Building a simple Emergency Lighting Circuit

Discussion in 'The Projects Forum' started by richiec_hatters, May 31, 2011.

  1. richiec_hatters

    Thread Starter New Member

    May 31, 2011
    Hope you guys can help me out with a small circuit I am looking to design.

    It is an Emergency Lighting Circuit which must have the following criteria:

    1. Detect an Interruption to a Light Source of 10 secs or more
    2. A delay in which will overcome the short intervals of darkenss.

    I have a 14V Lamp which will give a source of light when greater than 10 secs of darkness is monitored.


    LDR must incorporate sensitivity to detect a variety of light intensities.
    Relays are to be energised in the no fault condition
    Potential Dividers to be used to drive the Relays.

    Having looked at the various forums on here and understand the principles of the time delay circuit and Transister switching, am struggling how to merge the 2 together.

    Any help would be greatfully appreciated.
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
    Here is a brief outline: Assume a 14V DC battery supply with charging ckt. Q1 is an open collector comparator. R3, pot, sets darkness limit. When it is light, Q1 output is low, holding C1 near ground. Loss of light causes V on C1, 10 uF, to rise to 9V in 10 sec, resetting Q2, 555, supplying base current to Q3, turning on lamp of around 1 A@ 14V.