A Simple Time Delay Circuit

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About: Chemistry and electronics have been a staple in my life since I was 8 and have pretty much been my only hobbies although I have dabbled in herbalism, art, music, and various other areas as well.

I finally decided to add another line to my charge controller and I wanted a steady power output instead the PWM that comes off of the dump controller so I made this handy little circuit to take a PWM signal and change it to a constant DC signal.

Step 1: Gather the Materials

To build the basic circuit you will need:

  1. A MOSFET. I used an IRF3205
  2. A capacitor
  3. Two resistors
  4. Jumper wires

Step 2: Assemble the Circuit

Assemble the circuit according to the schematic.

Step 3: Testing and Tuning

Its now time to test the circuit and add the extras.

Add a button connecting the positive rail to the signal in line and connect an LED and a resistor to the signal out line. Apply power and push the button, if it lights up for a short time then fades out, the circuit is working properly and you can now add the relay if you do so choose. The way this circuit works is when the signal in, line goes high and charges up the capacitor and turns on the transistor. The resistor connected to ground slowly drains the capacitor of it's charge and when the capacitor reaches a certain voltage, the transistor fades out and shuts off. What the relay does is act as a type of quasi schmitt trigger and provides a nice transition from on to off without fading by breaking the contacts when the output from the transistor hits a certain voltage. This action would be handy for running electronic devices that can't really tolerate the in between voltages very well such as an induction motor driven pump (AC) or an inverter (DC).

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    13 Discussions

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    JohnO275

    Question 3 months ago on Step 3

    Hi - thanks for posting this. If you continue to perioidically trigger this circuit using the switch (or alternatively a relay) labeled "PAD2", will the output from the IRF3205 continue to be high as long as the capacitor doesn't discharge? I've been trying to build a circuit that keeps things on indefinitely, as long as it periodically receives an input pulse. The idea is that if I have a computer that controls many things (heaters, flows, valves, etc.) and something bad happens - like a computer crash or Windows update, then I want everything to shut off to prevent damage. So I would hook up the output to a large SSR,where all power would come from. It would stay on indefinitely to run the system, as long as it continues to receive the pulses from my program. I was thinking something like a 50 ms pulse every 5 s, so as long as the output voltage doesn't drop below ~3V (from 12) then it would keep the SSR on. I think the time can be adjusted based on the R and C values.

    5 answers
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    electricavemanJohnO275

    Answer 3 months ago

    Yes as long as the voltage on the capacitor doesn't drop below a certain point, the circuit will remain on. The delay can be tuned by changing the size of either the resistor, capacitor, or both, although a smaller capacitor and a higher value resistor will result in less current being drawn to drive it (not like it takes much to drive it anyways haha), also a 100k pot could be used in place of the 47k resistor for fine tuning. One word of warning though: the delay time is also greatly affected by the maximum input voltage of the signal, which with a few more components could actually be a crude ADC (it already kind of is), and length of the pulse.

    In your case, however, I'm not sure that this circuit, as is, would be the best fit considering, in the event of a software malfunction, there is a possibility that the signal pin driving this circuit can remain high until the power is removed from the computer (I've had it happen before) and this will prevent the circuit from turning off. A better circuit might be something like a 555 timer in one-shot mode and using the computer to continually reset the chip to keep the circuit on. In this case if the circuit stops receiving pulses from the computer, the 555 will not be reset and will shut off, cutting the signal to this circuit and then to the SSR(if I remember the workings of the 555 properly). I haven't tested a circuit like this but its something to ponder.

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    JohnO275electricaveman

    Reply 3 months ago

    Thanks for the reply! I tried a circuit using a 555 timer, but it always turned off. I wasn't able to figure out a way to keep the output high indefinitely. It could always be reset, and stay that way for a delay time, but it always would go off. I'll look into that more, as I know there are lot of different ways to use a 555 timer. I agree that just the RC circuit here isn't ideal in case that bit gets stuck high.

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    electricavemanJohnO275

    Reply 3 months ago

    Would a simple DC decoupling with a suitable non-polarized capacitor work?

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    JohnO275electricaveman

    Reply 3 months ago

    Hi, I'm not sure if that would work, as I'm not familiar with DC decoupling. In my old job, I saw somebody make this with a TI CD14538 (along with some resistors, capacitors and a transistor). I tried to duplicate it myself but wasn't successful. I should re-visit that and see where I went wrong and actually understand it better. Thanks.

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    electricavemanJohnO275

    Reply 3 months ago

    A non-polarized capacitor is hooked in series between the signal and the circuit, it will pass AC but not DC. After that a diode could be used to rectify the signal.

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    송진영

    1 year ago

    Can you take a picture about board without relay from upside?

    1 reply
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    electricaveman송진영

    Reply 1 year ago

    Well, I have since disassembled the circuit but I would be more than happy to answer any questions you have about it

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    송진영

    1 year ago

    can you take a picture about board from upside?

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    Fearce1

    1 year ago

    Awesome circuitry and very well explained.
    I believe you could also replace that relay with a comparator chip.
    Thanks for the post.

    3 replies
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    electricavemanFearce1

    Reply 1 year ago

    Well, this is what I came up with. It works perfectly and I must say I like it better than the relay. I changed the resistor value going to the gate of Q1 to a 1K as well.

    time delay full.png
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    electricavemanFearce1

    Reply 1 year ago

    That would most definitely be a more robust solution. One thing I was
    thinking is using an op amp wired up as a comparator considering I have a
    few of those laying around. I'll include a picture. Most of my parts
    are harvested from other electronics including the MOSFET, capacitor,
    and relay used in this instructable so I kind of just use what I have on
    hand.

    op amp.png