# Help Designing A Relay Circuit Please

#### 02CWRX

Joined Aug 3, 2012
11
First of all, I would like to say hello. This is my first post here, so I am looking forward to asking all kinds of silly questions. I am trying to accomplish what I believe to be a simple task. Unfortunately, my understanding and knowledge of circuit design is limited. I have a basic knowledge of what the individual components are, but as far as designing what I want, I've got nothing.

That said, I am looking to build a "time delay" relay. The circuit must accomplish the following things:

1. 12V DC is the trigger voltage
2. When 12V DC trigger is removed, 12V DC is still output from the circuit for a period of time of about 3 seconds fixed (adjustable between 1-10 seconds would be perfect though)
3. Ignores other trigger inputs during operation
4. Self "resetting" so it will do the same timed operation upon receiving the next trigger signal
5. Drive a relay with a 300 mA 12V DC coil
6. Status LED to show when the circuit is operating. (I have 12V DC built-in resistor LEDs on hand)

I have looked at quite a few "one shot" monostable 555 circuits, but unfortunately I just don't know which one is right. Am I correct in assuming there is more than one way to skin a cat on this subject? Obviously the time is accomplished by a combination of resistance and capacitance in most instances, so I know there are variations there, but I am seeing circuits that supposedly accomplish the same thing with various things hooked up to different pins on the 555 too. Not to mention most circuits I have seen look to use a low input on the 555 to trigger it, I need a high input trigger (or a way to convert a high input trigger to a low input).

I guess I just want to know what the "right way" is to accomplish what I'm doing. I bought an "off the shelf" timer kit for \$8 using a 555 timer at a local electronics shop to kind of "brush up" on what was going on, but it's an "intermittent" (bistable?) circuit (has adjustable delay for on time and off time via two pots) so it won't work. Well, that and I can't figure out how to take what I have and convert it into something that does what I want. Good news is, I have some bits and pieces I can use to build something new. I'm also not against buying new parts either of course.

So, if that's not too much to ask for a first post, I would greatly appreciate any and all help on this matter. Thank you in advance!

~T.J.

#### 02CWRX

Joined Aug 3, 2012
11
Here is what I have (had) before. Basically I am trying to build a "relay board". Originally I took all the parts from the timer kit and moved them to a new prototype board on the far right, then wired it all up. As I said, it worked, but not how I wanted, but I knew that going in. I just wanted "proof of concept" I suppose. Each of the 4 relays on the left coincide with a vertical wiring terminal, and the far right relay is the timer relay and uses the front horizontal wiring block. The wiring blocks are where everything comes in for each relay/circuit (power, ground, signal, input, output).

The idea is that I can connect my components to the four relays on the left how I need (which works fine, they're just relays), but I need the far right relay to operate as described above. Seems pretty simple in my mind, unfortunately, I am not an electrical engineer, haha.

~T.J.

EDIT: As a secondary question, what gauge of wire do you guys use to wire up these prototype boards? I had used 18 gauge on the back side for the relay input and outputs to the terminal blocks, and 22 gauge on the front for the signal and coil power. The relays will all be running 12V DC circuits at about 5A. I think 22 gauge for it all would have been safe, but I went with the 18 gauge for the load side of the relay to be safe. I you think 22 gauge would be safe for that also, that would be better since the 18 gauge was a bit of a pain to work with (large insulation).

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#### wayneh

Joined Sep 9, 2010
17,461
Maybe I'm missing something, but you want the relay to remain energized after the power has been removed? That's obviously a problem - the relay needs a fair amount of current (ie. more than a practical capacitor can store) and it needs to come from somewhere. If you've got a 12v supply, and a separate trigger signal, it'll be easy. But usually with a relay the "trigger" is the coil power itself.

#### 02CWRX

Joined Aug 3, 2012
11
Correct, there is 12V DC constant available, plus the 12 VDC trigger from the switch. I suppose a latching relay of some kind is an option too?

~T.J.

#### Bernard

Joined Aug 7, 2008
5,784
Here is a simple ckt if continuous power to 555 is allowed, there are ways around that if necessary.R2 may not be necessary if neg pulse is suffecent to trigger 555.

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#### 02CWRX

Joined Aug 3, 2012
11
Thanks for the diagram. Are there any special requirements of those diodes, that transistor, or that other capacitor? I see the 0.1 uF cap, but what is the other? Or does the C+ mean I have to calculate it somehow? Same with the R+ for the resistor pot size?

I have no transistors on hand, but I have a couple 1N4148s here from the kit I bought. Can I use those, or should I get different ones? I will need to get resistors at the store anyway because apparently I only have (3) 1K resistors from the kit. I have the 1M pot from the kit already, so that's good. I do have one small orange capacitor that was in the kit that's 100nF, and two other electrolytic that are 100uF - which one should I use, or do I need a different ones?

As you can tell, I'm not quite fluent in all this. I know enough to get myself in trouble it seems. Again, thanks for the help, it's very much appreciated.

~T.J.

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#### Bernard

Joined Aug 7, 2008
5,784
100nF = .1μF; Ct, timing, 100μF & 1 meg pot, Rt = about 100 sec. A 10 μF might be easier to adjust timing. 1N4148, 1000V, 300 mA are good for all 3. Put 1k between +12V & top of pot so +12 can not be directly fed to 555. Any NPN transistor with greater than 300 mA rating, 2N2222 etc.

#### 02CWRX

Joined Aug 3, 2012
11
I appreciate the help, but the way that was answered doesn't make a lot of sense to me. It looks like you're saying the Ct capacitor in the drawing could be either 100 uF for about 100 seconds of time, or 10 uF to make it easier to adjust the time? Either way the 1 meg pot I have will work it looks like. The diode and transistor answer makes sense, but when you say the 1K between top of pot and the 12V, I assume you mean an additional resistor not shown in that schematic? Also, where could I incorporate an LED into the circuit that will light when the relay is on?

Thanks SO much for your help, I really appreciate it! I hope these questions aren't too silly, but I'm trying to learn, haha.

~T.J.

#### Bernard

Joined Aug 7, 2008
5,784
Yes, the 1k needs to be added to the top of pot. Time is about Rt, meg Ω, X Ct, μF, so 1 X 100 = 100 sec. For 3 sec, only about bottom 3% of pot is used, making it a bit touchey; 10μF for 3 sec would be 30% of pot, and 10 sec would be full pot. Just parallel 12 V LED with relay coil, cathode to collector, or if only 555 time desired, connect between pin 3 and ground.

#### 02CWRX

Joined Aug 3, 2012
11
Thanks a lot, that makes sense. I will be picking up some supplies today to see if I can make this work on a breadboard. I really appreciate it!

~T.J.

#### 02CWRX

Joined Aug 3, 2012
11
Ok, keeping in mind I'm still new to this, I can't seem to get this to work. I have put it onto a bread board multiple times and it's just not working out. The most I have gotten from it is that as soon as I connect the 12V power, the relay clicks, which is not what I want. If I turn on the switch, it creates a short somehow. I'm fairly certain it's probably something I am doing, but I don't know.

Any ideas as to what I might be doing wrong?

~T.J.

#### 02CWRX

Joined Aug 3, 2012
11
For what it's worth, I was able to wire up this circuit (ignoring the reset switch and control capacitor) on the bread board using an LED as the output and it worked fine: http://www.kpsec.freeuk.com/555timer.htm#monostable

Then I changed out the LED for a transistor to switch the relay, and that worked fine. Then I added the LED to light along with the relay, and that works fine. Obviously this circuit is using a ground for a trigger which won't work in my application, but at least I know I can wire SOMETHING up. The question then becomes why can't I make the other circuit work?

~T.J.

#### Bernard

Joined Aug 7, 2008
5,784
Maybe it does not work because we are not on the same page.
# 2 "When 12 V trigger is removed" implies that 12 V must have been present beforehand or at turn on . SW is NC so power is applied to relay at powerup; when SW opens, 555 triggers , keeping relay energised for several seconds, thed drops out. Works like this on my bread board. Might define trigger operation in more detail.
If Trigger SW is suposed to be NO, operator closes SW to close relay, & when SW is opened relay stays closed for timeout period. Can be made to do this by adding reset at turn on. It is late- will check it out tomorrow.

#### 02CWRX

Joined Aug 3, 2012
11
Ok, thanks. I think we do have a little miscommunication, let me try and explain better.

The switch is normally open. On power up, the circuit should not do anything. Once the switch is closed momentarily, it sends out 12V DC. I need that action to activate the timer so it activates the relay for the set time.

The circuit I have built does what I want, but it does it by pulling pin 2 low using a ground. Can I just wire a transistor so that the 12V signal will pull pin 2 low? I might try that...

~T.J.

EDIT: I added a transistor to the circuit I had so that the collector was on pin 2 of the 555, the emiter was ground, and used my switch to sent 12DC into the base. This worked a couple times, but I think I fried the transistor. Any way to make this work permanently? Haha

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#### 02CWRX

Joined Aug 3, 2012
11
Ok, I found this while searching. This is basically what I think I need. I have built something similar now which works on my second attempt after killing one transistor, but the problem I have is that it triggers the relay as soon as power is applied, hopefully this circuit doesn't do that.

~T.J.

#### Bernard

Joined Aug 7, 2008
5,784
Still a bit of confusion; first was 555 triggers on removal of 12 V, as present schematic shows, or 555 triggers on application of 12 V which requires Bill's ckt with invertor. R5 & C3 provide a reset at turn on. C3 @ .1 is about a minimum & can be several uF, with larger Cs requiring longer time to discharge at power-off before next power-on.

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#### 02CWRX

Joined Aug 3, 2012
11
I need it to trigger on application of 12V from the switch.

His circuit looks like it will work for what I want, my only concern would be if it activates the timer each time power is connected. I will build it later and see.

The circuit I have built now is similar to what he shows, but mine activates the relay every time the power is connected - It can't work like this. I need something that will let me connect power and not do anything until it gets the 12V signal from the switch.

~T.J.

#### Bernard

Joined Aug 7, 2008
5,784
Try adding my ckt R5 & C3,to your ckt. making C3 a little larger, maybe 1μF.