Help/advice with 555 timer circuit

Discussion in 'The Projects Forum' started by Addo, Jun 23, 2008.

  1. Addo

    Thread Starter Member

    Jul 22, 2006
    Hi all,

    First up, my apologies for another 555 thread as I can't find a similar thread to tag onto...

    I'm attempting to make up a 555 circuit that will trigger a key press on a keyboard with the help of a solid state relay. I've attached a schematic of the actual 555 circuit below but without a few modifications that I'll mention later.

    Essentially, I'm taking a keyboard PCB and attaching the corresponding contacts for the space bar to a solid state relay. I'm planning on having 2 relays in series - one will trigger and simulate the space bar being pressed, one relay will light up an LED and the other will simply just close an external circuit for external expansion ( one of these relays are a dual SPST relay hence 2 relays!)

    So far, after about 2 days trawling through both Google here for inspiration I've come up with the circuit below but with a few modifications. For R1 I've decided to use a variable resistor (finding a 19.96KR resistor I discovered was impossible!!) which will be set and then left alone, and R2 I'm hoping to use a 432KR resistor and a 1MR Potentiometer in series so I can vary the time delay (currently from approx 3s upto approx 10s - this delay is not critical!!)

    I've lost track of what I've read where but I did read something about having a capacitor as a supply bypass due to spikes from the 555 when it changes state. I read from the datasheet of the LMC555CN, that I'm hoping to use, that as it's a CMOS chip, these spikes are reduced - but the datasheet doesn't go into any further detail. As the power is being tapped off the USB port in parallel to the keyboard am I guessing correctly that it would be a good idea to include this bypass and, if so, what capacitor should I use? Also, which type of capacitors should I be using for my project - tantalums, ceramic or electrolytic as I am a complete novice!

    From the LMC555CN datasheet I've noticed that the voltage output when it's in it's 'off' state is around 0.3V to 0.6V whereas the SSR's stated LED minimum forward voltage is 1.15V so am I correct is saying that when the 555 output goes low the SSR will in fact turn off? Or will there be something I need to add such as a resistor to ensure the SSR does turn off?

    I'm hoping I've included enough info here but if I've omitted anything let me know!!! Also, if there is an easier way to accomplish the same task please let me know I'm all ears!!!:)

    Many thanks!!!
  2. Wendy


    Mar 24, 2008
    You gave a lot of info, but I have to admit I'm still confused, so break it down. How long to you want this timer to work (in seconds), and what are you using to trigger it (a push button)?
  3. Addo

    Thread Starter Member

    Jul 22, 2006
    Hi Bill, thanks for the extremely quick reply!!

    After re-reading my post it looks like I completely forgot to mention how I'm going to use the timer!!

    I'm trying to get the 555 output to go low for 3 to 10 seconds (which I'm hoping can be tweaked by the potentiometer) and then I'm aiming for a 200mS duration for when the 555 output goes high (I pulled this figure out of thin air as I've really no idea how long to make the simulated key press - it just seemed to fit!)

    As for triggering the timer I was planning on using a simple on/off rocker switch in line with the Vcc pin so that the circuit is either on or it's off. Once the circuit is powered it will then be left alone maybe for a couple of hours without any further intervention.

    The idea behind this circuit is to automatically cue slides for presentations. Usually you would just set the slides to automatically advance and set them to loop. However, this company I'm working for have just gone over to Macs and currently the slideshow software they use, Keynote, has a strange bug/glitch when slideshows are set to auto advance - yet on manual advance they are fine and glitch free. So in a nutshell, my plan is to make this timer press the space bar at predetermined intervals

    Hopefully I haven't made things more confusing than they were before!! There's times when I just can't quite explain things simply and something tells me this could be one of those times so apologies if that's the case!!

    Many thanks again!! :)
  4. Audioguru


    Dec 20, 2007
    The Cmos 555 has a very low output high current especially when its supply voltage is only 5V. Its output high voltage is spec'd with an output current of only 2mA which might not be enough to turn on your solid state relay. An ordinary 555 has an output high voltage spec'd with an output current of 100mA when its supply voltage is only 5V.

    Use a 0.1uF ceramic capacitor as a supply bypass as is shown in most circuits in the datasheet. The supply current spike from a Cmos 555 operating at your low supply voltage is tiny compared to the up to 400mA supply current spike caused by an ordinary 555.
  5. Addo

    Thread Starter Member

    Jul 22, 2006
    Thanks Audioguru! I have to admit I did have a moment of horror when you mentioned the 2mA output current - I just checked the datasheet of the SSR and found that the stated maximum forward current is 2mA I had originally thought it was 3mA hence my brief moment of panic!!!

    After sifting through the datasheet of the 555 a few times I can't find any mention of the maximum output current so from here onwards it's 2mA! :)

    Thanks again for taking your time to reply! :)