555 TIMER : ASTABLE MODE:
KEY POINTS: 1) You need 1/3 of the supply voltage (3V in this case) on the TRIG pin (2) to start the timer. 2) You need 2/3 of the supply voltage on the discharge capacitor (22 nF here) to stop the timer. 3) Resistance between Pin 7 (DIS) in conjunction with the discharge capacitor (22 nF) that is connected to pin 6 (THR) drives the length of the pulse. EXPLANATION: Pin 2 gets a voltage < 3V and the timer starts = pin 3 (OUT) goes live and current flows through the LED. Meanwhile The discharge cap (22 nF) starts charging. As it does so, voltage on this cap rises until Pin 6 (THR) detects 6V and shuts off Pin 3 (OUT) and the current that was going to the LED. The cap now starts discharging through the pot, and a 10 K ohm back to Pin 7 (DIS). As it does so, the voltage decreases until we breach 3V. This is detected by Pin 2 (TRIG) once again and Pin 3 (OUT) lets current flow through once again. The cycle repeats itself. SIDE NOTES: 1) The charging of the cap goes through 2 resistors (10 K ohms each) and the pot. The discharge of the cap to pin 7 (DIS) goes through the pot and 1 resistor (10 K ohms). Therefore, it charges slower than it discharges. 2) the experiment in the book uses a speaker, I use a LED and a slow pulse cycle so that EveryCircuit can find a solution (it may crash otherwise). 3) The section with the discharge capacitor (22 nF) and the potentiomer + resistor is basically a RC network. If you understand RC networks and time constants, you can play around with the values of the cap & resistance in order to change the value of a time constant and therefore the length of the pulse. 4) Remember that the pin sequence of a 555 in EveryCircuit is very different from what you find on a typical 555 chip sold in the market and you need to keep that in mind in order to understand what is going on (do not mistake which pin is which). 5) You can see the frequency in Hz being generated in the graph at the top. You can play around with the % of the wiper on the potentiometer and see the impact on the frequency.
