Instructables

Timer Circuit

I found this simple circiut on jamesyawn.com, and was wondering if and how to change it so that it could time in the minutes, longer than it does already. I don't know anything about electronics, aside from soldering, and basic connections, so try to give a simple-ish answer. I also wondered about what transistor to use (like a radioshack part number). The circuit I was talking about is under "discrete timers".

(update, more questions)
What kind of timing (like how many seconds) would I get with this?
Using the same stuff shown on the schematic?

Also, how do I pick a transistor?
Are they interchangeable?
What specs should I be looking for?

If this seems like too much, could someone give me a link to a site that would be willing to help with (very) beginning electronics?

AND, how would you recommend making the transfer schematics for home-etched circuit boards? I can't get the component spacing right.

Picture of Timer Circuit
555 timer is the next step, look for it on the internet. Radioshack sells it. Its simple to use and I got up to 30min with it. But its not exact its more in between 25-35min.(it changes). You want the astable, or osillator mode when you research it, that will make more sense when you look for it.
Austringer6 years ago
A while back somebody offered me a heap of ego with very precisely zero advice on one of the Make comment threads. I'm going to give you the advise he would have given me had he not been all about letting me know how smart he was. Track down a copy of "The Art of Electronics" by Horowitz and Hill! It's a text book and, ergo, not exactly cheap, but I'm finding it worth it. To answer one of your questions up there, a capacitor charges in roughly 5RC where R is the resistance in ohms and C is the capacitance in Farads. In the diagram you have your looking at 150,000 ohms x .0002 Farads so 2. 5 minutes to FULLY charge the capacitor. Your transistor will do it's thing before that happens. What are you wanting to do with this? How exact does it need to be?
gmoon6 years ago
This uses a PNP transistor. Radio Shack sells the 2N3904, a PNP--but it can only handle 100mA (I've seen small relays that only draw 30-40mA, but an automotive relay is anywhere between 80-140mA....)

I'd use a 2N2222--cheap, easy to find, and can sink 1A. But they are NPN transistors, so change the circuit: Transistor circuits link.

You might consider using a 555 instead of the simple RC circuit....

Pretty old project there... Ironically, it's described as " Time delay relays are basic electronic circuits which serve to protect costly equip. from damage due to current surges ", but there's no surge-protection diode on the relay! Check out the link above for an explanation of the diode placement.
John Smith (author)  gmoon6 years ago
Ok, I'm lost. I need to find a simple electronics book.

Anyway, would a 2N3055 work? It is NPN, and I have one in the garage.
The specs of it are here. (at the end of the NPN section)

Thanks for the help.
I don't see why not...but that's serious overkill for a relay! (15 amp rating !!) It's gain (Hfe) is rather small, so it will draw more base current (no biggie.)

Most of the example on that link are NPN, so start there. As far as converting your original schematic (PNP), it's essentially upside-down compared to the PNP circuit on the 'Transistor Circuits' page.

I.E., (for your schem) the power source polarity-- the GND (-) is at the top, and the positive is at the bottom...Use the NPN circuits, and convert the 'timer' part by flipping it top-to-bottom...

Both Goodhart and I made the point about spike protection, so pay attention...;-)
John Smith (author)  gmoon6 years ago
GAH! I'm all confused!

...As far as converting your original schematic (PNP), it's essentially upside-down compared to the PNP circuit on the 'Transistor Circuits' page.
What do you mean "upside down"? To what PNP circuit on that page? I didn't see a timer there.

I.E., (for your schem) the power source polarity-- the GND (-) is at the top, and the positive is at the bottom...Use the NPN circuits, and convert the 'timer' part by flipping it top-to-bottom...
What NPN circuit?

Both Goodhart and I made the point about spike protection, so pay attention...;-)
*sigh* I guess now I have to ask what the heck you are talking about...
SO, what is "spike protection"?

I am SO sorry that I am making you do all of the work, I just don't understand this stuff. I've tried, but the second I get to understanding it, I get confused again!


The "that page" link reference...

1) What do you mean "upside down"? To what PNP circuit on that page? I didn't see a timer there.

--There is only one PNP circuit on "that page." Since you want to use an NPN instead, you'll need to convert your PNP circuit to the way they draw them on "that page..." The power source polarity on their drawing is upside-down compared to yours....

--Yep, no timer there, that's why you want to convert your schematic.

2) What NPN circuit?

--Any NPN circuit on "that page"--might as well get a working transistor-switched relay before tackling the timer...

3) SO, what is "spike protection"?

See the Protection diode section on "that page" :

If the load is a motor, relay or solenoid (or any other device with a coil) a diode must be connected across the load to protect the transistor (and chip) from damage when the load is switched off.....

There's a nice illustration next to the text....

Forget it, I drew you a picture and converted the circuit to NPN (No guarantees this will function, as I'm just copying someone else's circuit...) The right side is "their" drawing, the left is the timer circuit from yours...
npn.jpg
John Smith (author)  gmoon6 years ago
Ok, thanks for clearing that up for me. I just get confused with electronics and stuff. Oh, and thanks for the schematic, too.
Yep. Stick with it, it'll make sense eventually.

A couple things I didn't mention:

--The relay would rated for the supply voltage ( 5V, 9V , 12...whatever.)

--The 200uF cap is polarized (has a + and - side), so don't connect it backwards. The top in the drawing is connected to +V, so that's the + lead...

--There isn't a base resistor, per say, just the RC timer circuit connected to the base (maybe you should add a current limiting resistor to the base.) But also, the amount of base current required to switch on the transistor will vary according to the load (the relay.)

So it's pretty much impossible to predict what the delay time will be. Switch the load to an LED or motor, etc, and the delay will change...
John Smith (author)  gmoon6 years ago
Ok, I think I got it, I'll solder one up tomarrow...man, I wish I had my breadboard right not...I guess I'll have to wait for Christmas.
John Smith (author)  John Smith6 years ago
Hey, that's a NC relay, right?
Don't think it matters--choose whichever type suits your purpose. Also, should have mentioned--the relay voltage source doesn't need to match the voltage applied to the timer / transistor base. In that case, the 'initiate' switch wouldn't be connected to the relay voltage source. The switch could just be connected to the timer part. Both power supplies need to share the same GND, of course... For this circuit, it doesn't matter. But if you wanted to switch a 12V relay with a 5V microcontroller (instead of the RC timer) it would...
John Smith (author)  gmoon6 years ago
Any idea on how to design the traces for etching my own board? I can't seem to get the spacing right.
At this stage (if you don't have a solderless breadboard), I'd use one of those perfboards with the copper pads. Be easier to change components, since you haven't tested the timer, etc. Just solder point-to-point with some wire...
John Smith (author)  gmoon6 years ago
I did that, and tested it once..and...it WORKS! Well, I have a few kinks to work out, but I think it WORKS! I can hear the relay, but, it doesn't seem to stay on, just a short burst of clicks to begin with, then silence, even when I hold the battery on. I think it's because of a low battery; my 9v I'm using is giving out 8.35v (even thought that sounds about right). I'll try it with two 6v lantern batteries later. You don't think that'll be too much current, do you? Gah- I know I'm getting a breadboard for Christmas, because I saw the box from The Electronic Goldmine today! Dangit, I need it NOW.
Yeah, could be a weak battery (especially if it reads low without a load.) You need to choose the supply voltage based on the rating of the device. Running a 6V relay on 12V forces more current through than the relay can handle. Your transistor won't have a problem with 12V (max 60V.)
John Smith (author)  gmoon6 years ago
Don't think it matters--choose whichever type suits your purpose.
Ah, yep, stupid question.

For this circuit, it doesn't matter. But if you wanted to switch a 12V relay with a 5V microcontroller (instead of the RC timer) it would...
Ok, I understand that. Hopefully I can find a working relay somewhere, and make the circuit tomorrow...
NachoMahma7 years ago
. If you're talking about "A. Schematic from Radio Shack 150 Project kit (circa 1972)", then it's just a simple RC (resistance/capacitance) circuit. . The time can be changed by varying the resistors and the capacitor. Don't remember the details, but someone that knows will probably chime in. . It looks to me that just about any ?NPN? transistor that will handle the load of the relay coil should work - this is not a "precision" circuit.
John Smith (author)  NachoMahma7 years ago
Like I said, I don't know much about electronics.
Would a "mpf 102" transistor work?
Also, could I use a variable capacitor instead of the variable resistor? I don't have a variable resistor around, so...
Also, when I put in the transistor, which way does it go? According to this page (flat side to the left/right)?
The MPF102 is a MOSFET liinke to mpf102 specs
Goodhart6 years ago
http://jamesyawn.com/timers/index.html

Does appear to be a broken link.
John Smith (author)  Goodhart6 years ago
Oh, yep, sure is. his entire website seems to be gone...awww, I liked it... The schematic I was talking about is above, and there isn't much info on that specific timer on that page anyway.
The only thing I see that might be a problem is the lack of a diode across the coil of the relay: potential for the creation of a backwards surge when the coil opens each time. This would surely destroy the transistor.
John Smith (author) 6 years ago
*ahem* BUMP *ahem*
NachoMahma7 years ago
> Like I said, I don't know much about electronics. . It's been several decades since I worked with this stuff so I can't give you any details. :) . > Would a "mpf 102" transistor work? . Don't know. I no longer have the references to look it up. . > Also, could I use a variable capacitor instead of the variable resistor? I don't have a variable resistor around, so... . No. That one I do know the answer to. . > Also, when I put in the transistor, which way does it go? According to this page (flat side to the left/right)? . I don't remember. . . Since you're not getting an answer here, you might try a site devoted to beginning electronics.
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