade off circuit. While working on the above two circuits, I learned that an ultra-bright white LED can be made to fade on and off with different resistance values for R1. For more complex jobs, I found a site that helps one figure out which transistor should be used and why at. Your experience may be different than mine. The object is to get the 0.7V needed to turn the transistor on. ![]() I used a 100 KΩ variable resistor for some transistors. If you use different transistors, you may have to use different R1 and R2 values. If you need it, here’s Clue 2: Think current. Can you figure out why? Here’s Clue 1: In this case, it really doesn’t matter, but in some other cases it definitely could. NOTE: I was told by a friend that it would be better to put the variable resistor in the R2 position from the base to ground rather than in the R1 position. Q1 for the NPN was a BC549CTA NPN bipolar SI transistor I also tested 2N4401 and 2N3904.Q1 PNP = 2N3906 (I first tried a RadioShack Red PNP transistor, no other markings).I standardized on nine volts because of two circuits I found for making a square wave and a sine wave: I originally used 4.5 volts as the source and didn’t change the resistor values when I used nine volts. These both worked just fine, though the values that worked best for the resistors were a bit different for each. ![]() I came up with the test circuits shown in Schematics 1 and 2. I decided I’d use this information to build two circuits - one for the NPN and another for the PNP transistor - to simply drive an ultra-bright white LED. Specifically for the NPN, it was for the PNP, it was. ws for the two types of bipolar transistors. I started with the very basic transistor schematics I found on. I’ll start by showing the two circuits I put together which demonstrate that - at least sometimes - one can use NPN and PNP transistors for the same task. The only relationship between these two questions is a transistor. Does anyone have a circuit that does not require an IC?Ġ6193 - Transistor Confusion, Donald Bodine, Middleham, UK What determines which type of transistor to use in a given circuit? Are they interchangeable with those of a different type that I may already have on hand? The questions were:Ġ6192 - LED Fader, Cindi Carrillo I need a simple method to slowly fade an LED from bright to dim, then to bright again in about two seconds, then keep repeating. This article started as an attempt to answer two questions posed in the Tech Forum here in Nuts & Volts.
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