555 timer help

Discussion in 'The Projects Forum' started by ecjohnny, Oct 31, 2008.

  1. ecjohnny

    Thread Starter Senior Member

    Jul 16, 2005
    I need help. i have made a 555 timer from this site http://www.kpsec.freeuk.com/projects/timer.htm

    i used a lever switch to replace the switch there. i configure my lever switch as NC . i found out the 555 timer trigger my load (e.g LED) for 2 sec and off is fine which i thought off the 555 timer too BUT because my switch is in NO that doesn't off the 555 timer circuit which depleted my battery source really fast. HELP!!!

    My switch need to be in NO. how can i solve this problem.

    *i have been in stress in making this and think the circuit is wrong. and remaking it and also think the voltage is insufficient... i feel so dumb to find out actually the 555 timer circuit is continually depleting my battery. i really need HELP.
  2. Wendy


    Mar 24, 2008
    Actually it is an easy problem to fix, but are you just driving an LED? Are you using the entire circuit, as in the bleeper and 2 LEDs? If so this is contribuiting to your power drain. There is a CMOS version that should work just fine (available from Radio Shack) and would extend the battery a long time, but you need to get rid of the bleeper/red LED.


    It would be a really good idea to update the schmatic to show what you are doing. Are you trying to use a momentary contact switch?

    I've written an article that is in the final stages of proofreading here.
  3. ecjohnny

    Thread Starter Senior Member

    Jul 16, 2005
    My output is to a RF transmitter operating at 9vdc . Basically i just connect the pin3(output) to the vcc of RF transmitter and gnd to gnd.

    Everything works well, except the next day, i found out the circuit is not working due to the battery because i change a new one and it works again.

    The schematic is the same(link above) except i replaced :

    Variable resistor <--> 890k ohm (2sec delay for this value)

    470k resistor <---> 10k ohm

    switch <---> lever switch (NC NO COM)

    The rest remain same. except the output is a RF transmitter.

    How can solve it without using the CMOS version. Thanks.
  4. Kefka666

    Active Member

    Mar 4, 2008
    I've had a similar problem where a simple 555 timer circuit rapidly depleted a 9V battery while powering only 4 LEDs (@40mA). Haven't figured out why it happens.
  5. SgtWookie


    Jul 17, 2007
    Change your switch wiring from using the NC and COM terminals to using the NO and COM terminals.
    That way, when the switch has no pressure on it, there will be no drain on the battery. When the switch closes, the RF transmitter will be energized for 2 seconds, and then turn off. In order to stop current flow altogether, the switch needs to be released.

    Either that, or add a power switch so that you can turn the thing off when you're not using it.
  6. Wendy


    Mar 24, 2008
    We have a close idea as to what you're doing, but there are some inconsistancies. Their is neither a 470KΩ nor 10KΩ in this circuit. This is why drawing schematics is usually a good idea. You never answered the questions about the LEDs and buzzer, I assume their gone.

    One of the problems you're fighting is a standard 555 has a minimum of 15KΩ resistance for the voltage dividers, plus whatever the circuitry takes. The 7555 (cmos) doesn't, yet is a drop in replacement (no changes needed). The 555 is an old design, and works quite well within its limits, the CMOS version was created to overcome these limits (wider voltage range, lower current draw, much lower, with only a minor reduction in drive current on the output). The other resistors aren't contributing much overall to current draw, since the caps are basically open circuits once the surges are done.

    Baring that, all of Wookie's comments are also valid. You want extended battery life, remove all loads. 9V Batteries just aren't that deep.

    One last point, a 555 can not quite reach whatever it's supply voltage is, it drops around 1.3V internally. Your transmitter is getting around 7.7V when it is on.
  7. ecjohnny

    Thread Starter Senior Member

    Jul 16, 2005
    LEDs and buzzer is gone. i need the switch to be in NC. It activate the transmitter only when the pressure is released. But because it is is NC, i guess the current/voltage is still flowing in the 555 timer.

    If i apply pressure on the switch then only it will off the circuit completely which trouble me. i want it to to be completely off after i trigger my transmitter. and applying pressure on lever switch again will do nothing .
  8. bertus


    Apr 5, 2008

    Are you able to use a optocoupler in place of the relais.
    The optocoupler will draw less current.
    The place of the led from the optocoupler will determint NO or NC.

  9. Wendy


    Mar 24, 2008
    I don't think he has a relay, just a transmitter.

    Does this circuit work? If the switch is NC (Normally closed), when the door is opened does the switch open? Are you using a reed switch maybe?

    We are having some problem communicating. It's pretty common here, so don't sweat it, but we do need information. From what I'm hearing the switch closes when ever what you want to signal happens, you just want to limit to a 2 second burst, and you want the battery to last.
  10. ecjohnny

    Thread Starter Senior Member

    Jul 16, 2005
    The switch is what i mean by NO NC COM. The lever switch can be configure as NO NC COM.

    When i connect a battery to it at start, it automatically light up for 2 sec and off which is what the 555 timer does for me. WORK FINE.

    The problem is it doesn't completely off!(switch is NC) . So i guess current/voltage is still being drawn from the battery which depleted my battery so fast. Next day and the 9V battery is dead.

    i hope i clear any confuse.

    Here is a rough schematic of the connection http://img253.imageshack.us/my.php?image=555helprp6.jpg

    Thanks for the reply.
    Last edited: Nov 2, 2008
  11. ecjohnny

    Thread Starter Senior Member

    Jul 16, 2005
    i am quite sure is the NC switch that cause drain the battery.
    BUT i really need the switch to be in NC in my circuit.

    SWITCH = Lever Switch

    Basically what i am trying to do is a key hanger. when a key is hung onto(pressure) the switch, it will off the circuit/open circuit.
    when i remove the key, it will be NC (no pressure) and trigger my transmitter for 2 sec and off.

    BUT because of it being in NC, i guess it doesn't completely off, current/voltage is still being drawn. Hence, the next day or after couple of hours, the battery is dead.

    Any solution?