I am a beginner in electronics and have been playing around with simple LED circuits and using Ohm's law to determine what size of resistor to use to limit the current to the LED. I have also been studying how resistors and capacitors work in series and parallel. I think I have at least a fundamental grasp of these theories. I have since moved on to diodes and now transistors and how they are used in simple circuits, again using LEDS. I found the following circuit which uses two transistors, capacitors and resistors to alternately flash two LED’s but I am struggling to understand how it works.
How I think it works is this:
- Voltage is applied to the base of Q1 from capacitor C2 thus switching it “on” which allows charge to flow through to LED1 which lights it.
- At the same time, capacitor C1 is charged.
- When C1 is charged it begins to discharge into the base of Q2 thus switching it “on” which allows charge to flow through to LED2 which lights it.
- At the same time, C2 is fully discharged which switches Q1 “off” causing LED1 to go off.
- Eventually, C2 is again fully charged and the cycle repeats.
But I have a few questions which I cannot seem to answer.
- I believe that the resistors R1 and R2 are the current limiting resistors for the LEDs, but what are resistors R3 and R4? Do they control the rate at which the capacitors charge and discharge? Or do they have some other purpose that I am not grasping?
- There seems to be a “chicken and egg” situation here. When the circuit is unpowered and both capacitors are discharged, what causes either of the transistors to be “switched on”? Where does the voltage for the base of either of them come from?
- And a more general question: What "triggers" the capacitors to discharge?
I have read articles about how a transistor works at this link: http://amasci.com/amateur/transis.html and it has helped some, but the simple circuit above is still fuzzy to me.
Can someone help me to understand this so it “clicks” in my head?
I have also looked at the question below but it has not cleared it up for me.