# Blinking LED at two speeds using 555 Timer

I'm trying to build a circuit that will blink a single LED twice a second for one second and then once a second for two seconds. I can get the individual frequencies with two different 555 timers but I don't know how to make a circuit that would provide the two frequencies in succession. I know I will need to use two 555 timers but I can't wrap my head around the circuit that would make it work.

Any help is greatly appreciated.

• Any particular reason to not use a MCU? – Ignacio Vazquez-Abrams Dec 8 '15 at 1:58
• I'd like to expand my knowledge of 555 timers. I know they are really useful little chips but I'm not very good at utilizing them. – Anthony Rico Dec 8 '15 at 2:25

This is the most straightforward way that comes to mind. You could use another 555 timer (or an RC circuit equivalent) set to a period of 3 seconds and a duty cycle (ON-time) of 1 sec and OFF-time of 2 secs. The 555 output would be connected to an NPN transistor's (T1) base (standard CE config) to switch it on/off at intervals, according to its duty cycle. An inverter IC (or just another transistor) will also be needed. The supply pin of the 2-sec 555 will be connected to the collector of T1. The collector of T1 will also be connected to the inverter input, while the inverter's output should be connected to the supply pin of the 1-sec 555; this is so that both 555s are never ON at the same time. The outputs of both original 555s should be connected to the LED with a current-limiting resistor in series. A diode should be at each output to prevent current from flowing from one output to the other.

It should all work as follows: At first, the Master 555 output goes HIGH. During the Master 555's 1-sec ON-time, T1's collector is at 0V. Therefore, the 2-sec 555 is OFF and the 1-sec 555 is ON, because of the inverter. The 1-sec 555 generates its waveform within a second. T he Master 555 output then goes LOW, thereby turning off T1, and it begins its 2-sec OFF-time. T1's collector is HIGH, so the 2-sec 555 is ON and the 1-sec 555 is OFF, again because of the inverter. The 2-sec 555 generates its pulse. The Master 555 output goes HIGH again and so on.

Further tuning will be needed of course. An MCU would accomplish this with no more than a few lines and far lesser complexity so I presume you have a good reason for using this method instead.

Flash one: b-----b-----b-----b-----b-----b-----b-----b
Flash two: ---b-----------------b-----------------b---
time       0     1     2     3     4     5     6     7


...Assuming you want it to repeat.

Haven't really thought through triggering the start of timer two so it's properly offset from timer 1, and so it does not drift. Might involve a 3rd timer or a clever trick I'm not thinking of - the third timer seems a straightforward enough method, but the 555 inspires so many clever tricks I'm sure someone has a different way. You could also use some logic and the trigger/reset pins. Remember that there are 556's if you are using more than one.

If you are up for some additional components you could use the output of timer 2 to double the charge rate on the timer 1 capacitor (though due to timing limitations you either need some more externals or a method that inverts the sense of the output so that you double the charge rate for 1 second and not for 2 seconds. "Stock" 555 circuits don't do duty cycles below 50%, but as usual there are ways around that...

Use a single 555, and drive CD4017 decade counter IC. this will activate each of 10 outputs in sequence. Select outputs 1, 3, 6,7,8 to activate the LED.