# A Simple Time Delay Circuit

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## Introduction: A Simple Time Delay Circuit

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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Thanks for the instructable, very simple and easy and very well explained. I'm trying to learn about time delays, while trying to study your circuit i am unable to understand a couple of items, maybe you could explain.

Step 2 you have a schematic that shows MOSFET IRF3205, in the photos showing the assembly of the MOSFET is puzzling. Below is a pin-out diagram, it shows pin 1 as the Gate, 2 as the Drain, and pin 3 as the Source.

In the photos you have the resistor coming from the capacitor connected to pin 1, Pin 2 is connected to V+ in the top row, and "Signal Out" to pin 3.

Is the MOSFET a different type/version? I am having problems getting the circuit to work and don't have enough knowledge to troubleshoot.

What I am looking for is a delay before power off; the circuit is energized which energizes another circuit and the delay starts for about 30-45 seconds, then after the delay the other circuit is de-energized while power is still being supplied to the timing circuit, hope it's clear.

Thanks in advance, great work, I enjoyed going over your other work, you would make a good teacher :).

Yes it was a 3205. I looked at the schematic and it seems that the signal out and V+ should be switched around. The V+ should be attached to the drain and the signal out should be attached to the source pin. That was my error sorry. Have you tested the transistor to see if it works, checked connections, power supply, capacitor polarity, etc.? The voltage from the power supply should be at least 4v. The gate is always your main signal going to the transistor. In an N-channel MOSFET , the drain is the drain for the electrons (positive side of the circuit) and the source is the source for the electrons (negative side of the circuit)(electrons move from negative to positive). The signal out line would be where you attach the positive of your load to be switched by the MOSFET.

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.

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.

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.

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

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.

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.

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

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

can you take a picture about board from upside?

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

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.

IC1 should be LM741CS not LM318P. I just forgot to change the number.

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.