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Automatic 555 timer, delay off

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  • Automatic 555 timer, delay off


    I hope someone here may give me a little help, I'm building a car and misses a function.

    I need a timer, powered by the engine generator signal D+ (12v). When D+ is on, relay should go on. When engine switches off, D+ dissapears and the relay shold be hold for additional 0.5-2minutes aprox.

    Would be nice with a LED saying relay activated. And a switch to stop the timer and power off the relay. I have looked around the internet for a cheap kit or something, but most of the kits is either for 230v operation or with switch activated function.

    I thing the timer could be made of a very simple circuit board and a 555 timer. But I'm mostly an expert on automotive electrical wiering and not electronics..

    Thanks for any help

  • #2
    I've developed a relay controller than holds a latch for a selectable time period (by jumper)... Still interested? Send me an PM and I'll get you more info.
    Dynaudio 240GT
    PPI PC4100, PPI PC2350
    Zapco DSP6
    Testing new board (Albatron KI690-AM2)

    "I dont know if complaints were filed, or mistakes were made, but feelings were hurt." -Krystal Washington


    • #3
      Here's what you need:

      The two circuits above illustrate opening a relay contact a short time after the ignition or light switch is turned off. The capacitor is charged and the relay is closed when the voltage at the diode anode rises to +12 volts. The circuit on the left is a common collector or emitter follower and has the advantage of one less part since a resistor is not needed in series with the transistor base. However the voltage across the relay coil will be two diode drops less than the supply voltage, or about 11 volts for a 12.5 volt input. The common emitter configuration on the right offers the advantage of the full supply voltage across the load for most of the delay time, which makes the relay pull-in and drop-out voltages less of a concern but requires an extra resistor in series with transistor base. The common emitter (circuit on the right) is the better circuit since the series base resistor can be selected to obtain the desired delay time whereas the capacitor must be selected for the common collector (or an additional resistor used in parallel with the capacitor). The time delay for the common emitter will be approximately 3 time constants or 3*R*C. The capacitor/resistor values can be worked out from the relay coil current and transistor gain. For example a 120 ohm relay coil will draw 100 mA at 12 volts and assumming a transistor gain of 30, the base current will be 100/30 = 3 mA. The voltage across the resistor will be the supply voltage minus two diode drops or 12-1.4 = 10.6. The resistor value will be the voltage/current = 10.6/0.003 = 3533 or about 3.6K. The capacitor value for a 15 second delay will be 15/3R = 1327 uF. We can use a standard 1000 uF capacitor and increase the resistor proportionally to get 15 seconds.
      Originally posted by locked View Post
      Would be nice with a LED saying relay activated.
      Add a LED + current limiting series resistor, placed in parallel with the load you are switching.

      The value of the LED series resistor depends on how birght you want oyur LED to light up. A standard red, yellow or green LED has a typical voltage drop of approx 2V. This means there will be approx 10V across the series resistor. A typical LED current could be 10mA or 20mA.
      Resistor value @ 10mA: 10V/10mA=1kohm
      Resitor value @ 20mA: 10V/20mA=500ohm

      So just pick the resitor value you think gives the right brightness, but make sure not to exceed the maximum current from the LED datasheet.

      Originally posted by locked View Post
      And a switch to stop the timer and power off the relay.
      Not sure excatly what you want the switch function be. But if you want the switch to tunr of the delay function so it works like normal and tunrs on and off without any delay, then add a switch in series with the capacitor. When switched off the relay will shut down as soon as the ignition is turned off as there's no voltage to keep the relay turned on.

      I have added both the switch and LED + series resistor in the schematic below. The blue part symbolises the load you are going to switch. shold be connected differently if the load is connected directly to 12V instead of GND.

      If you want the delay to be very long you can use a darlingtontransistor or MOSFET trasnsistor which both draws much less base/gate current. This way you don't have to use a huge capacitor, but can instead add a resistor from in parallel with the capacitor to GND to control the discharge time.
      Attached Files


      • #4
        Here's a different circuit:
        A Power-Off Delay Circuit

        Originally, I had my MP3 box powered straight from the +12V line, and controlled it with the power switch on the inverter. That way, I could leave it running when stepping out of the car for just a few minutes. Of course, eventually I forgot to turn the thing off for the night, and found my battery dead the next day. That wasn't fun at all. So I decided to make a simple circuit to auto-power off the computer for me.

        This circuit delivers power to the computer as long as the key is in the ignition. After the key is removed, there is a delay of about 10 minutes before the power is cut. There's also a button in the dash to cut off the power immediately.

        The circuit is very simple & cheap, as you can see.

        IMPORTANT NOTE 1: there's a typo in the diagram - the "S" and "D" pins of the MOSFET are reversed.

        IMPORTANT NOTE 2: 50 Ohms for the first resistor is too small. Use 1 KOhm


        • #5
          What does NOTE 1 say? I don't get it, can someone help me please?
          There is nothing impossible in the world :-)


          • #6
            The MOSFET will have 3 pins; gate (G), drain (D) and source (S). It's saying that in the diagram they have accidentally put them the wrong way


            • #7
              oh ok, thanks I got it now
              There is nothing impossible in the world :-)


              • #8
                Here's a product which provides adjustable turn-on and turn-off delays:



                • #9
                  That's kind of odd having a variable resistor on the LED. Adjustable brightness was a concern and not time? hehe