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555 Timer Circuit

Discussion in 'General Electronics Discussion' started by Gammon, Feb 10, 2012.

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  1. Gammon


    Feb 10, 2012
    Hi there,

    I was wondering if anyone was able to help with a college question I have been stuck with for a while.

    The aim is to get a square wave output with a duty cycle of as close to 50% as possible using a 555 timer chip. The need for this circuit is to send pulses at the output at intervals of 10ms and I think I have a circuit that could work but i'm a bit unsure for cetain.

    The circuit used is similar to the one in this link:

    I have R1 at 100 ohms and R2 at 15k ohms, with C being 1uF.

    If supplying this circuit with a 10v supply am I right in thinking the output will be the current pulses I need at intervals of 10ms?

    Thanks in advance,

  2. Merlin3189


    Aug 4, 2011
    The 555 was a wonderfully versatile chip. A search on the internet or in amateur electronics books should quickly find you 100's of circuits and explanations of their operation. The question you raise is a common one and has several well known solutions.
    Your circuit will give a pretty good 50% duty cycle. But it should never be exactly 50%, because you charge via R1+R2 and discharge through R2. As long as you keep R2 much bigger than R1 - as you have here- you will be very near 50%.

    Two ways round this to obtain a theoretical 50%:
    -use diodes to direct the charging current through two separate paths, which are given exactly the same resistance. But then you have to carefully match or adjust the two resistances (and diodes).
    -charge and discharge through exactly the same path, so the charge & discharge path must be the same resistance. This can be done by using the output pin as the charge/discharge source, just as an op-amp astable does. (Op-amp astables are another standard circuit in any book.)

    If you are allowed to use another device as well, my favourite for an ideal 50% cycle is to feed the output to a divide by 2 stage (eg. a j-k flip flop) The duration of each half of the cycle should then equal the time between the pulses - which should be exactly the same if the frequency is constant.

    Whatever method you use, even if the output is theoretically a 50% square wave, real component limitations will probably make a very slight difference. Output stages often have a slightly different speed switching low to hi than hi to low. The high output Voltage and the low output Voltage are not necessarily symmetric relative to the switching points of the 555 (1/3 and 2/3 Vs) which themselves are subject to some tolerance. But we are talking here of differences of nano seconds or less. And with the 555 you are looking at pulses no shorter than about a microsecond (unless modern devices are much faster than those I remember.)

    Enjoy researching and using this chip. It's one of the heros of my lifetime. I suppose by comparison with modern VLSI and ASICs it is puny. But when it came out, the functionality that one could get for five bob in a little plastic spider, was miraculous to me!
  3. Gammon


    Feb 10, 2012
    Thanks for the reply, will head to the library today and do a bit more research before making the final choice on how I will have my circuit.
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