2N3904 PNP transistor
2N3906 NPN transistor
47 ohm - 1/4 Watt resistor
1K ohm - 1/4 Watt resistor
470K ohm - 1/4 Watt resistor
10uF electrolytic capacitor
0.01uF ceramic disc capacitor
5mm red LED
3V AA battery holderOptional:
10K ohm - 1/4 Watt resistor
This next schematic may look daunting, but it is actually rather straight-forward. It is using all of the parts that we have just gone over to automatically blink an LED.
Any general purpose NPN or PNP transistors should do for the circuit, but should you want to follow along at home, I am using 293904 (NPN) and 2N3906 (PNP) transistors. I learned their pin layouts by looking up their datasheets. A good source for quickly finding datasheets is Octopart.com
. Simply search for the part number and you should find a picture of the part and link to the datasheet.
For instance, from the datasheet for the 2N3904 transistor, I was quickly able to see that pin 1 was the emitter, pin 2 was the base, and pin 3 was the collector.
Aside from the transistors, all of the resistors, capacitors, and LED should be straight-forward to connect. However, there is one tricky bit in the schematic. Notice the half-arch near the transistor. This arch indicates that the capacitor jumps over the trace from the battery and connects to the base of the PNP transistor instead.
Also, when building the circuit, don't forget to keep in mind that the electrolytic capacitors and LED are polarized and will only work in one direction.
After you finish building the circuit and plug in the power, it should blink. If it does not blink, carefully check all of your connections and orientation of all of the parts.
A trick for quickly debugging the circuit is counting components in the schematic versus components on your breadboard. If they don't match, you left something out. You can also do the same counting trick for the number of things that connect to a particular point in the circuit.
Once it is working, try changing the value of 470K resistor. Notice that by increasing the value of this resistor, the LED blinks slower and that by decreasing it, the LED blinks faster.
The reason for this is that the resistor is controlling the rate at which the 10uF capacitor is filling and discharging. This is directly related to the blinking of the LED.
Replace this resistor with a 1M potentiometer that is in series with a 10K resistor. Wire it such that one side of the resistor connects to an outer pin on the potentiometer and the other side connects to the base of the PNP transistor. The center pin of the potentiometer should connect to ground. The rate of blinking now changes when you turn the knob and sweep through the resistance.