555 Timer - Monostable
August 6, 2013
When I was at school, I had a strong interest in electronics. This hobby eventually led me to doing a computing degree, and gave me the career I have today. Recently, I found my box of old electronic components, and discovered that it was 20 years since I last did anything with them. I decided it was time to refresh my knowledge.
I bought a copy of Make: Electronics: Learning Through Discovery
555 Monostable
I'm now going to use this blog to document some of the stuff I've re-learned. First of all, I'm going to make notes on using a 555 timer in Monostable mode. Monostable means that, when triggered, it emits one pulse and then resets itself. We can control the length of this pulse by changing the value of a capacitor and/or a resistor.
The 555 timer is an eight legged chip. The pins, numbered from top left anticlockwise are:
I've always driven my 555 timer circuits off a 9 volt PP3 battery, so that should be connected to pins 1 and 8.
Pin 2 is the trigger - this is what tells the timer to do something. When the voltage on this pin is below 1/3 of the power supply voltage, then this triggers the chip.
Pin 3 is the output - this will be high for the amount of time specified by the capacitor and resistor on the Discharge pin.
Pin 4 is the reset pin - if this has less than 1/3 the power supply voltage then the timer is reset, and the output is set low.
Pin 5 is the control pin - we don't need to use this so we isolate it using a small 0.1uF capacitor between it and ground.
Pin 6 is the threshold. We connect this to ground via a capacitor, and this (along with the resistor on pin 7) controls the time that the output is high for. We also connect this to pin 7.
Pin 7 is the discharge. We connect this to positive via a resistor. This (along with the capacitor on pin 6) controls how long the output is high for. This is also connected to pin 6.
Output
The output on pin 3 is sufficient for driving an LED. Connect the LED directly to the pin, then take the negative end of the LED and connect that to ground via a 1k resistor. This can also be taken, via a different 1k resistor, to the input pin of a logic gate if that's what you need.
