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jeever 26/11/2023
Easy to solder amd build LED Christmas tree As we are used to from such deliveries from China, the components are squeezed together in a far too small plastic bag. Fortunately, there are no fragile parts inside! No manual is included, but several building descriptions can be found on the internet. There is nothing to criticise about the quality of the components supplied, which is good. However, in some kits, all the LEDs supplied are transparent and then you cannot see what colour they emit. The Christmas tree consists of two identical circuits, built on two almost identical PCB. Once these PCBs are fully soldered, you need to slide them together and mount them on the lower PCB. Even such a cheap kit comes with double-sided, metallised PCBs. The standard schematic of an astable multivibrator with two transistors has been extended with an extra stage. Basically, three RC networks C1/R3, C2/R5 and C3/R1 control the base of the transistors. Such a capacitor puts a negative voltage on the base at some point in the cycle. That negative voltage flows through the resistor to the positive power supply. The moment the voltage at the base rises to about +0.65 V, the relevant transistor starts to conduct. The collector voltage goes to zero, the LEDs in the collector light up. The negative voltage jump on the collector creates the negative voltage on the base of the next transistor. In this way, the system maintains itself and the three transistors Q1, Q2 and Q3 take turns to conduct.
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