Resetting problem on 555 timer

Discussion in 'The Projects Forum' started by PhilTilson, Nov 30, 2009.

  1. PhilTilson

    Thread Starter Well-Known Member

    Nov 29, 2009
    Hi -

    I'll try to keep this as brief as possible, but some explanation is inevitable!

    I am developing a project whose purpose is to turn on some fluorescent tubes when it gets dark, and turn them off when it gets light again. Easy, huh? But I also wanted to include a facility to turn off the lights if the applied battery voltage got too low, so as not to damage the rechargeable lead-acid battery, and to protect against reverse polarity. So I came up with the following circuit, using a 556 dual timer:


    The operation is pretty simple: The group of diodes/LEDs at left provide indication of incorrect battery connection and D3 protects against reverse polarity. R2/R3 provide a potential divider which presents 2.7 volts to pins 2 and 6 of the 556 when the external battery voltage falls to approx 10v. Thus, given that the Vcc for the 556 is 8v, provided by REG1, at this point the 556 should latch.

    This raises pin 5 and forces the second half of the 556 to a reset condition, switching off the load. In the absence of a low battery voltage, the voltage on pins 8 and 12 of the 556 is set by the preset and the light level falling on TR1. When this drops below a set level, the 556 sets and the MOSFET conducts power through the load.

    I thought this would be pretty straightforward! The second half of the circuit works fine, but I have two problems with the battery voltage-sensing side.

    First, when the battery is connected, I cannot prevent the first half of the 556 going into a Set condition, whatever the battery voltage is. I have tried various tricks with capacitors on the reset pin (4) and the voltage inputs without success. If I apply a reset pulse to pin 4 then everything works fine and the lamps come on when the light level falls.

    However, when the battery level falls, the light stays on, even when it is well below the cut-off point of 10v. I can't actually measure this at pins 2 and 6 since, as soon as I apply a probe, the chip resets. Similarly, if I switch on an inspection light, the chip immediately resets!

    Can anyone offer any suggestions as to how to solve the two (probably connected) problems?
  2. lihle

    Active Member

    Apr 12, 2009
    hey man try using LDR they might solve your problem easy.

    i will try to look it up later.
  3. Duane P Wetick

    Senior Member

    Apr 23, 2009
    You might have to add a low voltage (VDC< 10.5) comparator like a LM-311 comparator to shutdown your circuit. This is what the commercial battery chargers use to sense a low battery condition.

    Keep trying, DPW [ Spent years making heaters out of op-amps].
  4. mik3

    Senior Member

    Feb 4, 2008
    Connect a 100uF capacitor between pin 4 and ground and a 5K resistor between pin 4 and 8V.
  5. PhilTilson

    Thread Starter Well-Known Member

    Nov 29, 2009
    Tried that! Unfortunately it didn't have the desired effect. :( Also tried a capacitor from the potential divider point to the 12v line to try to make sure that it was held high at switch-on. That didn't work either!